what are the main causes of climate change essay

What Are the Causes of Climate Change?

We can’t fight climate change without understanding what drives it.

A river runs through a valley between mountains, with brown banks visible on either side of the water

Low water levels at Shasta Lake, California, following a historic drought in October 2021

Andrew Innerarity/California Department of Water Resources

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At the root of climate change is the phenomenon known as the greenhouse effect , the term scientists use to describe the way that certain atmospheric gases “trap” heat that would otherwise radiate upward, from the planet’s surface, into outer space. On the one hand, we have the greenhouse effect to thank for the presence of life on earth; without it, our planet would be cold and unlivable.

But beginning in the mid- to late-19th century, human activity began pushing the greenhouse effect to new levels. The result? A planet that’s warmer right now than at any other point in human history, and getting ever warmer. This global warming has, in turn, dramatically altered natural cycles and weather patterns, with impacts that include extreme heat, protracted drought, increased flooding, more intense storms, and rising sea levels. Taken together, these miserable and sometimes deadly effects are what have come to be known as climate change .

Detailing and discussing the human causes of climate change isn’t about shaming people, or trying to make them feel guilty for their choices. It’s about defining the problem so that we can arrive at effective solutions. And we must honestly address its origins—even though it can sometimes be difficult, or even uncomfortable, to do so. Human civilization has made extraordinary productivity leaps, some of which have led to our currently overheated planet. But by harnessing that same ability to innovate and attaching it to a renewed sense of shared responsibility, we can find ways to cool the planet down, fight climate change , and chart a course toward a more just, equitable, and sustainable future.

Here’s a rough breakdown of the factors that are driving climate change.

Natural causes of climate change

Human-driven causes of climate change, transportation, electricity generation, industry & manufacturing, agriculture, oil & gas development, deforestation, our lifestyle choices.

Some amount of climate change can be attributed to natural phenomena. Over the course of Earth’s existence, volcanic eruptions , fluctuations in solar radiation , tectonic shifts , and even small changes in our orbit have all had observable effects on planetary warming and cooling patterns.

But climate records are able to show that today’s global warming—particularly what has occured since the start of the industrial revolution—is happening much, much faster than ever before. According to NASA , “[t]hese natural causes are still in play today, but their influence is too small or they occur too slowly to explain the rapid warming seen in recent decades.” And the records refute the misinformation that natural causes are the main culprits behind climate change, as some in the fossil fuel industry and conservative think tanks would like us to believe.

A black and white image of an industrial plant on the banks of a body of water, with black smoke rising from three smokestacks

Chemical manufacturing plants emit fumes along Onondaga Lake in Solvay, New York, in the late-19th century. Over time, industrial development severely polluted the local area.

Library of Congress, Prints & Photographs Division, Detroit Publishing Company Collection

Scientists agree that human activity is the primary driver of what we’re seeing now worldwide. (This type of climate change is sometimes referred to as anthropogenic , which is just a way of saying “caused by human beings.”) The unchecked burning of fossil fuels over the past 150 years has drastically increased the presence of atmospheric greenhouse gases, most notably carbon dioxide . At the same time, logging and development have led to the widespread destruction of forests, wetlands, and other carbon sinks —natural resources that store carbon dioxide and prevent it from being released into the atmosphere.

Right now, atmospheric concentrations of greenhouse gases like carbon dioxide, methane , and nitrous oxide are the highest they’ve been in the last 800,000 years . Some greenhouse gases, like hydrochlorofluorocarbons (HFCs) , do not even exist in nature. By continuously pumping these gases into the air, we helped raise the earth’s average temperature by about 1.9 degrees Fahrenheit during the 20th century—which has brought us to our current era of deadly, and increasingly routine, weather extremes. And it’s important to note that while climate change affects everyone in some way, it doesn’t do so equally: All over the world, people of color and those living in economically disadvantaged or politically marginalized communities bear a much larger burden , despite the fact that these communities play a much smaller role in warming the planet.

Our ways of generating power for electricity, heat, and transportation, our built environment and industries, our ways of interacting with the land, and our consumption habits together serve as the primary drivers of climate change. While the percentages of greenhouse gases stemming from each source may fluctuate, the sources themselves remain relatively consistent.

Four lanes of cars and trucks sit in traffic on a highway

Traffic on Interstate 25 in Denver

David Parsons/iStock

The cars, trucks, ships, and planes that we use to transport ourselves and our goods are a major source of global greenhouse gas emissions. (In the United States, they actually constitute the single-largest source.) Burning petroleum-based fuel in combustion engines releases massive amounts of carbon dioxide into the atmosphere. Passenger cars account for 41 percent of those emissions, with the typical passenger vehicle emitting about 4.6 metric tons of carbon dioxide per year. And trucks are by far the worst polluters on the road. They run almost constantly and largely burn diesel fuel, which is why, despite accounting for just 4 percent of U.S. vehicles, trucks emit 23 percent of all greenhouse gas emissions from transportation.

We can get these numbers down, but we need large-scale investments to get more zero-emission vehicles on the road and increase access to reliable public transit .

As of 2021, nearly 60 percent of the electricity used in the United States comes from the burning of coal, natural gas , and other fossil fuels . Because of the electricity sector’s historical investment in these dirty energy sources, it accounts for roughly a quarter of U.S. greenhouse gas emissions, including carbon dioxide, methane, and nitrous oxide.

That history is undergoing a major change, however: As renewable energy sources like wind and solar become cheaper and easier to develop, utilities are turning to them more frequently. The percentage of clean, renewable energy is growing every year—and with that growth comes a corresponding decrease in pollutants.

But while things are moving in the right direction, they’re not moving fast enough. If we’re to keep the earth’s average temperature from rising more than 1.5 degrees Celsius, which scientists say we must do in order to avoid the very worst impacts of climate change, we have to take every available opportunity to speed up the shift from fossil fuels to renewables in the electricity sector.

A graphic titled "Total U.S. Greenhouse Gas Emissions by Economic Sector (2020)"

The factories and facilities that produce our goods are significant sources of greenhouse gases; in 2020, they were responsible for fully 24 percent of U.S. emissions. Most industrial emissions come from the production of a small set of carbon-intensive products, including basic chemicals, iron and steel, cement and concrete, aluminum, glass, and paper. To manufacture the building blocks of our infrastructure and the vast array of products demanded by consumers, producers must burn through massive amounts of energy. In addition, older facilities in need of efficiency upgrades frequently leak these gases, along with other harmful forms of air pollution .

One way to reduce the industrial sector’s carbon footprint is to increase efficiency through improved technology and stronger enforcement of pollution regulations. Another way is to rethink our attitudes toward consumption (particularly when it comes to plastics ), recycling , and reuse —so that we don’t need to be producing so many things in the first place. And, since major infrastructure projects rely heavily on industries like cement manufacturing (responsible for 7 percent of annual global greenhouse gas), policy mandates must leverage the government’s purchasing power to grow markets for cleaner alternatives, and ensure that state and federal agencies procure more sustainably produced materials for these projects. Hastening the switch from fossil fuels to renewables will also go a long way toward cleaning up this energy-intensive sector.

The advent of modern, industrialized agriculture has significantly altered the vital but delicate relationship between soil and the climate—so much so that agriculture accounted for 11 percent of U.S. greenhouse gas emissions in 2020. This sector is especially notorious for giving off large amounts of nitrous oxide and methane, powerful gases that are highly effective at trapping heat. The widespread adoption of chemical fertilizers , combined with certain crop-management practices that prioritize high yields over soil health, means that agriculture accounts for nearly three-quarters of the nitrous oxide found in our atmosphere. Meanwhile, large-scale industrialized livestock production continues to be a significant source of atmospheric methane, which is emitted as a function of the digestive processes of cattle and other ruminants.

A man in a cap and outdoor vest in front of a wooden building holds a large squash

Stephen McComber holds a squash harvested from the community garden in Kahnawà:ke Mohawk Territory, a First Nations reserve of the Mohawks of Kahnawà:ke, in Quebec.

Stephanie Foden for NRDC

But farmers and ranchers—especially Indigenous farmers, who have been tending the land according to sustainable principles —are reminding us that there’s more than one way to feed the world. By adopting the philosophies and methods associated with regenerative agriculture , we can slash emissions from this sector while boosting our soil’s capacity for sequestering carbon from the atmosphere, and producing healthier foods.

A pipe sticks out of a hole in the ground in the center of a wide pit surrounded by crude fencing

A decades-old, plugged and abandoned oil well at a cattle ranch in Crane County, Texas, in June 2021, when it was found to be leaking brine water

Matthew Busch/Bloomberg via Getty Images

Oil and gas lead to emissions at every stage of their production and consumption—not only when they’re burned as fuel, but just as soon as we drill a hole in the ground to begin extracting them. Fossil fuel development is a major source of methane, which invariably leaks from oil and gas operations : drilling, fracking , transporting, and refining. And while methane isn’t as prevalent a greenhouse gas as carbon dioxide, it’s many times more potent at trapping heat during the first 20 years of its release into the atmosphere. Even abandoned and inoperative wells—sometimes known as “orphaned” wells —leak methane. More than 3 million of these old, defunct wells are spread across the country and were responsible for emitting more than 280,000 metric tons of methane in 2018.

Unsurprisingly, given how much time we spend inside of them, our buildings—both residential and commercial—emit a lot of greenhouse gases. Heating, cooling, cooking, running appliances, and maintaining other building-wide systems accounted for 13 percent of U.S. emissions overall in 2020. And even worse, some 30 percent of the energy used in U.S. buildings goes to waste, on average.

Every day, great strides are being made in energy efficiency , allowing us to achieve the same (or even better) results with less energy expended. By requiring all new buildings to employ the highest efficiency standards—and by retrofitting existing buildings with the most up-to-date technologies—we’ll reduce emissions in this sector while simultaneously making it easier and cheaper for people in all communities to heat, cool, and power their homes: a top goal of the environmental justice movement.

An aerial view show a large area of brown land surrounded by deep green land

An aerial view of clearcut sections of boreal forest near Dryden in Northwestern Ontario, Canada, in June 2019

River Jordan for NRDC

Another way we’re injecting more greenhouse gas into the atmosphere is through the clearcutting of the world’s forests and the degradation of its wetlands . Vegetation and soil store carbon by keeping it at ground level or underground. Through logging and other forms of development, we’re cutting down or digging up vegetative biomass and releasing all of its stored carbon into the air. In Canada’s boreal forest alone, clearcutting is responsible for releasing more than 25 million metric tons of carbon dioxide into the atmosphere each year—the emissions equivalent of 5.5 million vehicles.

Government policies that emphasize sustainable practices, combined with shifts in consumer behavior , are needed to offset this dynamic and restore the planet’s carbon sinks .

A passnger train crosses over a bridge on a river

The Yellow Line Metro train crossing over the Potomac River from Washington, DC, to Virginia on June 24, 2022

Sarah Baker

The decisions we make every day as individuals—which products we purchase, how much electricity we consume, how we get around, what we eat (and what we don’t—food waste makes up 4 percent of total U.S. greenhouse gas emissions)—add up to our single, unique carbon footprints . Put all of them together and you end up with humanity’s collective carbon footprint. The first step in reducing it is for us to acknowledge the uneven distribution of climate change’s causes and effects, and for those who bear the greatest responsibility for global greenhouse gas emissions to slash them without bringing further harm to those who are least responsible .

The big, climate-affecting decisions made by utilities, industries, and governments are shaped, in the end, by us : our needs, our demands, our priorities. Winning the fight against climate change will require us to rethink those needs, ramp up those demands , and reset those priorities. Short-term thinking of the sort that enriches corporations must give way to long-term planning that strengthens communities and secures the health and safety of all people. And our definition of climate advocacy must go beyond slogans and move, swiftly, into the realm of collective action—fueled by righteous anger, perhaps, but guided by faith in science and in our ability to change the world for the better.

If our activity has brought us to this dangerous point in human history, breaking old patterns can help us find a way out.

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Greenhouse Effect 101

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Causes of Climate Change

Graph: Human and Natural Influences on Global Temperature

Since the Industrial Revolution, human activities have released large amounts of carbon dioxide and other greenhouse gases into the atmosphere, which has changed the earth’s climate. Natural processes, such as changes in the sun's energy and volcanic eruptions, also affect the earth's climate. However, they do not explain the warming that we have observed over the last century. 1

Human Versus Natural Causes

It is unequivocal that human influence has warmed the atmosphere, ocean and land . - Intergovernmental Panel on Climate Change 4

Scientists have pieced together a record of the earth’s climate by analyzing a number of indirect measures of climate, such as ice cores, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in the earth’s orbit around the sun. 2 This record shows that the climate varies naturally over a wide range of time scales, but this variability does not explain the observed warming since the 1950s. Rather, it is extremely likely (> 95%) that human activities have been the dominant cause of that warming. 3

Human activities have contributed substantially to climate change through:

Greenhouse Gas Emissions

Reflectivity or Absorption of the Sun’s Energy

Heat-trapping greenhouse gases and the earth's climate, greenhouse gases.

Concentrations of the key greenhouse gases have all increased since the Industrial Revolution due to human activities. Carbon dioxide, methane, and nitrous oxide concentrations are now more abundant in the earth’s atmosphere than any time in the last 800,000 years. 5 These greenhouse gas emissions have increased the greenhouse effect and caused the earth’s surface temperature to rise . Burning fossil fuels changes the climate more than any other human activity.

Carbon dioxide: Human activities currently release over 30 billion tons of carbon dioxide into the atmosphere every year. 6 Atmospheric carbon dioxide concentrations have increased by more than 40 percent since pre-industrial times, from approximately 280 parts per million (ppm) in the 18th century 7 to 414 ppm in 2020. 8

Methane: Human activities increased methane concentrations during most of the 20th century to more than 2.5 times the pre-industrial level, from approximately 722 parts per billion (ppb) in the 18th century 9 to 1,867 ppb in 2019. 10

Nitrous oxide: Nitrous oxide concentrations have risen approximately 20 percent since the start of the Industrial Revolution, with a relatively rapid increase toward the end of the 20th century. Nitrous oxide concentrations have increased from a pre-industrial level of 270 ppb 11 to 332 ppb in 2019. 12

For more information on greenhouse gas emissions, see the Greenhouse Gas Emissions website. To learn more about actions that can reduce these emissions, see What You Can Do . To translate abstract greenhouse gas emissions measurements into concrete terms, try using EPA's Greenhouse Gas Equivalencies Calculator .

Graph showing concentrations of key greenhouse gases.

Activities such as agriculture, road construction, and deforestation can change the reflectivity of the earth's surface, leading to local warming or cooling. This effect is observed in heat islands , which are urban centers that are warmer than the surrounding, less populated areas. One reason that these areas are warmer is that buildings, pavement, and roofs tend to reflect less sunlight than natural surfaces. While deforestation can increase the earth’s reflectivity globally by replacing dark trees with lighter surfaces such as crops, the net effect of all land-use changes appears to be a small cooling. 13

Emissions of small particles, known as aerosols, into the air can also lead to reflection or absorption of the sun's energy. Many types of air pollutants undergo chemical reactions in the atmosphere to create aerosols. Overall, human-generated aerosols have a net cooling effect on the earth. Learn more about human-generated and natural aerosols .

Natural Processes

Natural processes are always influencing the earth’s climate and can explain climate changes prior to the Industrial Revolution in the 1700s. However, recent climate changes cannot be explained by natural causes alone.

Changes in the Earth’s Orbit and Rotation

Changes in the earth’s orbit and its axis of rotation have had a big impact on climate in the past. For example, the amount of summer sunshine on the Northern Hemisphere, which is affected by changes in the planet’s orbit, appears to be the primary cause of past cycles of ice ages, in which the earth has experienced long periods of cold temperatures (ice ages), as well as shorter interglacial periods (periods between ice ages) of relatively warmer temperatures. 14 At the coldest part of the last glacial period (or ice age), the average global temperature was about 11°F colder than it is today. At the peak of the last interglacial period, however, the average global temperature was at most 2°F warmer than it is today. 15

Variations in Solar Activity

Changes in the sun’s energy output can affect the intensity of the sunlight that reaches the earth’s surface. While these changes can influence the earth’s climate, solar variations have played little role in the climate changes observed in recent decades. 16 Satellites have been measuring the amount of energy the earth receives from the sun since 1978. These measurements show no net increase in the sun’s output, even as global surface temperatures have risen. 17

Measurements of Global Average Surface Temperature and the Sun’s Energy

Changes in the Earth’s Reflectivity

The amount of sunlight that is absorbed or reflected by the planet depends on the earth’s surface and atmosphere. Dark objects and surfaces, like the ocean, forests, and soil, tend to absorb more sunlight. Light-colored objects and surfaces, like snow and clouds, tend to reflect sunlight. About 70 percent of the sunlight that reaches the earth is absorbed. 18 Natural changes in the earth’s surface, like the melting of sea ice , have contributed to climate change in the past, often acting as feedbacks to other processes.

Volcanic Activity

Volcanoes have played a noticeable role in climate, and volcanic eruptions released large quantities of carbon dioxide in the distant past. Some explosive volcano eruptions can throw particles (e.g., SO 2 ) into the upper atmosphere, where they can reflect enough sunlight back to space to cool the surface of the planet for several years. 19 These particles are an example of cooling aerosols .

Volcanic particles from a single eruption do not produce long-term climate change because they remain in the atmosphere for a much shorter time than greenhouse gases. In addition, human activities emit more than 100 times as much carbon dioxide as volcanoes each year. 20

Changes in Naturally Occurring Carbon Dioxide Concentrations

Over the last several hundred thousand years, carbon dioxide levels varied in tandem with the glacial cycles. During warm interglacial periods, carbon dioxide levels were higher. During cool glacial periods, carbon dioxide levels were lower. 21 The heating or cooling of the earth’s surface and oceans can cause changes in the natural sources and sinks of these gases, and thus change greenhouse gas concentrations in the atmosphere. 22 These changing concentrations have acted as a positive climate feedback , amplifying the temperature changes caused by long-term shifts in the earth’s orbit. 23

A graph of atmospheric carbon dioxide concentrations.

1 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020 . The National Academies Press, Washington, DC, p. 5. doi: 10.17226/25733

2 Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, B. DeAngelo, S. Doherty, K. Hayhoe, R. Horton, J.P. Kossin, P.C. Taylor, A.M. Waple & C.P. Weaver. (2017). Executive summary. In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, pp. 12–34, doi: 10.7930/J0DJ5CTG

National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020 . The National Academies Press, Washington, DC, p. 5. doi: 10.17226/25733

3 IPCC (2013). Climate change 2013: The physical science basis . Working Group I contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, p. 869.

4 IPCC. (2021). Climate change 2021: The physical science basis . Working Group I contribution to the sixth assessment report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu & B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom, p. SPM-5.

5 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020 . The National Academies Press, Washington, DC, p. B-2. doi: 10.17226/25733

Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou & P.C. Taylor. (2017). Physical drivers of climate change . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 80, Figure 2.4. doi: 10.7930/J0513WCR

6 Hayhoe, K., D.J. Wuebbles, D.R. Easterling, D.W. Fahey, S. Doherty, J. Kossin, W. Sweet, R. Vose & M. Wehner. (2018). Our changing climate . In: Impacts, risks, and adaptation in the United States: Fourth national climate assessment, volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock & B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 76. doi: 10.7930/NCA4.2018

7 IPCC. (2013). Climate change 2013: The physical science basis . Working Group I contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, p. 166.

8 NOAA. (2021). Trends in atmospheric carbon dioxide . Retrieved 3/25/2021. esrl.noaa.gov/gmd/ccgg/trends/mlo.html

9 IPCC. (2013). Climate change 2013: The physical science basis . Working Group I contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, p. 167.

10 NOAA. (2021). Trends in atmospheric methane . Retrieved 3/25/2021. esrl.noaa.gov/gmd/ccgg/trends_ch4

11 IPCC. (2013). Climate change 2013: The physical science basis . Working Group I contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, p. 168.

12 NOAA. (2021). Trends in nitrous oxide . Retrieved 3/25/2021. esrl.noaa.gov/gmd/ccgg/trends_n2o/

13 Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou & P.C. Taylor. (2017). Physical drivers of climate change . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 78, Fig. 2.3 and p. 86. doi: 10.7930/J0513WCR

14 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020. The National Academies Press, Washington, DC, p. 9. doi: 10.17226/25733

15 Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou & P.C. Taylor. (2017). Our globally changing climate . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 53. doi: 10.7930/J08S4N35

16 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020. The National Academies Press, Washington, DC, p. 7. doi: 10.17226/25733

17 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020. The National Academies Press, Washington, DC, p. 7. doi: 10.17226/25733

18 Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou, & P.C. Taylor. (2017). Physical drivers of climate change . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 2. doi: 10.7930/J0513WCR

19 Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou, & P.C. Taylor. (2017). Physical drivers of climate change . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 79. doi: 10.7930/J0513WCR

20 Fahey, D.W., S.J. Doherty, K.A. Hibbard, A. Romanou & P.C. Taylor. (2017). Physical drivers of climate change . In: Climate science special report: Fourth national climate assessment, volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart & T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, p. 79. doi: 10.7930/J0513WCR

21 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020. The National Academies Press, Washington, DC, pp. 9–10. doi: 10.17226/25733

22 IPCC. (2013). Climate change 2013: The physical science basis . Working Group I contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, p. 399.

23 National Academy of Sciences. (2020). Climate change: Evidence and causes: Update 2020. The National Academies Press, Washington, DC, pp. 9–10. doi: 10.17226/25733

Frequently Asked Questions

Causes of global warming, explained

Human activity is driving climate change, including global temperature rise.

The average temperature of the Earth is rising at nearly twice the rate it was 50 years ago. This rapid warming trend cannot be explained by natural cycles alone, scientists have concluded. The only way to explain the pattern is to include the effect of greenhouse gases (GHGs) emitted by humans.

Current levels of the greenhouse gases carbon dioxide, methane, and nitrous oxide in our atmosphere are higher than at any point over the past 800,000 years , and their ability to trap heat is changing our climate in multiple ways .

IPCC conclusions

To come to a scientific conclusion on climate change and what to do about it, the United Nations in 1988 formed a group called the Intergovernmental Panel on Climate Change , or IPCC. The IPCC meets every few years to review the latest scientific findings and write a report summarizing all that is known about global warming. Each report represents a consensus, or agreement, among hundreds of leading scientists.

One of the first things the IPCC concluded is that there are several greenhouse gases responsible for warming, and humans emit them in a variety of ways. Most come from the combustion of fossil fuels in cars, buildings, factories, and power plants. The gas responsible for the most warming is carbon dioxide, or CO2. Other contributors include methane released from landfills, natural gas and petroleum industries, and agriculture (especially from the digestive systems of grazing animals); nitrous oxide from fertilizers; gases used for refrigeration and industrial processes; and the loss of forests that would otherwise store CO2.

Gaseous abilities

Different greenhouse gases have very different heat-trapping abilities. Some of them can trap more heat than an equivalent amount of CO2. A molecule of methane doesn't hang around the atmosphere as long as a molecule of carbon dioxide will, but it is at least 84 times more potent over two decades. Nitrous oxide is 264 times more powerful than CO2.

Other gases, such as chlorofluorocarbons, or CFCs—which have been banned in much of the world because they also degrade the ozone layer—have heat-trapping potential thousands of times greater than CO2. But because their emissions are much lower than CO2 , none of these gases trap as much heat in the atmosphere as CO2 does.

When those gases that humans are adding to Earth's atmosphere trap heat, it’s called the "greenhouse effect." The gases let light through but then keep much of the heat that radiates from the surface from escaping back into space, like the glass walls of a greenhouse. The more greenhouse gases in the atmosphere, the more dramatic the effect, and the more warming that happens.

Climate change continues

Despite global efforts to address climate change, including the landmark 2015 Paris climate agreement , carbon dioxide emissions from fossil fuels continue to rise, hitting record levels in 2018 .

Many people think of global warming and climate change as synonyms, but scientists prefer to use “climate change” when describing the complex shifts now affecting our planet’s weather and climate systems. Climate change encompasses not only rising average temperatures but also extreme weather events, shifting wildlife populations and and habitats, rising seas , and a range of other impacts.

Another weapon to fight climate change? Put carbon back where we found it

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What Is Climate Change?

Climate change is a long-term change in the average weather patterns that have come to define Earth’s local, regional and global climates. These changes have a broad range of observed effects that are synonymous with the term.

Changes observed in Earth’s climate since the mid-20th century are driven by human activities, particularly fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere, raising Earth’s average surface temperature. Natural processes, which have been overwhelmed by human activities, can also contribute to climate change, including internal variability (e.g., cyclical ocean patterns like El Niño, La Niña and the Pacific Decadal Oscillation) and external forcings (e.g., volcanic activity, changes in the Sun’s energy output , variations in Earth’s orbit ).

Scientists use observations from the ground, air, and space, along with computer models , to monitor and study past, present, and future climate change. Climate data records provide evidence of climate change key indicators, such as global land and ocean temperature increases; rising sea levels; ice loss at Earth’s poles and in mountain glaciers; frequency and severity changes in extreme weather such as hurricanes, heatwaves, wildfires, droughts, floods, and precipitation; and cloud and vegetation cover changes.

“Climate change” and “global warming” are often used interchangeably but have distinct meanings. Similarly, the terms "weather" and "climate" are sometimes confused, though they refer to events with broadly different spatial- and timescales.

What Is Global Warming?

Global warming is the long-term heating of Earth’s surface observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere. This term is not interchangeable with the term "climate change."

Since the pre-industrial period, human activities are estimated to have increased Earth’s global average temperature by about 1 degree Celsius (1.8 degrees Fahrenheit), a number that is currently increasing by more than 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade. The current warming trend is unequivocally the result of human activity since the 1950s and is proceeding at an unprecedented rate over millennia.

Weather vs. Climate

“if you don’t like the weather in new england, just wait a few minutes.” - mark twain.

Weather refers to atmospheric conditions that occur locally over short periods of time—from minutes to hours or days. Familiar examples include rain, snow, clouds, winds, floods, or thunderstorms.

Climate, on the other hand, refers to the long-term (usually at least 30 years) regional or even global average of temperature, humidity, and rainfall patterns over seasons, years, or decades.

Find Out More: A Guide to NASA’s Global Climate Change Website

This website provides a high-level overview of some of the known causes, effects and indications of global climate change:

Evidence. Brief descriptions of some of the key scientific observations that our planet is undergoing abrupt climate change.

Causes. A concise discussion of the primary climate change causes on our planet.

Effects. A look at some of the likely future effects of climate change, including U.S. regional effects.

Vital Signs. Graphs and animated time series showing real-time climate change data, including atmospheric carbon dioxide, global temperature, sea ice extent, and ice sheet volume.

Earth Minute. This fun video series explains various Earth science topics, including some climate change topics.

Other NASA Resources

Goddard Scientific Visualization Studio. An extensive collection of animated climate change and Earth science visualizations.

Sea Level Change Portal. NASA's portal for an in-depth look at the science behind sea level change.

NASA’s Earth Observatory. Satellite imagery, feature articles and scientific information about our home planet, with a focus on Earth’s climate and environmental change.

Header image is of Apusiaajik Glacier, and was taken near Kulusuk, Greenland, on Aug. 26, 2018, during NASA's Oceans Melting Greenland (OMG) field operations. Learn more here . Credit: NASA/JPL-Caltech

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The sum of Earth's plants, on land and in the ocean, changes slightly from year to year as weather patterns shift.

Facts About Earth

What Is Climate Change?

What Is Global Warming?

Weather vs. Climate

“If you don’t like the weather in New England, just wait a few minutes.” - Mark Twain

Weather refers to atmospheric conditions that occur locally over short periods of time—from minutes to hours or days. Familiar examples include rain, snow, clouds, winds, floods, or thunderstorms.

Climate, on the other hand, refers to the long-term (usually at least 30 years) regional or even global average of temperature, humidity, and rainfall patterns over seasons, years, or decades.

Find Out More: A Guide to NASA’s Global Climate Change Website

This website provides a high-level overview of some of the known causes, effects and indications of global climate change:

Evidence. Brief descriptions of some of the key scientific observations that our planet is undergoing abrupt climate change.

Causes. A concise discussion of the primary climate change causes on our planet.

Effects. A look at some of the likely future effects of climate change, including U.S. regional effects.

Vital Signs. Graphs and animated time series showing real-time climate change data, including atmospheric carbon dioxide, global temperature, sea ice extent, and ice sheet volume.

Earth Minute. This fun video series explains various Earth science topics, including some climate change topics.

Other NASA Resources

Goddard Scientific Visualization Studio. An extensive collection of animated climate change and Earth science visualizations.

Sea Level Change Portal. NASA's portal for an in-depth look at the science behind sea level change.

NASA’s Earth Observatory. Satellite imagery, feature articles and scientific information about our home planet, with a focus on Earth’s climate and environmental change.

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Causes of Climate Change

Climate change , periodic modification of Earth ’s climate brought about as a result of changes in the atmosphere as well as interactions between the atmosphere and various other geologic, chemical, biological, and geographic factors within the Earth system.

The atmosphere is a dynamic fluid that is continually in motion. Both its physical properties and its rate and direction of motion are influenced by a variety of factors, including solar radiation , the geographic position of continents , ocean currents , the location and orientation of mountain ranges, atmospheric chemistry , and vegetation growing on the land surface. All these factors change through time. Some factors, such as the distribution of heat within the oceans , atmospheric chemistry, and surface vegetation, change at very short timescales. Others, such as the position of continents and the location and height of mountain ranges, change over very long timescales. Therefore, climate, which results from the physical properties and motion of the atmosphere, varies at every conceivable timescale.

A more specific definition would state that climate is the mean state and variability of these features over some extended time period.

Climate is often defined loosely as the average weather at a particular place, incorporating such features as temperature , precipitation , humidity , and windiness . A more specific definition would state that climate is the mean state and variability of these features over some extended time period. Both definitions acknowledge that the weather is always changing, owing to instabilities in the atmosphere . And as weather varies from day to day, so too does climate vary, from daily day-and-night cycles up to periods of geologic time hundreds of millions of years long. In a very real sense, climate variation is a redundant expression—climate is always varying. No two years are exactly alike, nor are any two decades, any two centuries, or any two millennia ( see also Climate Change Throughout History ).

This article addresses the concept of climatic variation and change within the set of integrated natural features and processes known as the Earth system. The nature of the evidence for climate change is explained, as are the principal mechanisms that have caused climate change throughout the history of Earth. For a detailed description of the development of Earth’s atmosphere, see the article atmosphere, evolution of . For full treatment of the most critical issue of climate change in the contemporary world, see global warming .

The Earth system

The atmosphere is influenced by and linked to other features of Earth , including oceans , ice masses ( glaciers and sea ice ), land surfaces, and vegetation. Together, they make up an integrated Earth system, in which all components interact with and influence one another in often complex ways. For instance, climate influences the distribution of vegetation on Earth’s surface (e.g., deserts exist in arid regions, forests in humid regions), but vegetation in turn influences climate by reflecting radiant energy back into the atmosphere, transferring water (and latent heat ) from soil to the atmosphere, and influencing the horizontal movement of air across the land surface.

Earth scientists and atmospheric scientists are still seeking a full understanding of the complex feedbacks and interactions among the various components of the Earth system. This effort is being facilitated by the development of an interdisciplinary science called Earth system science. Earth system science is composed of a wide range of disciplines, including climatology (the study of the atmosphere), geology (the study of Earth’s surface and underground processes), ecology (the study of how Earth’s organisms relate to one another and their environment), oceanography (the study of Earth’s oceans), glaciology (the study of Earth’s ice masses), and even the social sciences (the study of human behaviour in its social and cultural aspects).

A full understanding of the Earth system requires knowledge of how the system and its components have changed through time. The pursuit of this understanding has led to development of Earth system history, an interdisciplinary science that includes not only the contributions of Earth system scientists but also paleontologists (who study the life of past geologic periods), paleoclimatologists (who study past climates ), paleoecologists (who study past environments and ecosystems ), paleoceanographers (who study the history of the oceans), and other scientists concerned with Earth history. Because different components of the Earth system change at different rates and are relevant at different timescales, Earth system history is a diverse and complex science. Students of Earth system history are not just concerned with documenting what has happened; they also view the past as a series of experiments in which solar radiation , ocean currents , continental configurations, atmospheric chemistry, and other important features have varied. These experiments provide opportunities to learn the relative influences of and interactions between various components of the Earth system. Studies of Earth system history also specify the full array of states the system has experienced in the past and those the system is capable of experiencing in the future.

Undoubtedly, people have always been aware of climatic variation at the relatively short timescales of seasons, years, and decades. Biblical scripture and other early documents refer to droughts , floods , periods of severe cold, and other climatic events. Nevertheless, a full appreciation of the nature and magnitude of climatic change did not come about until the late 18th and early 19th centuries, a time when the widespread recognition of the deep antiquity of Earth occurred. Naturalists of this time, including Scottish geologist Charles Lyell , Swiss-born naturalist and geologist Louis Agassiz , English naturalist Charles Darwin , American botanist Asa Gray , and Welsh naturalist Alfred Russel Wallace , came to recognize geologic and biogeographic evidence that made sense only in the light of past climates radically different from those prevailing today.

Geologists and paleontologists in the 19th and early 20th centuries uncovered evidence of massive climatic changes taking place before the Pleistocene —that is, before some 2.6 million years ago. For example, red beds indicated aridity in regions that are now humid (e.g., England and New England ), whereas fossils of coal -swamp plants and reef corals indicated that tropical climates once occurred at present-day high latitudes in both Europe and North America . Since the late 20th century the development of advanced technologies for dating rocks, together with geochemical techniques and other analytical tools, have revolutionized the understanding of early Earth system history.

The occurrence of multiple epochs in recent Earth history during which continental glaciers , developed at high latitudes, penetrated into northern Europe and eastern North America was recognized by scientists by the late 19th century. Scottish geologist James Croll proposed that recurring variations in orbital eccentricity (the deviation of Earth’s orbit from a perfectly circular path) were responsible for alternating glacial and interglacial periods. Croll’s controversial idea was taken up by Serbian mathematician and astronomer Milutin Milankovitch in the early 20th century. Milankovitch proposed that the mechanism that brought about periods of glaciation was driven by cyclic changes in eccentricity as well as two other orbital parameters: precession (a change in the directional focus of Earth’s axis of rotation) and axial tilt (a change in the inclination of Earth’s axis with respect to the plane of its orbit around the Sun ). Orbital variation is now recognized as an important driver of climatic variation throughout Earth’s history ( see below Orbital [Milankovitch] variations).

Evidence for climate change

All historical sciences share a problem: As they probe farther back in time, they become more reliant on fragmentary and indirect evidence. Earth system history is no exception. High-quality instrumental records spanning the past century exist for most parts of the world, but the records become sparse in the 19th century, and few records predate the late 18th century. Other historical documents, including ship’s logs, diaries, court and church records, and tax rolls, can sometimes be used. Within strict geographic contexts, these sources can provide information on frosts , droughts , floods , sea ice , the dates of monsoons , and other climatic features—in some cases up to several hundred years ago.

Ongoing climatic changes are being monitored by networks of sensors in space, on the land surface, and both on and below the surface of the world’s oceans.

Fortunately, climatic change also leaves a variety of signatures in the natural world. Climate influences the growth of trees and corals , the abundance and geographic distribution of plant and animal species, the chemistry of oceans and lakes , the accumulation of ice in cold regions, and the erosion and deposition of materials on Earth’s surface. Paleoclimatologists study the traces of these effects, devising clever and subtle ways to obtain information about past climates. Most of the evidence of past climatic change is circumstantial, so paleoclimatology involves a great deal of investigative work. Wherever possible, paleoclimatologists try to use multiple lines of evidence to cross-check their conclusions. They are frequently confronted with conflicting evidence, but this, as in other sciences, usually leads to an enhanced understanding of the Earth system and its complex history. New sources of data, analytical tools, and instruments are becoming available, and the field is moving quickly. Revolutionary changes in the understanding of Earth’s climate history have occurred since the 1990s, and coming decades will bring many new insights and interpretations.

Climatic changes of the past 200–300 years, especially since the early 1900s, are documented by instrumental records and other archives.

Ongoing climatic changes are being monitored by networks of sensors in space, on the land surface, and both on and below the surface of the world’s oceans. Climatic changes of the past 200–300 years, especially since the early 1900s, are documented by instrumental records and other archives. These written documents and records provide information about climate change in some locations for the past few hundred years. Some very rare records date back over 1,000 years. Researchers studying climatic changes predating the instrumental record rely increasingly on natural archives, which are biological or geologic processes that record some aspect of past climate. These natural archives, often referred to as proxy evidence, are extraordinarily diverse; they include, but are not limited to, fossil records of past plant and animal distributions, sedimentary and geochemical indicators of former conditions of oceans and continents, and land surface features characteristic of past climates. Paleoclimatologists study these natural archives by collecting cores, or cylindrical samples, of sediments from lakes, bogs , and oceans; by studying surface features and geological strata; by examining tree ring patterns from cores or sections of living and dead trees; by drilling into marine corals and cave stalagmites ; by drilling into the ice sheets of Antarctica and Greenland and the high-elevation glaciers of the Plateau of Tibet , the Andes , and other montane regions; and by a wide variety of other means. Techniques for extracting paleoclimatic information are continually being developed and refined, and new kinds of natural archives are being recognized and exploited.

Causes of climate change

It is much easier to document the evidence of climate variability and past climate change than it is to determine their underlying mechanisms. Climate is influenced by a multitude of factors that operate at timescales ranging from hours to hundreds of millions of years. Many of the causes of climate change are external to the Earth system. Others are part of the Earth system but external to the atmosphere. Still others involve interactions between the atmosphere and other components of the Earth system and are collectively described as feedbacks within the Earth system. Feedbacks are among the most recently discovered and challenging causal factors to study. Nevertheless, these factors are increasingly recognized as playing fundamental roles in climate variation. The most important mechanisms are described in this section.

Solar variability

The luminosity, or brightness, of the Sun has been increasing steadily since its formation. This phenomenon is important to Earth’s climate, because the Sun provides the energy to drive atmospheric circulation and constitutes the input for Earth’s heat budget. Low solar luminosity during Precambrian time underlies the faint young Sun paradox, described in the article Climate Change Throughout History .

Radiative energy from the Sun is variable at very small timescales, owing to solar storms and other disturbances, but variations in solar activity, particularly the frequency of sunspots , are also documented at decadal to millennial timescales and probably occur at longer timescales as well. The “ Maunder minimum ,” a period of drastically reduced sunspot activity between AD 1645 and 1715, has been suggested as a contributing factor to the Little Ice Age .

Volcanic activity

Volcanic activity can influence climate in a number of ways at different timescales. Individual volcanic eruptions can release large quantities of sulfur dioxide and other aerosols into the stratosphere , reducing atmospheric transparency and thus the amount of solar radiation reaching Earth’s surface and troposphere . A recent example is the 1991 eruption in the Philippines of Mount Pinatubo , which had measurable influences on atmospheric circulation and heat budgets. The 1815 eruption of Mount Tambora on the island of Sumbawa had more dramatic consequences, as the spring and summer of the following year (1816, known as “the year without a summer”) were unusually cold over much of the world. New England and Europe experienced snowfalls and frosts throughout the summer of 1816.

Volcanoes and related phenomena, such as ocean rifting and subduction, release carbon dioxide into both the oceans and the atmosphere. Emissions are low; even a massive volcanic eruption such as Mount Pinatubo releases only a fraction of the carbon dioxide emitted by fossil-fuel combustion in a year. At geologic timescales, however, release of this greenhouse gas can have important effects. Variations in carbon dioxide release by volcanoes and ocean rifts over millions of years can alter the chemistry of the atmosphere. Such changeability in carbon dioxide concentrations probably accounts for much of the climatic variation that has taken place during the Phanerozoic Eon .

Tectonic activity

Tectonic movements of Earth’s crust have had profound effects on climate at timescales of millions to tens of millions of years. These movements have changed the shape, size, position, and elevation of the continental masses as well as the bathymetry of the oceans. Topographic and bathymetric changes in turn have had strong effects on the circulation of both the atmosphere and the oceans . For example, the uplift of the Tibetan Plateau during the Cenozoic Era affected atmospheric circulation patterns, creating the South Asian monsoon and influencing climate over much of the rest of Asia and neighbouring regions.

Tectonic activity also influences atmospheric chemistry, particularly carbon dioxide concentrations. Carbon dioxide is emitted from volcanoes and vents in rift zones and subduction zones . Variations in the rate of spreading in rift zones and the degree of volcanic activity near plate margins have influenced atmospheric carbon dioxide concentrations throughout Earth’s history. Even the chemical weathering of rock constitutes an important sink for carbon dioxide. (A carbon sink is any process that removes carbon dioxide from the atmosphere by the chemical conversion of CO 2 to organic or inorganic carbon compounds.) Carbonic acid, formed from carbon dioxide and water , is a reactant in dissolution of silicates and other minerals. Weathering rates are related to the mass, elevation, and exposure of bedrock . Tectonic uplift can increase all these factors and thus lead to increased weathering and carbon dioxide absorption. For example, the chemical weathering of the rising Tibetan Plateau may have played an important role in depleting the atmosphere of carbon dioxide during a global cooling period in the late Cenozoic Era.

Orbital (Milankovich) variations

The orbital geometry of Earth is affected in predictable ways by the gravitational influences of other planets in the solar system. Three primary features of Earth’s orbit are affected, each in a cyclic, or regularly recurring, manner. First, the shape of Earth’s orbit around the Sun , varies from nearly circular to elliptical (eccentric), with periodicities of 100,000 and 413,000 years. Second, the tilt of Earth’s axis with respect to the Sun, which is primarily responsible for Earth’s seasonal climates, varies between 22.1° and 24.5° from the plane of Earth’s rotation around the Sun. This variation occurs on a cycle of 41,000 years. In general, the greater the tilt, the greater the solar radiation received by hemispheres in summer and the less received in winter . The third cyclic change to Earth’s orbital geometry results from two combined phenomena: (1) Earth’s axis of rotation wobbles, changing the direction of the axis with respect to the Sun, and (2) the orientation of Earth’s orbital ellipse rotates slowly. These two processes create a 26,000-year cycle, called precession of the equinoxes , in which the position of Earth at the equinoxes and solstices changes. Today Earth is closest to the Sun (perihelion) near the December solstice, whereas 9,000 years ago perihelion occurred near the June solstice.

Vegetation also influences greenhouse gas concentrations; living plants constitute an important sink for atmospheric carbon dioxide, whereas they act as sources of carbon dioxide when they are burned by wildfires or undergo decomposition.

These orbital variations cause changes in the latitudinal and seasonal distribution of solar radiation, which in turn drive a number of climate variations. Orbital variations play major roles in pacing glacial-interglacial and monsoonal patterns. Their influences have been identified in climatic changes over much of the Phanerozoic. For example, cyclothems —which are interbedded marine, fluvial, and coal beds characteristic of the Pennsylvanian Subperiod (318.1 million to 299 million years ago)—appear to represent Milankovitch-driven changes in mean sea level .

Greenhouse gases

Greenhouse gases are gas molecules that have the property of absorbing infrared radiation (net heat energy) emitted from Earth’s surface and reradiating it back to Earth’s surface, thus contributing to the phenomenon known as the greenhouse effect. Carbon dioxide , methane , and water vapour are the most important greenhouse gases, and they have a profound effect on the energy budget of the Earth system despite making up only a fraction of all atmospheric gases. Concentrations of greenhouse gases have varied substantially during Earth’s history, and these variations have driven substantial climate changes at a wide range of timescales. In general, greenhouse gas concentrations have been particularly high during warm periods and low during cold phases. A number of processes influence greenhouse gas concentrations. Some, such as tectonic activities, operate at timescales of millions of years, whereas others, such as vegetation, soil, wetland, and ocean sources and sinks, operate at timescales of hundreds to thousands of years. Human activities—especially fossil-fuel combustion since the Industrial Revolution—are responsible for steady increases in atmospheric concentrations of various greenhouse gases, especially carbon dioxide, methane, ozone , and chlorofluorocarbons (CFCs).

Perhaps the most intensively discussed and researched topic in climate variability is the role of interactions and feedbacks among the various components of the Earth system. The feedbacks involve different components that operate at different rates and timescales. Ice sheets, sea ice , terrestrial vegetation, ocean temperatures, weathering rates, ocean circulation, and greenhouse gas concentrations are all influenced either directly or indirectly by the atmosphere ; however, they also all feed back into the atmosphere, thereby influencing it in important ways. For example, different forms and densities of vegetation on the land surface influence the albedo , or reflectivity, of Earth’s surface, thus affecting the overall radiation budget at local to regional scales. At the same time, the transfer of water molecules from soil to the atmosphere is mediated by vegetation, both directly (from transpiration through plant stomata ) and indirectly (from shading and temperature influences on direct evaporation from soil). This regulation of latent heat flux by vegetation can influence climate at local to global scales. As a result, changes in vegetation, which are partially controlled by climate, can in turn influence the climate system. Vegetation also influences greenhouse gas concentrations; living plants constitute an important sink for atmospheric carbon dioxide, whereas they act as sources of carbon dioxide when they are burned by wildfires or undergo decomposition. These and other feedbacks among the various components of the Earth system are critical for both understanding past climate changes and predicting future ones.

Human activities

Recognition of global climate change as an environmental issue has drawn attention to the climatic impact of human activities. Most of this attention has focused on carbon dioxide emission via fossil-fuel combustion and deforestation . Human activities also yield releases of other greenhouse gases , such as methane (from rice cultivation, livestock , landfills, and other sources) and chlorofluorocarbons (from industrial sources). There is little doubt among climatologists that these greenhouse gases affect the radiation budget of Earth ; the nature and magnitude of the climatic response are a subject of intense research activity. Paleoclimate records from tree rings, coral , and ice cores indicate a clear warming trend spanning the entire 20th century and the first decade of the 21st century. In fact, the 20th century was the warmest of the past 10 centuries, and the decade 2001–10 was the warmest decade since the beginning of modern instrumental record keeping. Many climatologists have pointed to this warming pattern as clear evidence of human-induced climate change resulting from the production of greenhouse gases.

A second type of human impact, the conversion of vegetation by deforestation , afforestation, and agriculture, is receiving mounting attention as a further source of climate change. It is becoming increasingly clear that human impacts on vegetation cover can have local, regional, and even global effects on climate, due to changes in the sensible and latent heat flux to the atmosphere and the distribution of energy within the climate system. The extent to which these factors contribute to recent and ongoing climate change is an important, emerging area of study.

Rapid changes in an external factor can push the climate system into a new mode.

Written by Stephen T. Jackson , Professor Emeritus of Botany, University of Wyoming.

Top image credits: ©jzehnder/Fotolia

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You Asked, We Answered: Some Burning Climate Questions

Reporters from the Climate Desk gathered reader questions and are here to help explain some frequent puzzlers.

What’s one thing you want to know about climate change? We asked, and hundreds of you responded.

The topic, like the planet, is vast. Overwhelming. Complex. But there’s no more important time to understand what is happening and what can be done about it.

Why are extreme cold weather events happening if the planet is warming?

I understand that scientists believe that some extreme cold weather events are due to climate change, but I don’t quite understand how, especially if Earth is getting warmer overall. Could you explain this? — Gabriel Gutierrez, West Lafayette, Ind.

By Maggie Astor

The connection between climate change and extreme cold weather involves the polar jet stream in the Northern Hemisphere, strong winds that blow around the globe from west to east at an altitude of 5 to 9 miles. The jet stream naturally shifts north and south, and when it shifts south, it brings frigid Arctic air with it.

A separate wind system, called the polar vortex , forms a ring around the North Pole. When the vortex is temporarily disrupted — sometimes stretched or elongated, and other times broken into pieces — the jet stream tends to take one of those southward shifts. And research “suggests these disruptions to the vortex are happening more often in connection with a rapidly warming, melting Arctic, which we know is a clear symptom of climate change,” said Jennifer A. Francis, a senior scientist at the Woodwell Climate Research Center.

In other words, as climate change makes the Arctic warmer, the polar vortex is being more frequently disrupted in ways that allow Arctic air to escape south. And while temperatures are increasing on average, Arctic air is still frigid much of the time. Certainly frigid enough to cause extreme cold snaps in places like, say, Texas that are not accustomed to or prepared for them.

Where the extreme cold occurs depends on the nature of the disruption to the polar vortex. One type of disruption brings Arctic air into Europe and Asia. Another type brings Arctic air into the United States, and “that’s the type of polar vortex disruption that’s increasing the fastest,” said Judah L. Cohen, the director of seasonal forecasting at Atmospheric and Environmental Research, a private organization that works with government agencies.

It is important to note that these atmospheric patterns are extremely complicated, and while studies have shown a clear correlation between the climate-change-fueled warming of the Arctic and these extreme cold events, there is some disagreement among scientists about whether the warming of the Arctic is directly causing the extreme cold events. Research on that question is ongoing.

How will climate change affect biodiversity?

What impact will climate change have on biodiversity? How are they interlinked? How do the roles of developing versus developed countries differ, for example the United States and India? — A reader in India

By Catrin Einhorn

Warmer oceans are killing corals . Rising sea levels threaten the beaches that sea turtles need for nesting, and hotter temperatures are causing more females to be born. Changing seasons are increasingly out of step with the conditions species have evolved to depend on.

And then there are the polar bears , long a symbol of what could be lost in a warming world.

Climate change is already affecting plants and animals in ways that scientists are racing to understand. One study predicted sudden die offs , with large segments of ecosystems collapsing in waves. This has already started in coral reefs, scientists say, and could start in tropical forests by the 2040s.

Keeping global warming under 2 degrees Celsius, or 3.6 degrees Fahrenheit, the upper limit outlined by the Paris Agreement, would reduce the number of species exposed to dangerous climate change by 60 percent, the study found.

Despite these grim predictions, climate change isn’t yet the biggest driver of biodiversity loss. On land, the largest factor is the ways in which people have reshaped the terrain itself, creating farms and ranches, towns and cities, roads and mines from what was once habitat for myriad species. At sea, the main cause of biodiversity loss is overfishing. Also at play: pollution, introduced species that outcompete native ones, and hunting. A sobering report in 2019 by the leading international authority on biodiversity found that around a million species were at risk of extinction, many within decades.

While climate change will increasingly drive species loss, that’s not the only way in which the two are interlinked. Last year the same biodiversity panel joined with its climate change counterpart to issue a paper declaring that neither crisis could be addressed effectively on its own. For example, intact ecosystems like peatlands and forests both nurture biodiversity and sequester carbon; destroy them, and they turn into emitters of greenhouse gasses as well as lost habitat.

What to do? The science is clear that the world must transition away from fossil fuels far more quickly than is happening. Deforestation must stop . Consuming less meat and dairy would free up farmland for restoration , providing habitat for species and stashing away carbon. Ultimately, many experts say, we need a transformation from an extraction-based economy to a circular one. Like nature’s cycle, our waste — old clothes, old smartphones, old furniture — must be designed to provide the building blocks of what comes next.

Countries around the world are working on a new United Nations biodiversity agreement , which is expected to be approved later this year. One sticking point: How much money wealthy countries are willing to give poorer ones to protect intact natural areas, since wealthy countries have already largely exploited theirs.

What’s the status of U.S. climate legislation and emissions?

Where is the trimmed back version of climate legislation at? Joe Manchin reportedly said he would support such a bill. What do you know about the bill and will it pass with just Democrats? — Richard Buttny, Virgil, N.Y.

What is the current stated U.S. goal regarding reducing greenhouse gases and climate change, and how likely is it that we will achieve that goal? What do we need to do today to make progress toward achieving that goal? — Kathy Gray, Oak Ridge, Tenn.

By Lisa Friedman

Richard, as to the last part of your question, honestly, at this point your guess is as good as ours.

But here is what we know so far. Senator Joe Manchin III, Democrat of West Virginia, the most powerful man in Congress because his support in an evenly divided Senate is key, effectively killed President Biden’s Build Back Better climate and social spending legislation when he ended months of negotiations last year, saying he could not support the package .

A few weeks ago amid talks of revived discussions, Mr. Manchin was blunt. “There is no Build Back Better legislation,” he told reporters. Mr. Manchin also has not committed to passing a smaller version of the original $1 trillion spending plan. He has, however, voiced support for an “all of the above” energy package that increases oil and gas development.

Democrats hope that billions of dollars in tax incentives for wind, solar, geothermal and electric vehicle charging stations can also make its way into such a package. But relations between the White House and Mr. Manchin are rocky and it is unclear whether such a bill could pass before lawmakers leave town for an August recess.

To your emissions question, Kathy, Mr. Biden has pledged to cut United States emissions 50 to 52 percent below 2005 levels by 2030 . Energy experts say it is a challenging but realistic goal, and critical for helping the world avert the worst impacts of climate change.

It’s not going to be easy. So far there are few regulations and even fewer laws that can help achieve that target. Mr. Biden’s centerpiece legislation, the Build Back Better Act, includes $550 billion in clean energy tax incentives that researchers said could get the country about halfway to its goal. But, as noted, that bill is stalled in the Senate . Even if it manages to win approval this year, the administration will still have to enact regulations on things like power plants and automobile emissions to meet the target.

Will our drinking water be safe?

A lot of coverage on climate change deals with rising sea levels and extreme weather — droughts, floods, etc. My question is more about how climate change will affect drinking water and access to safe clean water. Are we in danger within our current lifetime to see an impact to safe water within the U.S. due to climate change? — Jessica, Silver Spring, Md.

By Christopher Flavelle

Climate change threatens Americans’ access to clean drinking water in a number of ways. The most obvious is drought: Rising temperatures are reducing the snowpack that supplies drinking water for much of the West.

But drought is far from the only climate-related threat to America’s water. Along the coast, cities like Miami that draw drinking water from underground aquifers have to worry about rising seas pushing saltwater into those aquifers , a process called saltwater intrusion. And rising seas also push up groundwater levels, which can cause septic systems to stop working, pushing unfiltered human waste into that groundwater.

Even in cities far from the coast, worsening floods are overwhelming aging sewer systems , causing untreated storm water and sewage to reach rivers and streams more frequently . And some 2,500 chemical sites are in areas at risk of flooding, which could cause those chemicals to leach into the groundwater.

In some cases, protecting drinking water from the effects of climate change is possible, so long as governments can find enough money to upgrade infrastructure — building new systems to contain storm water, for example, or better protect chemicals from being released during a flood.

Far harder will be finding new supplies of water to make up for what’s lost as temperatures rise. Some communities are responding by pumping more water from the ground. But if those aquifers are depleted faster than rainwater can replenish them, they will eventually run dry, a concern with the Ogallala Aquifer that supports much of the High Plains.

Even with significant reductions in water use, climate change could reduce the number of people that some regions can support, and leave more areas dependent on importing water.

Can you solve drought by piping water across the country?

Why don’t we create a national acequia system to capture excess rain falling primarily in the Eastern United States and pipeline it to the drought in the West? — Carol P. Chamberland, Albuquerque, N.M

The idea of taking water from one community and giving it to another has some basis in American history. In 1913, Los Angeles opened an aqueduct to carry water from Owens Valley, 230 miles north of the city, to sustain its growth.

But the project, in addition to costing some $23 million at the time, greatly upset Owens Valley residents, who so resented losing their water that they took to dynamiting the aqueduct. Repeatedly .

Today, there are some enormous water projects in the United States, though building a pipeline that spanned a significant stretch of the country would be astronomically more difficult. The distance between Albuquerque, for example, and the Mississippi River — perhaps the closest hypothetical starting point for such a pipeline — is about 1,000 miles, crossing at least three states along the way. Moving that water all the way to Los Angeles would mean piping it at least 1,800 miles across five states.

So the engineering and permitting challenges alone would be daunting. And that’s assuming the local and state governments that would have to give up their water would be willing to do so.

China dealt with similar challenges to build a colossal network of waterways that is transferring water from the country’s humid south to its dry north. But of course, China’s system of government makes engineering feats of that scale somewhat more feasible to pull off.

For the United States, it would be easier to just build a series of desalination plants along the West coast, according to Greg Pierce, director of the Human Right to Water Solutions Lab at the University of California, Los Angeles. And before turning to desalination, which is itself energy-intensive and thus expensive, communities in the West should work harder at other steps, such as water conservation and recycling, he said.

“It’s not worth it,” Dr. Pierce said of the pipeline idea. “You’d have to exhaust eight other options first.”

Is the weather becoming more extreme than scientists predicted?

How can we have faith in climate modeling when extreme events are much worse than predicted? Given “unexpected” extreme events like the 2021 Pacific Northwest heat wave and extreme heat in Antarctica that appear to shock scientists, it’s difficult for me to trust the I.P.C.C.’s framing that we haven’t run out of time. — Kevin, Herndon, Va.

By Raymond Zhong

Climate scientists have said for a long time that global warming is causing the intensity and frequency of many types of extreme weather to increase. And that’s exactly what has been happening. But global climate models aren’t really designed to simulate extreme events in individual regions. The factors that shape individual heat waves, for instance, are very local. Large-scale computer models simply can’t handle that level of detail quite yet.

That said, sometimes there are events that seem so anomalous that they make scientists wonder if they reflect something totally new and unforeseen, a gap in our understanding of the climate. Some researchers put the 2021 Pacific Northwest heat wave in that category, and are working to figure out whether they need to re-evaluate some of their assumptions.

For its part, the I.P.C.C. has hardly failed to acknowledge what’s happening with extreme weather. But its mandate is to assess the whole range of climate research, which might make it lean toward the middle of the road in its summaries. A decade ago, when a group of researchers looked back at the panel’s assessments from the early 2000s, they found that it generally underestimated the actual changes in sea level rise, increases in surface temperatures, intensity of rainfall and more. They blamed the instinct of scientists to avoid making conclusions that seem “excessively dramatic,” perhaps out of fear of being called alarmist.

The panel’s latest report, from April , concluded that we haven’t run out of time to slow global warming, but only if nations and societies make some huge changes right away. That’s a big if.

How can I hear from climate scientists themselves?

Why are climate change scientists faceless, aloof, terrible communicators and absent from social media? — A reader in Dallas

Climate science may not yet have its Bill Nye or its Neil deGrasse Tyson, but plenty of climate scientists are passionate about communicating their work to the public. Lots of them are on Twitter. Here’s a (very small) cross-section of people to follow, in alphabetical order:

Alaa Al Khourdajie : Senior scientist in London with the Intergovernmental Panel on Climate Change, the body of experts convened by the United Nations that puts out regular, authoritative surveys of climate research. Tweets on climate change economics and climate diplomacy.

Andrew Dessler : Professor of atmospheric sciences at Texas A&M University. Elucidator of energy and renewables, climate models and Texas.

Zeke Hausfather : Climate research lead at the payment processing company Stripe and scientist at Berkeley Earth, a nonprofit research group. A seemingly tireless chronicler, charter and commentator on all things climate.

David Ho : Climate scientist at the University of Hawaii at Manoa and École Normale Supérieure in Paris. Talks oceans and carbon dioxide removal, with wry observations on transit, cycling and life in France, too.

Twila Moon : Deputy lead scientist at the National Snow and Ice Data Center in Boulder, Colo. Covers glaciers, polar regions and giant ice sheets, and why we should all care about what happens to them.

Maisa Rojas : Climatologist at the University of Chile and Chile’s current environment minister. Follow along for slices of life at the intersection of science and government policy.

Sonia I. Seneviratne : Professor of land-climate dynamics at ETH Zurich in Switzerland. Tweets on extreme weather, greenhouse gas emissions and European energy policy.

Chandni Singh : Researcher on climate adaptation at the Indian Institute for Human Settlements in Bangalore. Posts about how countries and communities are coping with climate change, in both helpful ways and not so helpful ones.

Kim Wood : Geoscientist and meteorologist at Mississippi State University. A fount of neat weather maps and snarky GIFs.

What kind of trees are best to plant for the planet?

The world is trying to reforest the planet by planting nonnative trees like eucalyptus. Is this another disastrous plan? Shouldn’t they be planting native trees? — Katy Green, Nashville

Ecologists would say planting native trees is the best choice. We recently published an article on this very topic , examining how tree planting can resurrect or devastate ecosystems, depending on what species are planted and where.

To be sure, people need wood and other tree products for all kinds of reasons, and sometimes nonnative species make sense. But even when the professed goal is to help nature, the commercial benefits of certain trees, like Australian eucalyptus in Africa and South America or North American Sitka spruce in Europe, often win out.

A new standard is in development that would score tree planting projects on how well they’re doing with regard to biodiversity, with the aim of helping those with poor scores to improve.

The same ecological benefit of planting native species also holds true for people’s yards. Doug Tallamy, a professor of entomology at the University of Delaware, worked with the National Wildlife Federation to develop this tool to help people find native trees, shrubs and flowers that support the most caterpillars, which in turn feed baby birds .

Can we engineer solutions to atmospheric warming?

Why are we not investing in scalable solutions that can remove carbon or reduce solar radiation? — Hayes Morehouse, Hayward, Calif.

By Henry Fountain

As a group, these types of solutions are referred to as geoengineering, or intentional manipulation of the climate. Geoengineering generally falls into two categories: removing some of the carbon dioxide already in the atmosphere so Earth traps less heat, known as direct air capture, or reducing how much sunlight reaches Earth’s surface so that there is less heat to begin with, usually called solar radiation management.

There are a few companies developing direct air capture machines, and some have deployed them on a small scale. According to the International Energy Agency, these projects capture a total of about 10 thousand tons of CO2 a year, a tiny fraction of the roughly 35 billion tons of annual energy-related emissions. Removing enough CO2 to have a climate impact would take a long time and require many thousands of machines, all of which would need energy to operate.

The captured gas would also have to be securely stored to keep it from re-entering the atmosphere. Those hurdles make direct air capture a long shot, especially since, for now at least, there are few financial incentives to overcome them. No one wants to pay to remove carbon dioxide from the air and bury it underground.

Solar radiation management is a different story. The basics of how to do it are known: inject some kind of chemical (perhaps sulfur dioxide) into the upper atmosphere, where it would reflect more of the sun’s rays. Relatively speaking, it wouldn’t be all that expensive (a fleet of high-flying planes would probably suffice) although once started it would have to continue indefinitely.

The major hurdle to developing the technology has been grave concern among many scientists, policymakers and others about unintended consequences that might result, and about the lack of a structure to govern its deployment. To date, there have been almost no real-world studies of the technology .

How do we know how warm the planet was in the 1800s?

One key finding of climate science is that global temperatures have increased by 2 degrees Fahrenheit since the late 1800s. How can we possibly have reliable measures of global temperatures from back then, keeping in mind that oceans cover about 70 percent of the globe and that a large majority of land has never been populated by humans to any significant degree? — Robert, Madison, Wis.

The mercury thermometer was invented in the early 1700s, and by the mid- to late 19th century, local temperatures were being monitored continuously in many locations, predominantly in the United States, Europe and the British colonies. By 1900, there were hundreds of recording stations worldwide, but over half of the Southern Hemisphere still wasn’t covered. And the techniques could be primitive. To measure temperatures at the sea’s surface, for instance, the most common method before about 1940 was to toss a bucket overboard a ship, haul it back up with a rope and read the temperature of the water inside.

To turn these spotty local measurements into estimates of average temperatures globally, across both land and ocean, climate scientists have had to perform some highly delicate analysis . They’ve used statistical models to fill in the gaps in direct readings. They’ve taken into account when weather stations changed locations or were situated close to cities that were hot for reasons unrelated to larger temperature trends.

They have also used some clever techniques to try to correct for antiquated equipment and methods. Those bucket readings , for example, might be inaccurate because the water in the bucket cooled down as it was pulled aboard. So scientists have scoured various nations’ maritime archives to determine what materials their sailors’ buckets were made of — tin, wood, canvas, rubber — during different periods in history and adjusted the way they incorporate those temperature recordings into their computations.

Such analysis is fiendishly tricky. The numbers that emerge are uncertain estimates, not gospel truth. Scientists are working constantly to refine them. Today’s global temperature measurements are based on a much broader and more quality-controlled set of readings, including from ships and buoys in the oceans.

But having a historical baseline, even an imperfect one, is important. As Roy L. Jenne, a researcher at the National Center for Atmospheric Research, wrote in a 1975 report on the institution’s collections of climate data: “Although they are not perfect, if they are used wisely they can help us find answers to a number of problems.”

Does producing batteries for electric cars damage the environment more than gas vehicles do?

Is the environmental damage collecting metals/producing batteries for electric cars more dangerous to the environment than gas powered vehicles? — Sandy Rogers, San Antonio, Texas

By Hiroko Tabuchi

There’s no question that mining the metals and minerals used in electric car batteries comes with sizable costs that are not just environmental but also human.

Much of the world’s cobalt, for example, is mined in the Democratic Republic of Congo , where corruption and worker exploitation has been widespread. Extracting the metals from their ores also requires a process called smelting, which can emit sulfur oxide and other harmful air pollution.

Beyond the minerals required for batteries, electric grids still need to become much cleaner before electric vehicles are emissions free.

Most electric vehicles sold today already produce significantly fewer planet-warming emissions than most cars fueled with gasoline, but a lot still depends on how much coal is being burned to generate the electricity they use.

Still, consider that batteries and other clean technology require relatively tiny amounts of these critical minerals, and that’s only to manufacture them. Once a battery is in use, there are no further minerals necessary to sustain it. That’s a very different picture from oil and gas, which must constantly be drilled from the ground, transported via pipelines and tankers, refined and combusted in our gasoline cars to keep those cars moving, said Jim Krane, a researcher at Rice University’s Baker Institute for Public Policy in Houston. In terms of environmental and other impacts, he said, “There’s just no comparison.”

How close are alternatives to fuel-powered aircraft?

As E. V.s are to gas-powered cars, are there greener alternatives to fuel-powered planes that are close to commercialization? — Rashmi Sarnaik, Boston

There are alternatives to fossil-fuel-powered aircraft in development, but whether they are close to commercialization depends on how you define “close.” It’s probably fair to say that the day when a significant amount of air travel is on low- or zero-emissions planes is still far-off.

There has been some work on using hydrogen , including burning it in modified jet engines. Airbus and the engine manufacturer CFM International expect to begin flight testing a hydrogen-fueled engine by the middle of the decade.

As with cars, though, most of the focus in aviation has been on electric power and batteries. The main problem with batteries is how little energy they supply relative to their weight. In cars that’s less of an obstacle (they don’t have to get off the ground, after all) but in aviation, batteries severely limit the size of the plane and how far it can fly.

One of the biggest battery-powered planes to fly so far was a modified Cessna Grand Caravan, test-flown by two companies, Magnix and Aerotec. Turboprop Grand Caravans can carry 10 or more people up to 1,200 miles. The companies said theirs could fly four or five people 100 miles or less.

The limitations of batteries, at least for now, have led some companies to work on other designs. Some use fuel cells, which work like batteries but can continuously supply electricity using hydrogen or other fuel. Others use hybrid systems — like hybrid cars, combining batteries and fossil-fuel-powered engines. In one approach, the engines provide some power and also keep the batteries charged. In another, the engines are used in takeoff and descent, when more power is needed, and the batteries for cruising, which requires less power. That keeps the number of batteries, and the weight, down.

Can countries meet the goals they set in the Paris agreement?

What countries, if any, have a realistic chance of meeting their Paris agreement pledges? — Michael Svetly, Philadelphia

According to Climate Action Tracker , a research group that analyzes climate goals and policies, very few. Ahead of United Nations talks in Glasgow last year, the organization found most major emitters of carbon dioxide, including the United States and China, are falling short of their pledge to stabilize global warming around 1.5 degrees Celsius, or 2.7 degrees Fahrenheit.

A few are doing better than most, including Costa Rica and the United Kingdom. Just one country was on track to meet its promises: Gambia, a small West African nation that has been bolstering its renewable energy use.

What will happen to N.Y.C.?

How is N.Y.C. planning for relocation or redevelopment, or both, of its many low-lying neighborhoods as floodwaters become too high to levee? — A reader in North Bergen, N.J.

New York City has yet to announce plans to fully relocate entire neighborhoods threatened by climate change, with all the steps that would entail: determining which homes to buy, getting agreement from homeowners, finding a new patch of land for the community, building new infrastructure, securing funding and so on.

Relocation projects on that scale, often described as “managed retreat,” remain extremely rare in the United States. What projects have been attempted so far have mostly been in rural areas or small towns , and their success has been mixed.

And the idea of pulling back from the water, while never easy, is especially fraught in New York City, which has some of the highest real estate values in the country. Those high values have been used to justify fantastically expensive projects to protect low-lying land in the city, rather than abandon it — like a $10 billion berm along the South Street Seaport , or a $119 billion sea wall in New York Harbor .

Perhaps unsurprisingly, then, the city’s most recent Comprehensive Waterfront Plan , issued in December, makes no mention of managed retreat. But the plan does include what it calls “housing mobility” — policies aimed at helping individual households move to safer areas, for example by giving people money to buy a new home on higher ground, as well as paying for moving and other costs. The city also says it is limiting the density of new development in high-risk areas.

Robert Freudenberg, a vice president of the Regional Plan Association, a nonprofit planning group in New York, New Jersey and Connecticut, gave city officials credit for beginning to talk about the idea that some areas can’t be protected forever.

“It’s an extremely challenging topic,” Mr. Freudenberg said. But as flooding gets worse, he added, “we can’t not talk about it.”

As oceans rise, will the Great Lakes, too?

The oceans are predicted to rise and affect coastal areas and cities, however, does this rise also affect the coastal areas of the Great Lakes, as the lakes are connected to the Atlantic Ocean via the St. Lawrence River and one would have to assume they would eventually be impacted? — Terri Messinides, Madison, Wis.

The Great Lakes are not directly threatened by rising oceans because of their elevation: The lowest of them, Lake Ontario, is about 240 feet above sea level. The St. Lawrence River carries water from the lakes to the Atlantic Ocean, but because of the elevation change, rising waters in the Atlantic can’t travel in the other direction.

That said, climate change is causing increasingly frequent and intense storms in the Great Lakes region, and the effects, including higher water levels and more flooding, are in many respects the same as those caused by rising seas. It’s just a different manifestation of climate change.

When it comes to precipitation, the past five years, from April 2017 through March 2022, the last month for which complete data is available, have been the second-wettest on record for the Great Lakes Basin, according to records kept by the National Oceanic and Atmospheric Administration . The water has risen accordingly. In 2019, water levels in the lakes hit 100-year highs , causing severe flooding and shoreline erosion.

At the same time, higher temperatures increase the rate of evaporation, which can lead to abnormally low water levels. People who live around the Great Lakes can expect to see both extremes — high water driven by severe rainfall, and low water driven by evaporation — happen more often as the climate continues to warm.

What is the environmental cost of cryptocurrency?

Can you tell us about the damage being done to our environment by crypto mining? I’ve heard the mining companies are trying to switch to renewable energy, yet at the same time reopening old coal power plants to provide the huge amounts of electricity they need. — Barry Engelman, Santa Monica, Calif.

Cryptomining, the enigmatic way in which virtual cryptocurrencies like Bitcoin are created (and which is also behind technology like NFTs ), requires a whole lot of computing power, is highly energy-intensive and generates outsize emissions. We delved into that process, and its environmental impact in this article — but suffice to say the problem isn’t going away soon.

The way Bitcoin is set up, using a process called “proof of work,” means that as interest in cryptocurrencies grows and more people start mining, more energy is required to mine a single Bitcoin. Researchers at Cambridge University estimate that mining Bitcoin uses more electricity than midsize countries like Norway. In New York, an influx of Bitcoin miners has led to the reopening of mothballed power plants.

But you might wonder about the traditional financial system: doesn’t that use energy, too? Yes, of course. But Bitcoin, for all its hype, still makes up just a few percent of all the world’s money or its transactions. So even though one industry study estimated that Bitcoin consumes about a 10th of the energy required by the traditional banking system, that still means Bitcoin’s energy use is outsize.

To address its high emissions footprint, cryptomining has increasingly tapped into renewable forms of energy, like hydroelectric power. But figuring out exactly just how much renewable energy Bitcoin miners use can be tricky. For one, we don’t exactly know where many of these miners are. We do know a lot of crypto miners used to be in China, where they had access to large amounts of hydro power. But now that they’ve largely been kicked out, cryptomining’s global climate impact has likely gotten worse .

In the United States, cryptominers have started to tap an unconventional new energy source: drilled gas, collected at oil and gas wells. The miners argue that this gas would otherwise have been flared or vented into the atmosphere, so no excess emissions are created. The reality is not that clear cut: If the presence of those cryptominers disincentivizes oil and gas companies from piping away that gas to be used elsewhere, any savings effect is blunted.

Other efforts are afoot to make cryptomining less damaging for the environment, including an alternative way of cryptomining involving a process called “proof of stake,” that doesn’t require miners to use as much energy. But unless Bitcoin, the most popular cryptocurrency, switches over, that’s going to do little to dent miners’ energy use.

How much do volcanoes contribute to global warming?

What does the data look like for greenhouse gas emissions in the last 200 years if volcanic activity was subtracted out? — Haley Rowlands, Boston

Volcanic activity generates 130 million to 440 million tons of carbon dioxide per year, according to the United States Geological Survey . Human activity generates about 35 billion tons of carbon dioxide per year — 80 times as much as the high-end estimate for volcanic activity, and 270 times as much as the low-end estimate. And that’s carbon dioxide. Human activity also emits other greenhouse gases, like methane, in far greater quantities than volcanoes.

The largest volcanic eruption in the past century was the 1991 eruption of Mount Pinatubo in the Philippines; if an explosion that size happened every day, NASA has calculated , it would still release only half as much carbon dioxide as daily human activity does. The annual emissions from cement production alone, one small component of planet-warming human activity, are greater than the annual emissions from every volcano in the world.

There is also no evidence that volcanic activity has increased over the past 200 years. While there have been more documented eruptions, researchers at the Smithsonian Institution’s Global Volcanism Program found that this was attributable not to an actual trend, but rather to “increases in populations living near volcanoes to observe eruptions and improvements in communication technologies to report those eruptions.”

All told, volcanic activity accounts for less than 1 percent of greenhouse gas emissions, which is not enough to contribute in any meaningful way to the increase we’ve seen over the past 200 years. The Intergovernmental Panel on Climate Change found in 2013 (see Page 56 of its report ) that the climatic effects of volcanic activity were “inconsequential” over the scale of a century.

Do carbon dioxide concentrations vary around the globe?

Why is the concentration of carbon dioxide in the atmosphere at Mauna Loa Observatory in Hawaii used as the global reference? It’s only one point on Earth. Do concentrations vary between different parts of the world? — Evan, Boston

At any given moment, levels of carbon dioxide in the air vary from place to place, depending on the amount of vegetation and human activity nearby. Which is why, as a location to monitor the average state of the atmosphere, at least over a large part of the Northern Hemisphere, a barren volcano in the middle of the Pacific has much to offer. It’s high above the ground and far enough from major sources of industrial pollution but still relatively accessible to researchers.

Today, the National Oceanic and Atmospheric Administration studies global carbon dioxide levels by looking at readings from Mauna Loa Observatory and a variety of other sources. These include observatories in Alaska, American Samoa and the South Pole, tall towers across the United States, and samples collected by balloons, aircraft and volunteers around the world. ( Here’s a map of all those sites.)

NOAA also checks its measurements at Mauna Loa against others from the same location, including ones taken independently, using different methods, by the Scripps Institution of Oceanography . On average, the difference in their monthly estimates is tiny.

Could a ‘new ice age’ offset global warming?

Will increases in global temperature associated with climate change be mitigated by the coming of a new “ice age?” — Suzanne Smythe, Essex, Conn.

In a “mini ice age,” if it occurred, average worldwide temperatures would drop, thus offsetting the warming that has been caused by emissions of greenhouse gases from the burning of fossil fuels in the last century and a half.

It’s a nice thought: a natural phenomenon comes to our rescue. But it’s not happening, nor is it expected to.

The idea is linked to the natural variability in the amount of the sun’s energy that reaches Earth. The sun goes through regular cycles, lasting about 11 years, when activity swings from a minimum to a maximum. But there are also longer periods of reduced activity, called grand solar minimums. The last one began in the mid-17th century and lasted seven decades.

There is some debate among scientists whether we are entering a new grand minimum . But even if we are, and even if it lasted for a century, the reduction in the sun’s output would not have a significant effect on temperatures. NASA scientists, among others, have calculated that any cooling effect would be overwhelmed by the warming effect of all the greenhouse gases we have pumped, and continue to pump, into the atmosphere.

Causes of Climate Change

Includes natural changes (cyclic variability, volcanic eruptions, solar output) and human-caused changes (due to GHG emissions and land use changes)

November 2020: The year is ending as it began, on a hot streak

November 2020 was the second-warmest November on record, which increases 2020's chance of becoming the warmest year on record to over 50%.

September 2020: Another record-setting month for global heat

September 2020 was the hottest September on record for the globe, continuing a sweltering year.

Climate.gov tweet chat: Talk with heat experts on mapping urban heat islands

Join three heat experts to talk about how we map, monitor, and lessen the impacts of urban heat islands.

June 2020: third-hottest June on record

June 2020 broke the streak of each month being warmest or second warmest on record by being only the third-warmest June on record.

Climate.gov tweet chat: Talk with education experts on climate change education

Join three education experts to talk about how engaging, empowering and educating others can lead to powerful climate action.

Nature’s archives: piecing together 12,000 years of Earth’s climate story

The most comprehensive database ever assembled of paleoclimate proxies that tell scientists about temperatures since the last ice age ended around 12,000 years ago has been released to the public.

What is NOAA's climate mission?

NOAA is an agency that enriches life through science. From the surface of the sun to the bottom of the ocean, NOAA advances scientific understanding of Earth's environment, climate, and weather.

Are humans causing or contributing to global warming?

Yes, human activity is putting carbon dioxide into the atmosphere faster than natural processes take it out. Rising carbon dioxide levels are strengthening Earth's greenhouse effect and causing global warming.

High temperatures smash all-time records in Alaska in early July 2019

July 16, 2019

¿Cuál es la diferencia entre el calentamiento global y el cambio climático?

El calentamiento global es un síntoma del problema mucho más grande del cambio climático causado por los humanos.

Science News

Abdulhamid Hosbas/Anadolu Agency via Getty Images

Century of Science: Theme

Our climate change crisis

The climate change emergency.

Even in a world increasingly battered by weather extremes, the summer 2021 heat wave in the Pacific Northwest stood out. For several days in late June, cities such as Vancouver, Portland and Seattle baked in record temperatures that killed hundreds of people. On June 29 Lytton, a village in British Columbia, set an all-time heat record for Canada, at 121° Fahrenheit (49.6° Celsius); the next day, the village was incinerated by a wildfire.

Within a week, an international group of scientists had analyzed this extreme heat and concluded it would have been virtually impossible without climate change caused by humans. The planet’s average surface temperature has risen by at least 1.1 degree Celsius since preindustrial levels of 1850–1900 — because people are loading the atmosphere with heat-trapping gases produced during the burning of fossil fuels, such as coal and gas, and from cutting down forests.

A little over 1 degree of warming may not sound like a lot. But it has already been enough to fundamentally transform how energy flows around the planet. The pace of change is accelerating, and the consequences are everywhere. Ice sheets in Greenland and Antarctica are melting, raising sea levels and flooding low-lying island nations and coastal cities. Drought is parching farmlands and the rivers that feed them. Wildfires are raging. Rains are becoming more intense, and weather patterns are shifting .

Australian Wildfires. Research links the fires to human-caused climate change.

The roots of understanding this climate emergency trace back more than a century and a half. But it wasn’t until the 1950s that scientists began the detailed measurements of atmospheric carbon dioxide that would prove how much carbon is pouring from human activities. Beginning in the 1960s, researchers began developing comprehensive computer models that now illuminate the severity of the changes ahead.

Global average temperature change, 1850–2021

Long-term climate datasets show that Earth’s average surface temperature (combined land and ocean) has increased by more than 1 degree Celsius since preindustrial times. Temperature change is the difference from the 1850–1900 average.

Today we know that climate change and its consequences are real, and we are responsible. The emissions that people have been putting into the air for centuries — the emissions that made long-distance travel, economic growth and our material lives possible — have put us squarely on a warming trajectory . Only drastic cuts in carbon emissions, backed by collective global will, can make a significant difference.

“What’s happening to the planet is not routine,” says Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “We’re in a planetary crisis.” — Alexandra Witze

Tracking a Greenland glacier

The calving front of Greenland’s Helheim Glacier, which flows toward the sea where it crumbles into icebergs, held roughly the same position from the 1970s until 2001 (left, the calving front is to the far right of the image). But by 2005 (right), it had retreated 7.5 kilometers toward its source.

The first climate scientists

One day in the 1850s, Eunice Newton Foote, an amateur scientist and women’s rights activist living in upstate New York, put two glass jars in sunlight. One contained regular air — a mix of nitrogen, oxygen and other gases including carbon dioxide — while the other contained just CO 2 . Both had thermometers in them. As the sun’s rays beat down, Foote observed that the jar of CO 2 alone heated more quickly, and was slower to cool, than the one containing plain air.

Illustration of Eunice Newton Foote. Hers were some of the first studies of climate change.

The results prompted Foote to muse on the relationship between CO 2 , the planet and heat. “An atmosphere of that gas would give to our earth a high temperature,” she wrote in an 1856 paper summarizing her findings .

Three years later, working independently and apparently unaware of Foote’s discovery, Irish physicist John Tyndall showed the same basic idea in more detail. With a set of pipes and devices to study the transmission of heat, he found that CO 2 gas, as well as water vapor, absorbed more heat than air alone. He argued that such gases would trap heat in Earth’s atmosphere, much as panes of glass trap heat in a greenhouse, and thus modulate climate. “As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth’s surface,” he wrote in 1862.

Today Tyndall is widely credited with the discovery of how what are now called greenhouse gases heat the planet, earning him a prominent place in the history of climate science. Foote faded into relative obscurity — partly because of her gender, partly because her measurements were less sensitive. Yet their findings helped kick off broader scientific exploration of how the composition of gases in Earth’s atmosphere affects global temperatures.

Carbon floods in

Humans began substantially affecting the atmosphere around the turn of the 19th century, when the Industrial Revolution took off in Britain. Factories burned tons of coal; fueled by fossil fuels, the steam engine revolutionized transportation and other industries. In the decades since, fossil fuels including oil and natural gas have been harnessed to drive a global economy. All these activities belch gases into the air.

Yet Svante Arrhenius, a Swedish physical chemist, wasn’t worried about the Industrial Revolution when he began thinking in the late 1800s about changes in atmospheric CO 2 levels. He was instead curious about ice ages — including whether a decrease in volcanic eruptions, which can put CO 2 into the atmosphere, would lead to a future ice age. Bored and lonely in the wake of a divorce, Arrhenius set himself to months of laborious calculations involving moisture and heat transport in the atmosphere at different zones of latitude. In 1896 he reported that halving the amount of CO 2 in the atmosphere could indeed bring about an ice age — and that doubling CO 2 would raise global temperatures by around 5 to 6 degrees C.

It was a remarkably prescient finding for work that, out of necessity, had simplified Earth’s complex climate system down to just a few variables. Today, estimates for how much the planet will warm through a doubling of CO 2 — a measure known as climate sensitivity — range between 1.5 degrees and 4.5 degrees Celsius. (The range remains broad in part because scientists now incorporate their understanding of many more planetary feedbacks than were recognized in Arrhenius’ day.)

But Arrhenius’ findings didn’t gain much traction with other scientists at the time. The climate system seemed too large, complex and inert to change in any meaningful way on a timescale that would be relevant to human society. Geologic evidence showed, for instance, that ice ages took thousands of years to start and end. What was there to worry about? And other laboratory experiments — later shown to be flawed — appeared to indicate that changing levels of CO 2 would have little impact on heat absorption in the atmosphere. Most scientists aware of the work came to believe that Arrhenius had been proved wrong.

One researcher, though, thought the idea was worth pursuing. Guy Stewart Callendar, a British engineer and amateur meteorologist, had tallied weather records over time, obsessively enough to determine that average temperatures were increasing at 147 weather stations around the globe. In 1938, in a paper in a Royal Meteorological Society journal , he linked this temperature rise to the burning of fossil fuels. Callendar estimated that fossil fuel burning had put around 150 billion metric tons of CO 2 into the atmosphere since the late 19th century.

Like many of his day, Callendar didn’t see global warming as a problem. Extra CO 2 would surely stimulate plants to grow and allow crops to be farmed in new regions. “In any case the return of the deadly glaciers should be delayed indefinitely,” he wrote. But his work revived discussions tracing back to Tyndall and Arrhenius about how the planetary system responds to changing levels of gases in the atmosphere. And it began steering the conversation toward how human activities might drive those changes.

When World War II broke out the following year, the global conflict redrew the landscape for scientific research. Hugely important wartime technologies, such as radar and the atomic bomb, set the stage for “big science” studies that brought nations together to tackle high-stakes questions of global reach. And that allowed modern climate science to emerge.

The Keeling curve and climate change

One major postwar effort was the International Geophysical Year, an 18-month push in 1957–1958 that involved a wide array of scientific field campaigns including exploration in the Arctic and Antarctica. Climate change wasn’t a high research priority during the IGY, but some scientists in California, led by Roger Revelle of the Scripps Institution of Oceanography in La Jolla, used the funding influx to begin a project they’d long wanted to do. The goal was to measure CO 2 levels at different locations around the world, accurately and consistently.

The job fell to geochemist Charles David Keeling, who put ultraprecise CO 2 monitors in Antarctica and on the Hawaiian volcano of Mauna Loa. Funds soon ran out to maintain the Antarctic record, but the Mauna Loa measurements continued. Thus was born one of the most iconic datasets in all of science — the “Keeling curve,” which tracks the rise of atmospheric CO 2 . When Keeling began his measurements in 1958, CO 2 made up 315 parts per million of the global atmosphere. Within just a few years it became clear that the number was increasing year by year. Because plants take up CO 2 as they grow in spring and summer and release it as they decompose in fall and winter, CO 2 concentrations rose and fell each year in a sawtooth pattern — but superimposed on that pattern was a steady march upward.

Monthly average CO 2 concentrations at Mauna Loa Observatory

Atmospheric carbon dioxide measurements collected continuously since 1958 at Mauna Loa volcano in Hawaii show the rise due to human activities. The visible sawtooth pattern is due to seasonal plant growth: Plants take up CO 2 in the growing seasons, then release it as they decompose in fall and winter.

“The graph got flashed all over the place — it was just such a striking image,” says Ralph Keeling, who is Charles David Keeling’s son. Over the years, as the curve marched higher, “it had a really important role historically in waking people up to the problem of climate change.” The Keeling curve has been featured in countless earth science textbooks, congressional hearings and in Al Gore’s 2006 documentary on climate change, An Inconvenient Truth . Each year the curve keeps going up: In 2016 it passed 400 ppm of CO 2 in the atmosphere, as measured during its typical annual minimum in September. In 2021, the annual minimum was 413 ppm. (Before the Industrial Revolution, CO 2 levels in the atmosphere had been stable for centuries at around 280 ppm.)

Around the time that Keeling’s measurements were kicking off, Revelle also helped develop an important argument that the CO 2 from human activities was building up in Earth’s atmosphere. In 1957 he and Hans Suess, also at Scripps at the time, published a paper that traced the flow of radioactive carbon through the oceans and the atmosphere. They showed that the oceans were not capable of taking up as much CO 2 as previously thought; the implication was that much of the gas must be going into the atmosphere instead. “Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future,” Revelle and Suess wrote in the paper. It’s one of the most famous sentences in earth science history.

“Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future.”

Here was the insight underlying modern climate science: Atmosheric CO 2 is increasing, and humans are causing the buildup. Revelle and Suess became the final piece in a puzzle dating back to Svante Arrhenius and John Tyndall.

“I tell my students that to understand the basics of climate change, you need to have the cutting-edge science of the 1860s, the cutting-edge math of the 1890s and the cutting-edge chemistry of the 1950s,” says Joshua Howe, an environmental historian at Reed College in Portland, Ore.

Environmental awareness grows

As this scientific picture began to emerge in the late 1950s, Science News was on the story. A March 1, 1958 article in Science News Letter , “Weather May Be Warming,” described a warm winter month in the Northern Hemisphere. It posits three theories, including that “carbon dioxide poured into the atmosphere by a booming industrial civilization could have caused the increase. By burning up about 100 billion tons of coal and oil since 1900, man himself may be changing the climate.” By 1972, the magazine was reporting on efforts to expand global atmospheric greenhouse gas monitoring beyond Keeling’s work; two years later, the U.S. National Oceanic and Atmospheric Administration launched its own CO 2 monitoring network, now the biggest in the world.

Environmental awareness on other issues grew in the 1960s and 1970s. Rachel Carson catalyzed the modern U.S. environmental movement in 1962 when she published a magazine series and then a book, Silent Spring , condemning the pesticide DDT for its ecological impacts. 1970 saw the celebration of the first Earth Day , in the United States and elsewhere, and in India in 1973 a group of women led a series of widely publicized protests against deforestation. This Chipko movement explicitly linked environmental protection with protecting human communities, and helped seed other environmental movements.

The fragility of global energy supplies was also becoming more obvious through the 1970s. The United States, heavily dependent on other countries for oil imports, entered a gas shortage in 1973–74 when Arab members of the Organization of the Petroleum Exporting Countries cut off oil supplies because of U.S. government support for Israel. The shortage prompted more people to think about the finiteness of natural resources and the possibility of overtaxing the planet. — Alexandra Witze

Welland, Ontario environmental movement pic

Climate change evidence piles up

Observational data collected throughout the second half of the 20th century helped researchers gradually build their understanding of how human activities were transforming the planet. “It was a sort of slow accretion of evidence and concern,” says historian Joshua Howe of Reed College.

Environmental records from the past, such as tree rings and ice cores, established that the current changes in climate are unusual compared with the recent past. Yet such paleoclimatology data also showed that climate has changed quickly in the deep past — driven by triggers other than human activity, but with lessons for how abrupt planetary transformations can be.

Ice cores pulled from ice sheets, such as that atop Greenland, offer some of the most telling insights for understanding past climate change. Each year snow falls atop the ice and compresses into a fresh layer of ice representing climate conditions at the time it formed. The abundance of certain forms, or isotopes, of oxygen and hydrogen in the ice allows scientists to calculate the temperature at which it formed, and air bubbles trapped within the ice reveal how much carbon dioxide and other greenhouse gases were in the atmosphere at that time. So drilling down into an ice sheet is like reading the pages of a history book that go back in time the deeper you go.

Scientists began reading these pages in the early 1960s, using ice cores drilled at a U.S. military base in northwest Greenland . Contrary to expectations that past climates were stable, the cores hinted that abrupt climate shifts had happened over the last 100,000 years. By 1979, an international group of researchers was pulling another deep ice core from a second location in Greenland — and it, too, showed that abrupt climate change had occurred in the past. In the late 1980s and early 1990s a pair of European- and U.S.-led drilling projects retrieved even deeper cores from near the top of the ice sheet, pushing the record of past temperatures back a quarter of a million years.

Together with other sources of information, such as sediment cores drilled from the seafloor and molecules preserved in ancient rocks, the ice cores allowed scientists to reconstruct past temperature changes in extraordinary detail. Many of those changes happened alarmingly fast. For instance, the climate in Greenland warmed abruptly more than 20 times in the last 80,000 years, with the changes occurring in a matter of decades. More recently, a cold spell that set in around 13,000 years ago suddenly came to an end around 11,500 years ago — and temperatures in Greenland rose 10 degrees Celsius in a decade.

Evidence for such dramatic climate shifts laid to rest any lingering ideas that global climate change would be slow and unlikely to occur on a timescale that humans should worry about. “It’s an important reminder of how ‘tippy’ things can be,” says Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson.

More evidence of global change came from Earth-observing satellites, which brought a new planet-wide perspective on global warming beginning in the 1960s. From their viewpoint in the sky, satellites have measured the steady rise in global sea level — currently 3.4 millimeters per year and accelerating, as warming water expands and as ice sheets melt — as well as the rapid decline in ice left floating on the Arctic Ocean each summer at the end of the melt season. Gravity-sensing satellites have ‘weighed’ the Antarctic and Greenlandic ice sheets from above since 2002, reporting that more than 400 billion metric tons of ice are lost each year.

Temperature observations taken at weather stations around the world also confirm that we are living in the hottest years on record. The 10 warmest years since record keeping began in 1880 have all occurred since 2005. And nine of those 10 have come since 2010.

What’s more, extreme weather is hammering the planet more and more frequently. That 2021 heat wave in the Pacific Northwest, for instance, is just a harbinger of what’s to come. — Alexandra Witze

Worrisome predictions from climate models

By the 1960s, there was no denying that the planet was warming. But understanding the consequences of those changes — including the threat to human health and well-being — would require more than observational data. Looking to the future depended on computer simulations: complex calculations of how energy flows through the planetary system. Such models of the climate system have been crucial to developing projections for what we can expect from greenhouse warming.

A first step in building climate models was to connect everyday observations of weather to the concept of forecasting future climate. During World War I, the British mathematician Lewis Fry Richardson imagined tens of thousands of meteorologists working to forecast the weather, each calculating conditions for a small part of the atmosphere but collectively piecing together a global forecast. Richardson published his work in 1922, to reviews that called the idea “of almost quixotic boldness.”

Charney paper (first weather predictions with ENIAC)

But it wasn’t until after World War II that computational power turned Richardson’s dream into reality. In the wake of the Allied victory, which relied on accurate weather forecasts for everything from planning D-Day to figuring out when and where to drop the atomic bombs, leading U.S. mathematicians acquired funding from the federal government to improve predictions. In 1950 a team led by Jule Charney, a meteorologist at the Institute for Advanced Study in Princeton, N.J., used the ENIAC, the first general-purpose, programmable electronic computer, to produce the first computer-driven regional weather forecast . The forecasting was slow and rudimentary, but it built on Richardson’s ideas of dividing the atmosphere into squares, or cells, and computing the weather for each of those. With the obscure title “Numerical integration of the barotropic vorticity equation,” the paper reporting the results set the stage for decades of climate modeling to follow.

By 1956 Norman Phillips, a member of Charney’s team, had produced the world’s first general circulation model, which captured how energy flows between the oceans, atmosphere and land. Phillips ran the calculations on a computer with just 5 kilobytes of memory, yet it was able to reproduce monthly and seasonal patterns in the lower atmosphere. That meant scientists could begin developing more realistic models of how the planet responds to factors such as increasing levels of greenhouse gases. The field of climate modeling was born.

The work was basic at first, because early computers simply didn’t have much computational power to simulate all aspects of the planetary system. “People thought that it was stupid to try to study this greenhouse-warming issue by three-dimensional model[s], because it cost so much computer time,” meteorologist Syukuro Manabe told physics historian Spencer Weart in a 1989 oral history .

Climate models have predicted how much ice the Ilulissat region of the Greenland ice sheet might lose by 2300 based on different scenarios for greenhouse gas emissions. The models are compared to 2008 (first image). In a best-case scenario, in which emissions peak by mid-century, the speed at which the glacier is sending ice out into the ocean is much lower (second image) than with a worst-case scenario, in which emissions rise at a high rate (third image).

An important breakthrough came in 1967, when Manabe and Richard Wetherald — both at the Geophysical Fluid Dynamics Laboratory in Princeton, a lab born from Charney’s group — published a paper in the Journal of the Atmospheric Sciences that modeled connections between Earth’s surface and atmosphere and calculated how changes in carbon dioxide would affect the planet’s temperature. Manabe and Wetherald were the first to build a computer model that captured the relevant processes that drive climate , and to accurately simulate how the Earth responds to those processes. (Manabe shared the 2021 Nobel Prize in physics for his work on climate modeling; Wetherald died in 2011.)

The rise of climate modeling allowed scientists to more accurately envision the impacts of global warming. In 1979, Charney and other experts met in Woods Hole, Mass., to try to put together a scientific consensus on what increasing levels of CO 2 would mean for the planet. They analyzed climate models from Manabe and from James Hansen of NASA. The resulting “Charney report” concluded that rising CO 2 in the atmosphere would lead to additional and significant climate change. The ocean might take up much of that heat, the scientists wrote — but “it appears that the warming will eventually occur, and the associated regional climatic changes so important to the assessment of socioeconomic consequence may well be significant.”

In the decades since, climate modeling has gotten increasingly sophisticated . Scientists have drawn up a variety of scenarios for how carbon emissions might change in the future, depending on the stringency of emissions cuts. Modelers use those scenarios to project how climate and weather will change around the globe, from hotter croplands in China to melting glaciers in the Himalayas. Climate simulations have also allowed researchers to identify the fingerprints of human impacts on extreme weather that is already happening, by comparing scenarios that include the influence of human activities with those that do not.

And as climate science firmed up and the most dramatic consequences became clear, the political battles raged. — Alexandra Witze

Climate science meets politics

With the development of climate science tracing back to the early Cold War, perhaps it shouldn’t be a surprise that the science of global warming became enmeshed in broader societal and political battles. A complex stew of political, national and business interests mired society in debates about the reality of climate change, and what to do about it, decades after the science became clear that humans are fundamentally altering the planet’s atmosphere.

Society has pulled itself together before to deal with global environmental problems, such as the Antarctic ozone hole. In 1974 chemists Mario Molina and F. Sherwood Rowland, both of the University of California, Irvine, reported that chlorofluorocarbon chemicals, used in products such as spray cans and refrigerants, caused a chain of reactions that gnawed away at the atmosphere’s protective ozone layer . The resulting ozone hole, which forms over Antarctica every spring, allows more ultraviolet radiation from the sun to make it through Earth’s atmosphere and reach the surface, where it can cause skin cancer and eye damage.

Governments ultimately worked under the auspices of the United Nations to craft the 1987 Montreal Protocol, which strictly limited the manufacture of chlorofluorocarbons . In the years following, the ozone hole began to heal. But fighting climate change would prove to be far more challenging. Chlorofluorocarbons were a suite of chemicals with relatively limited use and for which replacements could be found without too much trouble. But the greenhouse gases that cause global warming stem from a wide variety of human activities, from energy development to deforestation. And transforming entire energy sectors to reduce or eliminate carbon emissions is much more difficult than replacing a set of industrial chemicals.

In 1980, though, researchers took an important step toward banding together to synthesize the scientific understanding of climate change and bring it to the attention of international policy makers. It started at a small scientific conference in Villach, Austria. There, experts met under the auspices of the World Meteorological Organization, the International Council of Scientific Unions and the United Nations Environment Program to discuss the seriousness of climate change. On the train ride home from the meeting, Swedish meteorologist Bert Bolin talked with other participants about how a broader, deeper and more international analysis was needed. In 1985, a second conference was held at Villach to highlight the urgency, and in 1988, the Intergovernmental Panel on Climate Change, the IPCC, was born. Bolin was its first chairperson.

The IPCC became a highly influential and unique body. It performs no original scientific research; instead, it synthesizes and summarizes the vast literature of climate science for policy makers to consider — primarily through massive reports issued every couple of years. The first IPCC report , in 1990, predicted that the planet’s global mean temperature would rise more quickly in the following century than at any point in the last 10,000 years, due to increasing greenhouse gases in the atmosphere. Successive IPCC reports showed more and more confidence in the link between greenhouse emissions and rising global temperatures — and explored how society might mitigate and adapt to coming changes.

IPCC reports have played a key role in providing scientific information for nations discussing how to stabilize greenhouse gas concentrations. This process started with the Rio Earth Summit in 1992 , which resulted in the U.N. Framework Convention on Climate Change. Annual U.N. meetings to tackle climate change led to the first international commitments to reduce emissions, the Kyoto Protocol of 1997. Under it, developed countries committed to reduce emissions of CO 2 and other greenhouse gases. By 2007 the IPCC declared that the reality of climate warming is “unequivocal ”; the group received the Nobel Peace Prize that year along with Al Gore for their work on climate change.

The IPCC process ensured that policy makers had the best science at hand when they came to the table to discuss cutting emissions. “If you go back and look at the original U.N. framework on climate change, already you see the core of the science represented there,” says Rachel Cleetus, a climate policy expert with the Union of Concerned Scientists in Cambridge, Mass. Of course, nations did not have to abide by that science — and they often didn’t.

Throughout the 2000s and 2010s, international climate meetings discussed less hard-core science and more issues of equity. Countries such as China and India pointed out that they needed energy to develop their economies, and that nations responsible for the bulk of emissions through history, such as the United States, needed to lead the way in cutting greenhouse gases. Meanwhile, residents of some of the most vulnerable nations, such as low-lying islands that are threatened by sea level rise, gained visibility and clout at international negotiating forums. “The issues around equity have always been very uniquely challenging in this collective action problem,” says Cleetus.

By 2015, the world’s nations had made some progress on the emissions cuts laid out in the Kyoto Protocol, but it was still not enough to achieve substantial global reductions. That year, a key U.N. climate conference in Paris produced an international agreement to try to limit global warming to 2 degrees C , and preferably 1.5 degrees C, above preindustrial levels.

Every country has its own approach to the challenge of addressing climate change. In the United States, which gets approximately 80 percent of its energy from fossil fuels, sophisticated efforts to downplay and critique the science led to major delays in climate action. For decades U.S. fossil fuel companies such as ExxonMobil worked to influence politicians to take as little action on emissions reductions as possible. Working with a small group of influential scientists, this well-funded, well-orchestrated campaign took many of its tactics from earlier tobacco-industry efforts to cast doubt on the links between smoking and cancer, as historians Naomi Oreskes and Erik Conway documented in their book Merchants of Doubt.

Perhaps the peak of U.S. climate denialism came in the late 1980s and into the 1990s — roughly a century after Swedish physical chemist Svante Arrhenius laid out the consequences of putting too much carbon dioxide into the atmosphere. In 1988 NASA scientist James Hansen testified to lawmakers about the consequences of global warming. “It is already happening now,” Hansen said, summarizing what scientists had long known.

The high-profile nature of Hansen’s testimony, combined with his NASA expertise, vaulted global warming into the public eye in the United States like never before. “It really hit home with a public who could understand that there are reasons that Venus is hot and Mars is cold,” says Joshua Howe, a historian at Reed College. “And that if you use that same reasoning, we have some concerns about what is happening here on Earth.” But Hansen also kicked off a series of bitter public battles about the reality of human-caused climate change that raged for years.

One common approach of climate skeptics was to attack the environmental data and models that underlie climate science. In 1998, scientist Michael Mann, then at the University of Massachusetts–Amherst, and colleagues published a detailed temperature record that formed the basis of what came to be known as the “hockey stick” graph, so named because the chart showed a sharp rise in temperatures (the hockey blade) at the end of a long, much flatter period (the hockey stick). Skeptics soon demanded the data and software processing tools Mann used to create the graph. Bloggers and self-proclaimed citizen scientists created a cottage industry of questioning new climate science papers under the guise of “audits.” In 2009 hackers broke into a server at the University of East Anglia, a leading climate-research hub in Norwich, England, and released more than 1,000 e-mails between climate scientists. This “Climategate” scandal purported to reveal misconduct on the part of the researchers, but several reviews largely exonerated the scientists.

The graph that launched climate skeptic attacks

This famous graph, produced by scientist Michael Mann and colleagues, and then reproduced in a 2001 report by the Intergovernmental Panel on Climate Change, dramatically captures temperature change over time. Climate change skeptics made it the center of an all-out attack on climate science.

image of the "hockey stick" graph showing the increase in temperature from 1961 to 1990

Such tactics undoubtedly succeeded in feeding politicians’ delay on climate action in the United States, most of it from Republicans. President George W. Bush withdrew the country from the Kyoto Protocol in 2001 ; Donald Trump similarly rejected the Paris accord in 2017 . As late as 2015, the chair of the Senate’s environment committee, James Inhofe of Oklahoma, brought a snowball into Congress on a cold winter’s day in order to continue his argument that human-caused global warming is a “hoax.” In Australia, a similar mix of right-wing denialism and fossil fuel interests has kept climate change commitments in flux, as prime ministers are voted in and out over fierce debates about how the nation should act on climate.

Yet other nations have moved forward. Some European countries such as Germany aggressively pursued renewable energies, such as wind and solar, while activists such as the Swedish teenager Greta Thunberg — the vanguard of a youth-action movement — pressured their governments for more.

In recent years the developing economies of China and India have taken center stage in discussions about climate action. Both nations argue that they must be allowed extra time to wean themselves off fossil fuels in order to continue economic growth. They note that historically speaking, the United States is the largest total emitter of carbon by far.

Total carbon dioxide emissions by country, 1850–2021

These 20 nations have emitted the largest cumulative amounts of carbon dioxide since 1850. Emissions are shown in in billions of metric tons and are broken down into subtotals from fossil fuel use and cement manufacturing (blue) as well as from land use and forestry (green).

China, whose annual CO 2 emissions surpassed those of the United States in 2006, declared several moderate steps in 2021 to reduce emissions, including that it would stop building coal-burning power plants overseas. India announced it would aim for net-zero emissions by 2070, the first time it has set a date for this goal.

Yet such pledges continue to be criticized. At the 2021 U.N. Climate Change Conference in Glasgow, Scotland, India was globally criticized for not committing to a complete phaseout of coal — although the two top emitters, China and the United States, have not themselves committed to phasing out coal. “There is no equity in this,” says Aayushi Awasthy, an energy economist at the University of East Anglia. — Alexandra Witze

Facing a warmer future

Climate change creeps up gradually on society, except when it doesn’t. The slow increase in sea level, for instance, causes waters to lap incrementally higher at shorelines year after year. But when a big storm comes along — which may be happening more frequently due to climate change — the consequences become much more obvious. Storm surge rapidly swamps communities and wreaks disproportionate havoc. That’s why New York City installed floodgates in its subway and tunnel system in the wake of 2012’s Superstorm Sandy , and why the Pacific island nation of Tuvalu has asked Australia and New Zealand to be prepared to take in refugees fleeing from rising sea levels.

The list of climate impacts goes on and on — and in many cases, changes are coming faster than scientists had envisioned a few decades ago. The oceans are becoming more acidic as they absorb carbon dioxide, harming tiny marine organisms that build protective calcium carbonate shells and are the base of the marine food web. Warmer waters are bleaching coral reefs. Higher temperatures are driving animal and plant species into areas in which they previously did not live, increasing the risk of extinction for many. “It’s no longer about impacts in the future,” says Rachel Cleetus, a climate policy expert at the Union of Concerned Scientists. “It’s about what’s happening in the U.S. here and now, and around the world.”

No place on the planet is unaffected. In many areas, higher temperatures have led to major droughts, which dry out vegetation and provide additional fuel for wildfires such as those that have devastated Australia , the Mediterranean and western North America in recent years. The Colorado River , the source of water for tens of millions of people in the western United States , came under a water-shortage alert in 2021 for the first time in history.

Then there’s the Arctic, where temperatures are rising at more than twice the global average and communities are at the forefront of change. Permafrost is thawing, destabilizing buildings, pipelines and roads. Caribou and reindeer herders worry about the increased risk of parasites to the health of their animals. With less sea ice available to buffer the coast from storm erosion, the Inupiat village of Shishmaref, Alaska, risks crumbling into the sea. It will need to move from its sand-barrier island to the mainland .

“We know these changes are happening and that the Titanic is sinking,” says Louise Farquharson, a geomorphologist at the University of Alaska in Fairbanks who monitors permafrost and coastal change around Alaska. Like many Arctic scientists, she is working with Indigenous communities to understand the shifts they’re experiencing and what can be done when buildings start to slump and water supplies start to drain away. “A big part is just listening to community members and understanding what they’re seeing change,” she says.

All around the planet, those who depend on intact ecosystems for their survival face the greatest threat from climate change. And those with the least resources to adapt to climate change are the ones who feel it first .

“We are going to warm,” says Claudia Tebaldi, a climate scientist at Lawrence Berkeley National Laboratory in California. “There is no question about it. The only thing that we can hope to do is to warm a little more slowly.”

That’s one reason why the IPCC report released in 2021 focuses on anticipated levels of global warming. There is a big difference between the planet warming 1.5 degrees versus 2 degrees or 2.5 degrees. Consider that we are now at least 1.1 degrees above preindustrial levels of CO 2 and are already seeing dramatic shifts in climate. Given that, keeping further global temperature increases as low as possible will make a big difference in the climate impacts the planet faces. “With every fraction of a degree of warming, everything gets a little more intense,” says paleoclimatologist Jessica Tierney. “There’s no more time to beat around the bush.”

Historical and projected global temperature change

Various scenarios for how greenhouse gas emissions might change going forward help scientists predict future climate change. This graph shows the simulated historical temperature trend along with future projections of global surface temperature based on five scenarios from the Intergovernmental Panel on Climate Change. Temperature change is the difference from the 1850–1900 average.

The future rests on how much nations are willing to commit to cutting emissions and whether they will stick to those commitments. It’s a geopolitical balancing act the likes of which the world has never seen.

Science can and must play a role going forward. Improved climate models will illuminate what changes are expected at the regional scale, helping officials prepare. Governments and industry have crucial parts to play as well. They can invest in technologies, such as carbon sequestration, to help decarbonize the economy and shift society toward more renewable sources of energy. “We can solve these problems — most of the tools are already there,” says Cascade Tuholske, a geographer at Columbia University. “We just have to do it.”

Huge questions remain. Do voters have the will to demand significant energy transitions from their governments? How can business and military leaders play a bigger role in driving climate action? What should be the role of low-carbon energy sources that come with downsides, such as nuclear energy ? How can developing nations achieve a better standard of living for their people while not becoming big greenhouse gas emitters? How can we keep the most vulnerable from being disproportionately harmed during extreme events, and incorporate environmental and social justice into our future?

These questions become more pressing each year, as CO 2 accumulates in our atmosphere. The planet is now at higher levels of CO 2 than at any time in the last 3 million years. Yet Ralph Keeling, keeper of the iconic Mauna Loa record tracking the rise in atmospheric CO 2 , is already optimistically thinking about how scientists would be able to detect a slowdown, should the world actually start cutting emissions by a few percent per year. “That’s what the policy makers want to see — that there’s been some large-scale impact of what they did,” he says.

At the 2021 U.N. climate meeting in Glasgow diplomats from around the world agreed to work more urgently to shift away from using fossil fuels. They did not, however, adopt targets strict enough to keep the world below a warming of 1.5 degrees Celsius. It’s been well over a century since Svante Arrhenius recognized the consequences of putting extra carbon dioxide into the atmosphere, and yet world leaders have yet to pull together to avoid the most dangerous consequences of climate change.

Time is running out. — Alexandra Witze

Climate change facts

We know that climate change and its consequences are real, and we are responsible. Here’s what the science tells us.

How much has the planet warmed over the past century?

The planet’s average surface temperature has risen by at least 1.1 degree Celsius since preindustrial levels of 1850–1900.

What is causing climate change?

People are loading the atmosphere with carbon dioxide and other heat-trapping gases produced during the burning of fossil fuels, such as coal and gas, and cutting down forests.

What are some of the effects of climate change?

Ice sheets in Greenland and Antarctica are melting, raising sea levels and flooding low-lying island nations and coastal cities. Drought is parching farmlands and the rivers that feed them. Wildfires are raging. Rains are becoming more intense, and weather patterns are shifting.

What is the greenhouse effect?

In the 19th century, Irish physicist John Tyndall found that carbon dioxide gas, as well as water vapor, absorbed more heat than air alone. He argued that such gases would trap heat in Earth’s atmosphere, much as panes of glass trap heat in a greenhouse, and thus modulate climate.

What is the Keeling curve?

line graph showing increasing monthly average CO2 concentrations at Mauna Loa Observatory from 1958 to 2022

One of the most iconic datasets in all of science, the Keeling curve tracks the rise of atmospheric CO 2 . When geochemist Charles David Keeling began his measurements in 1958 on the Hawaiian volcano of Mauna Loa, CO 2 made up 315 parts per million of the global atmosphere. Each year the curve keeps going up: In 2016 it passed 400 ppm of CO 2 in the atmosphere, as measured during its typical annual minimum in September. In 2021, the annual minimum was 413 ppm.

Does it get hotter every year?

Average global temperatures fluctuate from year to year, but temperature observations taken at weather stations around the world confirm that we are living in the hottest years on record. The 10 warmest years since record keeping began in 1880 have all occurred since 2005. And nine of those 10 have come since 2010.

What countries emit the most carbon dioxide?

The United States has been the largest total emitter of carbon dioxide by far, followed by China and Russia. China’s annual CO 2 emissions surpassed those of the United States in 2006.

What places are impacted by climate change?

No place on the planet is unaffected. Higher temperatures have led to major droughts, providing fuel for wildfires such as those that have devastated Australia , the Mediterranean and western North America in recent years. The Colorado River came under a water-shortage alert in 2021 for the first time in history. In the Arctic, where temperatures are rising at more than twice the global average, permafrost is thawing, destabilizing buildings, pipelines and roads. With less sea ice available to buffer the coast from storm erosion, the Inupiat village of Shishmaref, Alaska, risks crumbling into the sea. All around the planet, those who depend on intact ecosystems for their survival face the greatest threat from climate change. And those with the least resources to adapt to climate change are the ones who feel it first .

Editor’s note: This story was published March 10, 2022.

British mathematician Lewis Fry Richardson (shown at center) proposes forecasting the weather by piecing together the calculations of tens of thousands of meteorologists working on small parts of the atmosphere.

Geochemist Charles David Keeling (shown in 1988) begins tracking the rise in atmospheric carbon dioxide at Mauna Loa in Hawaii. The record, which continues through today, has become one of the most iconic datasets in all of science.

Rachel Carson (shown) publishes the book Silent Spring , raising alarm over the ecological impacts of the pesticide DDT. The book helps catalyze the modern U.S. environmental movement.

The first Earth Day, organized by U.S. senator Gaylord Nelson and graduate student Denis Hayes, is celebrated.

Image of rocket on the base set to launch Landsat

The first Landsat satellite launched (shown), opening the door to continuous monitoring of Earth and its features from above.

A powerful eruption from the Philippines’ Mount Pinatubo (shown) ejects millions of tons of sulfur dioxide into the stratosphere, temporarily cooling the planet.

World leaders gathered (shown) at the United Nations Conference on Environment and Development in Rio de Janeiro to address how to pursue economic development while also protecting the Earth. The meeting resulted in an international convention on climate change.

Activist Greta Thunberg initiates the “School Strike for Climate” movement by protesting outside the Swedish parliament. Soon, students around the world join a growing movement demanding action on climate change . (Activists at the 2021 U.N. Climate Change Conference are shown.)

From the archive

Climate change foreseen.

In an early mention of climate change in Science News-Letter , the predecessor of Science News , British meteorologist C.E.P. Brooks warns that present warming trends could lead to “important economic and political effects.”

IGY Brings Many Discoveries

Science News Letter lists the Top 8 accomplishments of the International Geophysical Year.

Chilling possibilities

Science News explores the tentative idea that global temperatures are cooling and that a new ice age could be imminent, which is later shown to be inaccurate.

Long Hot Future: Warmer Earth Appears Inevitable

“The planet earth will be a warmer place in the 21st century, and there is no realistic strategy that can prevent the change,” Science News reports.

Ozone and Global Warming: What to Do?

Policy makers discuss how to solve the dual problems of ozone depletion and global warming.

Looking for Mr. Greenhouse

Science writer Richard Monastersky reports on scientists’ efforts to evaluate how to connect increasing greenhouse gases and a warming climate.

World Climate Panel Charts Path for Action

The Intergovernmental Panel on Climate Change reports that “the fingerprint of man in the past temperature record” is now apparent.

Animals on the Move

A warming climate means shifting ranges and ecosystem disruptions for a lot of species, Nancy Ross-Flanigan reports.

Changing climate: 10 years after ‘An Inconvenient Truth’

A decade after former vice president Al Gore releases the documentary film An Inconvenient Truth , Science News looks back at how climate science has advanced.

With nowhere to hide from rising seas, Boston prepares for a wetter future

Mary Caperton Morton reports for Science News on how Boston is taking action to prepare for rising seas.

The new UN climate change report shows there’s no time for denial or delay

Earth & climate writer Carolyn Gramling covers the sixth assessment report from the Intergovernmental Panel on Climate Change, which documents how climate change is already affecting every region on Earth.

Climate change disinformation is evolving. So are efforts to fight back

Researchers are testing games and other ways to help people recognize climate change denial.

photo of cars backed up on a freeway with a sign above that reads, "EXTREME HEAT SAVE POWER 4-9PM STAY COOL"

Extreme weather in 2022 showed the global impact of climate change

Heat waves, floods, wildfires and drought around the world were exacerbated by Earth’s changing climate.

A line of wind turbines disappearing into the distance with an out of focus wheat field in the foreground.

It’s possible to reach net-zero carbon emissions. Here’s how

Cutting carbon dioxide emissions to curb climate change and reach net zero is possible but not easy.

This image shows a man in Houston wiping sweat from his brow amid a record-breaking heat wave in June.

The last 12 months were the hottest on record

The planet’s average temperature was about 1.3 degrees Celsius higher than the 1850–1900 average, a new report finds.

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How Do We Know that Humans Are the Major Cause of Global Warming?

Published Jul 14, 2009 Updated Jan 21, 2021

Humans cause climate change. How do we know?

Scientists agree that global warming is caused mainly by human activity. Specifically, the evidence shows that certain heat-trapping gases, such as carbon dioxide, are warming the world—and that we release those gases when we burn fossil fuels like coal, oil, and gas.

As scientific models and methods grow more sophisticated, and as we collect more data, our confidence in human-caused climate change only grows higher. Here’s what we know.

A line chart showing co2 in the last century (keeling curve).

Climate science 101

Climate science encompasses a range of disciplines, from oceanography and meteorology, to chemistry, physics, biology and computer science.

Generally speaking, scientists working on climate change compare the climate patterns they observe with patterns developed using sophisticated models of Earth's systems (such as the atmosphere and ocean). By comparing the observed and modeled patterns, scientists can positively identify " human fingerprints ” and attribute a proportion of observed warming to human activities.

These fingerprints are found in a diverse range of records from the atmosphere, the ocean, and Earth’s surface. They include rising levels of carbon dioxide, unprecedented warming, and a distinct atomic signature left behind in the atmosphere when fossil fuels are burned.

A line chart showing co2 over time. It spikes at the end.

Rising carbon dioxide levels

Carbon dioxide (CO 2 ) is the heat-trapping gas in our atmosphere responsible for most of the warming measured over the past several decades. It’s released during cement manufacturing and when coal, gas, and oil are burned—something humans started doing a great deal of during the Industrial Revolution through to today.

The concentration of CO 2 in the Earth’s atmosphere has increased dramatically over the last 150 years, from a pre-industrial era concentration of approximately 280 parts per million (ppm) to more than 410 ppm currently. CO 2 concentration levels are unlikely to dip below these annual averages for hundreds of years .

In fact, measurements from ancient ice cores show that CO 2 is now at its highest levels in over 800,000 years.

A line chart showing temperatures going up over time

Unprecedented warming

As the concentration of CO 2 and other heat-trapping gases in the atmosphere rises, the Earth warms up. In fact, Earth has recently undergone unprecedented warming , particularly since the 1950s.

Every single year since 1977 has been warmer than the 20th century average. Nineteen of the 20 warmest years have occurred since 2001. The period of 2015 to 2019 included the top five hottest years on record .

Analysis of these trends strongly suggests that without the emissions from burning coal and oil, it is very unlikely that 13 out of the 15 warmest years on record would have occurred either on their own or in such quick succession. This also makes sense from the known causes.

It is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. 2018 US National Climate Assessment

Fossil fuel fingerprinting

We know that CO 2 warms the world, that CO 2 concentrations are at record-breaking levels, and that global temperatures are rising. But how do we know for certain that humans are the cause?

The answer is in the science. The CO 2 produced from fossil fuels carries a unique signature that differentiates it from CO 2 produced from other sources. In brief, it carries a specific ratio of carbon isotopes that is only found in the atmosphere when coal, oil, or gas is burned.

Scientists call this δ 13 C (pronounced "delta C thirteen"), and it’s a smoking gun. Since the 1880s, δ 13 C has changed in a way that could only happen if CO 2 was increasingly coming from fossil fuel sources.

This information tells scientists that human-caused fossil fuel emissions have been the main contributor to the rise in CO 2 concentrations since the pre-industrial era.

Data viz showing how 97percent of scientists agree on climate

The scientific consensus

Within the scientific community, there is essentially no disagreement on the causes of climate change. Multiple studies have shown that at least 97 percent of scientists agree that global warming is happening and that human activity is the primary cause.

Major scientific assessments also agree. The 2018 US National Climate Assessment —whose authors include 300 leading scientists and thirteen federal government agencies—concludes that “it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”

Similarly, an authoritative 2014 report by the Intergovernmental Panel on Climate Change, written and reviewed by hundreds of climate experts and scientists from countries around the world, states unambiguously: “Human influence on the climate system is clear, and recent anthropogenic emissions of green-house gases are the highest in history. […] Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia.”

A line graph comparing observed temperature changes with the sum of natural processes

Natural factors

We know that warming and cooling have happened in the past , long before humans were around. Natural “climate drivers” include the energy from the sun; aerosols from periodic volcanic eruptions, dust, and salt spray; natural ecological phenomena like methane-emitting termite mounds and CO 2 uptake by plants; and variations in snow and ice cover that change how much the Earth’s surface reflects the sun’s energy back into space (referred to as albedo).

All of these factors have profoundly shaped the Earth’s climate. However, none of them sufficiently explain the recent, dramatic increase in global temperatures. For that, scientists need to include human influences.

Natural drivers + human drivers best match reality

When natural and human-induced climate drivers are compared to one another, the human influences are so large that they crowd out other climate drivers over the past half century, producing the warming that we’re all experiencing. Put another way, when climate scientists focus only on natural climate drivers, their models cannot accurately reproduce the observed warming of the past half century. But when the models also include human-induced climate drivers, they accurately capture recent temperature increases in the atmosphere and in the oceans.

In fact, studies show that human activity is responsible for more than half of the warming observed since 1951.

This evidence has led organizations like the IPCC to conclude that the effects of heat-trapping gases and other human activities are both detectable throughout the climate system and “extremely likely to have been the dominant cause of the observed warming since the mid-20th century.”

A graphic showing extreme weather and its probably relationship to climate change

Are humans contributing to more extreme weather?

In recent years, the field of attribution science has become more sophisticated, and scientists are now able to quantify how much more likely an extreme event—such as a heat wave or a massive downpour—was as a result of human-caused climate change.

For example, in a landmark 2004 paper , researchers determined that climate change had at least doubled the risk of occurrence of the record-breaking 2003 European summer heat wave, which resulted in the deaths of tens of thousands of people. A 2016 study of the same heat wave concluded that human-caused climate change had increased the risk of heat-related mortality during the event by about 70 percent in central Paris and about 20 percent in London.

Similarly, researchers found that the record-breaking precipitation Texas experienced in 2017 during Hurricane Harvey was made three times more likely and 15 percent more intense by climate change.

More recently, scientists have found that fingerprinting of climate change can be detected in global daily weather patterns since 2012 and yearly patterns since 1999.

While some types of events are more readily attributable to global warming than others, attribution science is becoming increasingly robust. Several authoritative scientific institutions and government agencies have confirmed both the rigor and the validity of attributing individual extreme events to human-caused climate change.

We are the cause, and we are the solution

Knowing that human activities are the main driver of global warming helps us understand how and why our climate is changing, and it clearly defines the problem as one that is within our power to address.

We cannot avoid some level of warming caused by the heat-trapping gases already present in the atmosphere. Some of the gases (such as carbon dioxide and nitrous oxide) will last for more than a century. Many of the people who have contributed the least to global warming—including much of the Global South, and marginalized and low-income communities across the world—are facing the most severe impacts.

But with aggressive measures to reduce future emissions and adapt to those climate impacts we cannot avoid, we have a small window to avoid the worst climate change harms and build a better world.

What does that mean? It means investing in a clean energy economy and healthier and more livable and resilient communities. It means transforming transportation , reorienting our food system , and shifting to lower-carbon lifestyles. It means battling the disinformation spread by fossil fuel companies and other wealthy interests. And it means fighting for equity and environmental justice for all.

The Paris Agreement of 2015 called on nations to keep global warming under the dangerous threshold of 2°C, aiming for 1.5°C. In recent years, the global climate movement—and young people in particular—have been demanding we do even better. With the right policies, practices, vision and cooperation, acting boldly now will give us the best chance of creating the safe and promising future the youth of the world deserve.

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Follow the Money

Attacks on Science

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The Basics of Climate Change

Greenhouse gases affect Earth’s energy balance and climate

The Sun serves as the primary energy source for Earth’s climate. Some of the incoming sunlight is reflected directly back into space, especially by bright surfaces such as ice and clouds, and the rest is absorbed by the surface and the atmosphere. Much of this absorbed solar energy is re-emitted as heat (longwave or infrared radiation). The atmosphere in turn absorbs and re-radiates heat, some of which escapes to space. Any disturbance to this balance of incoming and outgoing energy will affect the climate. For example, small changes in the output of energy from the Sun will affect this balance directly.

If all heat energy emitted from the surface passed through the atmosphere directly into space, Earth’s average surface temperature would be tens of degrees colder than today. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, act to make the surface much warmer than this because they absorb and emit heat energy in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm [Figure B1]. Without this greenhouse effect, life as we know it could not have evolved on our planet. Adding more greenhouse gases to the atmosphere makes it even more effective at preventing heat from escaping into space. When the energy leaving is less than the energy entering, Earth warms until a new balance is established.

Greenhouse gases emitted by human activities alter Earth’s energy balance and thus its climate. Humans also affect climate by changing the nature of the land surfaces (for example by clearing forests for farming) and through the emission of pollutants that affect the amount and type of particles in the atmosphere.

Scientists have determined that, when all human and natural factors are considered, Earth’s climate balance has been altered towards warming, with the biggest contributor being increases in CO 2 .

Figure b1. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, absorb heat energy and emit it in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm. Adding more greenhouse gases to the atmosphere enhances the effect, making Earth’s surface and lower atmosphere even warmer. Image based on a figure from US EPA.

Human activities have added greenhouse gases to the atmosphere

The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began. In the case of carbon dioxide, the average concentration measured at the Mauna Loa Observatory in Hawaii has risen from 316 parts per million (ppm) in 1959 (the first full year of data available) to more than 411 ppm in 2019 [Figure B2]. The same rates of increase have since been recorded at numerous other stations worldwide. Since preindustrial times, the atmospheric concentration of CO 2 has increased by over 40%, methane has increased by more than 150%, and nitrous oxide has increased by roughly 20%. More than half of the increase in CO 2 has occurred since 1970. Increases in all three gases contribute to warming of Earth, with the increase in CO 2 playing the largest role. See page B3 to learn about the sources of human emitted greenhouse gases. Learn about the sources of human emitted greenhouse gases.

Scientists have examined greenhouse gases in the context of the past. Analysis of air trapped inside ice that has been accumulating over time in Antarctica shows that the CO 2 concentration began to increase significantly in the 19th century [Figure B3], after staying in the range of 260 to 280 ppm for the previous 10,000 years. Ice core records extending back 800,000 years show that during that time, CO 2 concentrations remained within the range of 170 to 300 ppm throughout many “ice age” cycles - learn about the ice ages - and no concentration above 300 ppm is seen in ice core records until the past 200 years.

Measurements of the forms (isotopes) of carbon in the modern atmosphere show a clear fingerprint of the addition of “old” carbon (depleted in natural radioactive 14 C) coming from the combustion of fossil fuels (as opposed to “newer” carbon coming from living systems). In addition, it is known that human activities (excluding land use changes) currently emit an estimated 10 billion tonnes of carbon each year, mostly by burning fossil fuels, which is more than enough to explain the observed increase in concentration. These and other lines of evidence point conclusively to the fact that the elevated CO 2 concentration in our atmosphere is the result of human activities.

Fig b2. Measurements of atmospheric CO 2 since 1958 from the Mauna Loa Observatory in Hawaii (black) and from the South Pole (red) show a steady annual increase in atmospheric CO 2 concentration. The measurements are made at remote places like these because they are not greatly influenced by local processes, so therefore they are representative of the background atmosphere. The small up-and-down saw-tooth pattern reflects seasonal changes in the release and uptake of CO 2 by plants. Source: Scripps CO2 Program

Figure b3. CO 2 variations during the past 1,000 years, obtained from analysis of air trapped in an ice core extracted from Antarctica (red squares), show a sharp rise in atmospheric CO 2 starting in the late 19th century. Modern atmospheric measurements from Mauna Loa are superimposed in gray. Source: figure by Eric Wolff, data from Etheridge et al., 1996; MacFarling Meure et al., 2006; Scripps CO 2 Program.

Climate records show a warming trend

Estimating global average surface air temperature increase requires careful analysis of millions of measurements from around the world, including from land stations, ships, and satellites. Despite the many complications of synthesising such data, multiple independent teams have concluded separately and unanimously that global average surface air temperature has risen by about 1 °C (1.8 °F) since 1900 [Figure B4]. Although the record shows several pauses and accelerations in the increasing trend, each of the last four decades has been warmer than any other decade in the instrumental record since 1850.

Going further back in time before accurate thermometers were widely available, temperatures can be reconstructed using climate-sensitive indicators “proxies” in materials such as tree rings, ice cores, and marine sediments. Comparisons of the thermometer record with these proxy measurements suggest that the time since the early 1980s has been the warmest 40-year period in at least eight centuries, and that global temperature is rising towards peak temperatures last seen 5,000 to 10,000 years ago in the warmest part of our current interglacial period.

Many other impacts associated with the warming trend have become evident in recent years. Arctic summer sea ice cover has shrunk dramatically. The heat content of the ocean has increased. Global average sea level has risen by approximately 16 cm (6 inches) since 1901, due both to the expansion of warmer ocean water and to the addition of melt waters from glaciers and ice sheets on land. Warming and precipitation changes are altering the geographical ranges of many plant and animal species and the timing of their life cycles. In addition to the effects on climate, some of the excess CO 2 in the atmosphere is being taken up by the ocean, changing its chemical composition (causing ocean acidification).

Figure b4. Earth’s global average surface temperature has risen, as shown in this plot of combined land and ocean measurements from 1850 to 2019 derived from three independent analyses of the available data sets. The top panel shows annual average values from the three analyses, and the bottom panel shows decadal average values, including the uncertainty range (grey bars) for the maroon (HadCRUT4) dataset. The temperature changes are relative to the global average surface temperature, averaged from 1961−1990. Source: Based on IPCC AR5, data from the HadCRUT4 dataset (black), NOAA Climate.gov; data from UK Met Office Hadley Centre (maroon), US National Aeronautics and Space Administration Goddard Institute for Space Studies (red), and US National Oceanic and Atmospheric Administration National Centers for Environmental Information (orange).

Many complex processes shape our climate

Based just on the physics of the amount of energy that CO 2 absorbs and emits, a doubling of atmospheric CO 2 concentration from pre-industrial levels (up to about 560 ppm) would by itself cause a global average temperature increase of about 1 °C (1.8 °F). In the overall climate system, however, things are more complex; warming leads to further effects (feedbacks) that either amplify or diminish the initial warming.

The most important feedbacks involve various forms of water. A warmer atmosphere generally contains more water vapour. Water vapour is a potent greenhouse gas, thus causing more warming; its short lifetime in the atmosphere keeps its increase largely in step with warming. Thus, water vapour is treated as an amplifier, and not a driver, of climate change. Higher temperatures in the polar regions melt sea ice and reduce seasonal snow cover, exposing a darker ocean and land surface that can absorb more heat, causing further warming. Another important but uncertain feedback concerns changes in clouds. Warming and increases in water vapour together may cause cloud cover to increase or decrease which can either amplify or dampen temperature change depending on the changes in the horizontal extent, altitude, and properties of clouds. The latest assessment of the science indicates that the overall net global effect of cloud changes is likely to be to amplify warming.

The ocean moderates climate change. The ocean is a huge heat reservoir, but it is difficult to heat its full depth because warm water tends to stay near the surface. The rate at which heat is transferred to the deep ocean is therefore slow; it varies from year to year and from decade to decade, and it helps to determine the pace of warming at the surface. Observations of the sub-surface ocean are limited prior to about 1970, but since then, warming of the upper 700 m (2,300 feet) is readily apparent, and deeper warming is also clearly observed since about 1990.

Surface temperatures and rainfall in most regions vary greatly from the global average because of geographical location, in particular latitude and continental position. Both the average values of temperature, rainfall, and their extremes (which generally have the largest impacts on natural systems and human infrastructure), are also strongly affected by local patterns of winds.

Estimating the effects of feedback processes, the pace of the warming, and regional climate change requires the use of mathematical models of the atmosphere, ocean, land, and ice (the cryosphere) built upon established laws of physics and the latest understanding of the physical, chemical and biological processes affecting climate, and run on powerful computers. Models vary in their projections of how much additional warming to expect (depending on the type of model and on assumptions used in simulating certain climate processes, particularly cloud formation and ocean mixing), but all such models agree that the overall net effect of feedbacks is to amplify warming.

Human activities are changing the climate

Rigorous analysis of all data and lines of evidence shows that most of the observed global warming over the past 50 years or so cannot be explained by natural causes and instead requires a significant role for the influence of human activities.

In order to discern the human influence on climate, scientists must consider many natural variations that affect temperature, precipitation, and other aspects of climate from local to global scale, on timescales from days to decades and longer. One natural variation is the El Niño Southern Oscillation (ENSO), an irregular alternation between warming and cooling (lasting about two to seven years) in the equatorial Pacific Ocean that causes significant year-to-year regional and global shifts in temperature and rainfall patterns. Volcanic eruptions also alter climate, in part increasing the amount of small (aerosol) particles in the stratosphere that reflect or absorb sunlight, leading to a short-term surface cooling lasting typically about two to three years. Over hundreds of thousands of years, slow, recurring variations in Earth’s orbit around the Sun, which alter the distribution of solar energy received by Earth, have been enough to trigger the ice age cycles of the past 800,000 years.

Fingerprinting is a powerful way of studying the causes of climate change. Different influences on climate lead to different patterns seen in climate records. This becomes obvious when scientists probe beyond changes in the average temperature of the planet and look more closely at geographical and temporal patterns of climate change. For example, an increase in the Sun’s energy output will lead to a very different pattern of temperature change (across Earth’s surface and vertically in the atmosphere) compared to that induced by an increase in CO 2 concentration. Observed atmospheric temperature changes show a fingerprint much closer to that of a long-term CO 2 increase than to that of a fluctuating Sun alone. Scientists routinely test whether purely natural changes in the Sun, volcanic activity, or internal climate variability could plausibly explain the patterns of change they have observed in many different aspects of the climate system. These analyses have shown that the observed climate changes of the past several decades cannot be explained just by natural factors.

How will climate change in the future?

Scientists have made major advances in the observations, theory, and modelling of Earth’s climate system, and these advances have enabled them to project future climate change with increasing confidence. Nevertheless, several major issues make it impossible to give precise estimates of how global or regional temperature trends will evolve decade by decade into the future. Firstly, we cannot predict how much CO 2 human activities will emit, as this depends on factors such as how the global economy develops and how society’s production and consumption of energy changes in the coming decades. Secondly, with current understanding of the complexities of how climate feedbacks operate, there is a range of possible outcomes, even for a particular scenario of CO 2 emissions. Finally, over timescales of a decade or so, natural variability can modulate the effects of an underlying trend in temperature. Taken together, all model projections indicate that Earth will continue to warm considerably more over the next few decades to centuries. If there were no technological or policy changes to reduce emission trends from their current trajectory, then further globally-averaged warming of 2.6 to 4.8 °C (4.7 to 8.6 °F) in addition to that which has already occurred would be expected during the 21st century [Figure B5]. Projecting what those ranges will mean for the climate experienced at any particular location is a challenging scientific problem, but estimates are continuing to improve as regional and local-scale models advance.

Figure b5. The amount and rate of warming expected for the 21st century depends on the total amount of greenhouse gases that humankind emits. Models project the temperature increase for a business-as-usual emissions scenario (in red) and aggressive emission reductions, falling close to zero 50 years from now (in blue). Black is the modelled estimate of past warming. Each solid line represents the average of different model runs using the same emissions scenario, and the shaded areas provide a measure of the spread (one standard deviation) between the temperature changes projected by the different models. All data are relative to a reference period (set to zero) of 1986-2005. Source: Based on IPCC AR5

Climate change and biodiversity

Human activities are changing the climate. Science can help us understand what we are doing to habitats and the climate, but also find solutions.

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Climate change widespread, rapid, and intensifying – ipcc.

GENEVA, Aug 9 – Scientists are observing changes in the Earth’s climate in every region and across the whole climate system, according to the latest Intergovernmental Panel on Climate Change (IPCC) Report, released today. Many of the changes observed in the climate are unprecedented in thousands, if not hundreds of thousands of years, and some of the changes already set in motion—such as continued sea level rise—are irreversible over hundreds to thousands of years.

However, strong and sustained reductions in emissions of carbon dioxide (CO 2 ) and other greenhouse gases would limit climate change. While benefits for air quality would come quickly, it could take 20-30 years to see global temperatures stabilize, according to the IPCC Working Group I report, Climate Change 2021: the Physical Science Basis , approved on Friday by 195 member governments of the IPCC, through a virtual approval session that was held over two weeks starting on July 26.

The Working Group I report is the first instalment of the IPCC’s Sixth Assessment Report (AR6), which will be completed in 2022.

“This report reflects extraordinary efforts under exceptional circumstances,” said Hoesung Lee, Chair of the IPCC. “The innovations in this report, and advances in climate science that it reflects, provide an invaluable input into climate negotiations and decision-making.”

Faster warming

The report provides new estimates of the chances of crossing the global warming level of 1.5°C in the next decades, and finds that unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming to close to 1.5°C or even 2°C will be beyond reach.

The report shows that emissions of greenhouse gases from human activities are responsible for approximately 1.1°C of warming since 1850-1900, and finds that averaged over the next 20 years, global temperature is expected to reach or exceed 1.5°C of warming. This assessment is based on improved observational datasets to assess historical warming, as well progress in scientific understanding of the response of the climate system to human-caused greenhouse gas emissions.

“This report is a reality check,” said IPCC Working Group I Co-Chair Valérie Masson-Delmotte. “We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done, and how we can prepare.”

Every region facing increasing changes

Many characteristics of climate change directly depend on the level of global warming, but what people experience is often very different to the global average. For example, warming over land is larger than the global average, and it is more than twice as high in the Arctic.

“Climate change is already affecting every region on Earth, in multiple ways. The changes we experience will increase with additional warming,” said IPCC Working Group I Co-Chair Panmao Zhai.

The report projects that in the coming decades climate changes will increase in all regions. For 1.5°C of global warming, there will be increasing heat waves, longer warm seasons and shorter cold seasons. At 2°C of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health, the report shows.

But it is not just about temperature. Climate change is bringing multiple different changes in different regions – which will all increase with further warming. These include changes to wetness and dryness, to winds, snow and ice, coastal areas and oceans. For example:

Climate change is intensifying the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions.
Climate change is affecting rainfall patterns. In high latitudes, precipitation is likely to increase, while it is projected to decrease over large parts of the subtropics. Changes to monsoon precipitation are expected, which will vary by region.
Coastal areas will see continued sea level rise throughout the 21st century, contributing to more frequent and severe coastal flooding in low-lying areas and coastal erosion. Extreme sea level events that previously occurred once in 100 years could happen every year by the end of this century.
Further warming will amplify permafrost thawing, and the loss of seasonal snow cover, melting of glaciers and ice sheets, and loss of summer Arctic sea ice.
Changes to the ocean, including warming, more frequent marine heatwaves, ocean acidification, and reduced oxygen levels have been clearly linked to human influence. These changes affect both ocean ecosystems and the people that rely on them, and they will continue throughout at least the rest of this century.
For cities, some aspects of climate change may be amplified, including heat (since urban areas are usually warmer than their surroundings), flooding from heavy precipitation events and sea level rise in coastal cities.

For the first time, the Sixth Assessment Report provides a more detailed regional assessment of climate change, including a focus on useful information that can inform risk assessment, adaptation, and other decision-making, and a new framework that helps translate physical changes in the climate – heat, cold, rain, drought, snow, wind, coastal flooding and more – into what they mean for society and ecosystems.

This regional information can be explored in detail in the newly developed Interactive Atlas interactive-atlas.ipcc.ch as well as regional fact sheets, the technical summary, and underlying report.

Human influence on the past and future climate

“It has been clear for decades that the Earth’s climate is changing, and the role of human influence on the climate system is undisputed,” said Masson-Delmotte. Yet the new report also reflects major advances in the science of attribution – understanding the role of climate change in intensifying specific weather and climate events such as extreme heat waves and heavy rainfall events.

The report also shows that human actions still have the potential to determine the future course of climate. The evidence is clear that carbon dioxide (CO 2 ) is the main driver of climate change, even as other greenhouse gases and air pollutants also affect the climate.

“Stabilizing the climate will require strong, rapid, and sustained reductions in greenhouse gas emissions, and reaching net zero CO 2 emissions. Limiting other greenhouse gases and air pollutants, especially methane, could have benefits both for health and the climate,” said Zhai.

For more information contact:

IPCC Press Office [email protected] , +41 22 730 8120

Katherine Leitzell [email protected]

Nada Caud (French) [email protected]

Notes for Editors

Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

The Working Group I report addresses the most updated physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, global and regional climate simulations. It shows how and why climate has changed to date, and the improved understanding of human influence on a wider range of climate characteristics, including extreme events. There will be a greater focus on regional information that can be used for climate risk assessments.

The Summary for Policymakers of the Working Group I contribution to the Sixth Assessment Report (AR6) as well as additional materials and information are available at https://www.ipcc.ch/report/ar6/wg1/

Note : Originally scheduled for release in April 2021, the report was delayed for several months by the COVID-19 pandemic, as work in the scientific community including the IPCC shifted online. This is first time that the IPCC has conducted a virtual approval session for one of its reports.

AR6 Working Group I in numbers

234 authors from 66 countries

31 – coordinating authors
167 – lead authors
36 – review editors
517 – contributing authors

Over 14,000 cited references

A total of 78,007 expert and government review comments

(First Order Draft 23,462; Second Order Draft 51,387; Final Government Distribution: 3,158)

More information about the Sixth Assessment Report can be found here .

About the IPCC

The Intergovernmental Panel on Climate Change (IPCC) is the UN body for assessing the science related to climate change. It was established by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) in 1988 to provide political leaders with periodic scientific assessments concerning climate change, its implications and risks, as well as to put forward adaptation and mitigation strategies. In the same year the UN General Assembly endorsed the action by the WMO and UNEP in jointly establishing the IPCC. It has 195 member states.

Thousands of people from all over the world contribute to the work of the IPCC. For the assessment reports, IPCC scientists volunteer their time to assess the thousands of scientific papers published each year to provide a comprehensive summary of what is known about the drivers of climate change, its impacts and future risks, and how adaptation and mitigation can reduce those risks.

The IPCC has three working groups: Working Group I , dealing with the physical science basis of climate change; Working Group II , dealing with impacts, adaptation and vulnerability; and Working Group III , dealing with the mitigation of climate change. It also has a Task Force on National Greenhouse Gas Inventories that develops methodologies for measuring emissions and removals. As part of the IPCC, a Task Group on Data Support for Climate Change Assessments (TG-Data) provides guidance to the Data Distribution Centre (DDC) on curation, traceability, stability, availability and transparency of data and scenarios related to the reports of the IPCC.

IPCC assessments provide governments, at all levels, with scientific information that they can use to develop climate policies. IPCC assessments are a key input into the international negotiations to tackle climate change. IPCC reports are drafted and reviewed in several stages, thus guaranteeing objectivity and transparency. An IPCC assessment report consists of the contributions of the three working groups and a Synthesis Report. The Synthesis Report integrates the findings of the three working group reports and of any special reports prepared in that assessment cycle.

About the Sixth Assessment Cycle

At its 41st Session in February 2015, the IPCC decided to produce a Sixth Assessment Report (AR6). At its 42nd Session in October 2015 it elected a new Bureau that would oversee the work on this report and the Special Reports to be produced in the assessment cycle.

Global Warming of 1.5°C , an IPCC special report on the impacts of global warming of 1.5 degrees Celsius above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty was launched in October 2018.

Climate Change and Land , an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems was launched in August 2019, and the Special Report on the Ocean and Cryosphere in a Changing Climate was released in September 2019.

In May 2019 the IPCC released the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories , an update to the methodology used by governments to estimate their greenhouse gas emissions and removals.

The other two Working Group contributions to the AR6 will be finalized in 2022 and the AR6 Synthesis Report will be completed in the second half of 2022.

For more information go to www.ipcc.ch

The website includes outreach materials including videos about the IPCC and video recordings from outreach events conducted as webinars or live-streamed events.

Most videos published by the IPCC can be found on our YouTube and Vimeo channels.

Climate Change: Causes & Effects|UPSC Notes

Climate change is a global issue that affects every living being on Earth. In recent years, discussions on climate change have intensified due to its profound impact on our environment and societies. The term climate change refers to significant and lasting changes in the average weather patterns over long periods, usually decades or longer. These changes can occur naturally, but human activities have accelerated the process, leading to more severe consequences.

Climate change impacts weather patterns.
Rising global temperatures contribute to climate change.
Human activities are a major cause of climate change.
Climate change has widespread effects on ecosystems.


Topics for UPSC Prelims	Global Warming, United Nations Framework Convention on Climate Change (UNFCCC), United Nations Framework Convention on Biological Diversity (UNCBD), Conventions and Protocols on Environment,Rio Earth Summit, Paris Agreement
Topics for UPSC Mains	Global Warming, Climate Change, United Nations Framework Convention on Climate Change (UNFCCC), United Nations Framework Convention on Biological Diversity (UNCBD), Conventions and Protocols on Environment, Rio Earth Summit, Paris Agreement

Table of Contents

What is Climate Change?

What is climate change? At its core, climate change is the long-term alteration of temperature and typical weather patterns in a place. While the Earth’s climate has always fluctuated due to natural causes, such as volcanic eruptions and solar cycles, the current trend of climate change is primarily driven by human activities. The Industrial Revolution marked the beginning of a new era where human actions, particularly the burning of fossil fuels, started contributing to the accumulation of greenhouse gases in the atmosphere.

Causes of Climate Change

The causes of climate change are both natural and anthropogenic. However, in recent times, human activities have become the dominant force behind climate change. The major causes of climate change include:

Burning Fossil Fuels

The use of coal, oil, and natural gas for electricity, transportation, and heating generates a large volume of carbon dioxide (CO2). This gas traps heat in the atmosphere, leading to global warming. Power plants, vehicles, and factories are major sources of these emissions, making fossil fuel combustion the largest contributor to climate change.

Deforestation

Forests play a crucial role in absorbing CO2 from the atmosphere. However, widespread deforestation, often for agriculture, urban development, and logging, reduces this capacity. The loss of trees means less CO2 is absorbed, leading to higher atmospheric CO2 levels. Additionally, the act of cutting and burning trees releases stored carbon, further exacerbating climate change.

Agricultural Practices

Agriculture contributes to climate changes through various activities. Livestock, such as cows and sheep, produce methane during digestion, a potent greenhouse gas. The use of nitrogen-based fertilizers leads to the release of nitrous oxide (N2O), another powerful greenhouse gas. Moreover, rice paddies, due to their waterlogged conditions, emit methane as well.

Industrial Processes

Many industries emit greenhouse gases during production processes. For example, cement manufacturing releases CO2 through the chemical transformation of limestone into lime. The steel and chemical industries also produce large amounts of CO2 and other greenhouse gases, contributing significantly to global emissions.

Waste Management

Poor waste management practices, particularly in landfills, lead to the emission of methane as organic waste decomposes. This methane, along with other greenhouse gases emitted from waste treatment and disposal, contributes to the overall greenhouse effect, accelerating climate changes.

Effects of Climate Change

The effects of climate changes are evident across the globe and manifest in various ways. Some of the most significant effects of climate changes include:

Rising Global Temperatures

Over the past century, the Earth’s average temperature has increased by about 1.2°C, primarily due to human activities. This rise leads to more frequent and prolonged heatwaves, which can cause droughts, wildfires, and severe impacts on agriculture and water supply. The warming also affects global climate patterns, such as the jet stream, leading to unpredictable and extreme weather.

Melting Ice Caps and Glaciers

The Arctic and Antarctic regions are experiencing rapid ice loss due to warming temperatures. Glaciers in mountainous regions are also retreating, contributing to rising sea levels. This melting disrupts habitats for polar species, reduces freshwater availability in some regions, and contributes to more significant global sea-level rise.

Sea-Level Rise

Sea levels have risen by about 20 centimeters (8 inches) since 1900, with the rate of increase accelerating in recent decades. This rise, driven by melting ice and the thermal expansion of seawater, leads to coastal erosion, inundation of low-lying areas, and increased salinity of freshwater aquifers. Coastal cities and small island nations are particularly vulnerable, facing threats to infrastructure, housing, and freshwater resources.

Extreme Weather Events

Climate change is making extreme weather events more frequent and severe. Hurricanes and typhoons are becoming more intense, with stronger winds and heavier rainfall, leading to catastrophic flooding and damage. Prolonged droughts are affecting regions like Sub-Saharan Africa and the western United States, while unseasonal and intense rainfall leads to floods in other parts of the world. These events disrupt economies, displace populations, and cause significant loss of life.

Disruption of Ecosystems

As temperatures rise, many species are forced to migrate to cooler areas, but not all species can adapt quickly enough. This shift leads to changes in species distribution, with some facing extinction. Coral reefs, for instance, are experiencing bleaching events due to warmer ocean temperatures, threatening marine biodiversity. Forests, wetlands, and other ecosystems are also at risk, which in turn affects the species that depend on them and disrupts ecological balance.

Impact on Agriculture

Climate change is altering growing seasons and reducing the predictability of weather patterns, which is crucial for agriculture. Crops are sensitive to temperature changes, and extreme weather events, such as droughts and floods, can devastate harvests. In regions dependent on rain-fed agriculture, changes in precipitation patterns are threatening food security. Additionally, increased CO2 levels can reduce the nutritional value of some crops, exacerbating malnutrition in vulnerable populations.

Human Health Risks

The health impacts of climate changes are profound and multifaceted. Heatwaves increase the risk of heatstroke and other heat-related illnesses, particularly in the elderly and those with preexisting conditions. Climate changes is also expanding the range of vector-borne diseases, such as malaria, dengue, and Zika virus, as warmer temperatures allow mosquitoes and other vectors to thrive in previously unsuitable areas.

Strategies to Mitigate Climate Change

To combat climate changes, it is essential to adopt and implement effective strategies that reduce greenhouse gas emissions and enhance the resilience of communities and ecosystems. Key strategies to mitigate climate changes include:

Transitioning to Renewable Energy: Shifting from fossil fuels to renewable energy sources such as solar, wind, and hydropower can significantly reduce carbon emissions.
Reforestation and Afforestation: Planting trees and restoring forests can help absorb CO2 from the atmosphere, acting as a natural climate solution.
Improving Energy Efficiency: Enhancing energy efficiency in buildings, transportation, and industry reduces the overall energy demand, leading to lower emissions.
Promoting Sustainable Agriculture: Adopting sustainable farming practices, such as organic farming and agroforestry, can reduce greenhouse gas emissions from the agricultural sector.
Reducing Waste: Implementing waste reduction strategies, such as recycling and composting, can decrease methane emissions from landfills.
Supporting Climate Policies: Governments play a crucial role in enacting policies that limit emissions, promote clean energy, and support climate adaptation initiatives.
Encouraging Individual Action: Individuals can contribute by reducing their carbon footprint, adopting sustainable lifestyles, and advocating for climate action.

Climate change is one of the most pressing issues of our time. Understanding climate changes and its causes is the first step towards addressing it. The effects of climate changes are already being felt worldwide, and without decisive action, the situation will only worsen. However, by implementing strategies to mitigate climate changes, we can reduce its impact and protect the planet for future generations. The time to act is now, and every individual, community, and nation must play their part in this global effort.

1. Climate change refers to significant and lasting changes in global weather patterns, primarily driven by human activities.
2. Burning fossil fuels, deforestation, and industrial processes are major causes of climate change due to their contribution to greenhouse gas emissions.
3. Rising global temperatures due to climate change lead to extreme weather events, melting ice caps, and rising sea levels.
4. Climate change disrupts ecosystems, forcing species migration, altering habitats, and increasing the risk of extinction.
5. Agricultural productivity is severely affected by climate change, leading to food security threats due to altered growing seasons and extreme weather.
6. Human health risks increase with climate change, including heat-related illnesses, the spread of vector-borne diseases, and respiratory issues.
7. Effective mitigation strategies include transitioning to renewable energy, improving energy efficiency, and supporting sustainable agricultural practices.
8. Global cooperation, such as the Paris Agreement, is essential to combat climate change and limit global warming to below 2°C.

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What Is Climate Change?

Climate change refers to long-term shifts in temperatures and weather patterns. Such shifts can be natural, due to changes in the sun’s activity or large volcanic eruptions. But since the 1800s, human activities have been the main driver of climate change , primarily due to the burning of fossil fuels like coal, oil and gas.

Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures.

The main greenhouse gases that are causing climate change include carbon dioxide and methane. These come from using gasoline for driving a car or coal for heating a building, for example. Clearing land and cutting down forests can also release carbon dioxide. Agriculture, oil and gas operations are major sources of methane emissions. Energy, industry, transport, buildings, agriculture and land use are among the main sectors causing greenhouse gases.

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Humans are responsible for global warming

Climate scientists have showed that humans are responsible for virtually all global heating over the last 200 years. Human activities like the ones mentioned above are causing greenhouse gases that are warming the world faster than at any time in at least the last two thousand years.

The average temperature of the Earth’s surface is now about 1.2°C warmer than it was in the late 1800s (before the industrial revolution) and warmer than at any time in the last 100,000 years. The last decade (2011-2020) was the warmest on record , and each of the last four decades has been warmer than any previous decade since 1850.

Many people think climate change mainly means warmer temperatures. But temperature rise is only the beginning of the story. Because the Earth is a system, where everything is connected, changes in one area can influence changes in all others.

The consequences of climate change now include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.

People are experiencing climate change in diverse ways

Climate change can affect our health , ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where whole communities have had to relocate, and protracted droughts are putting people at risk of famine. In the future, the number of people displaced by weather-related events is expected to rise.

Every increase in global warming matters

In a series of UN reports , thousands of scientists and government reviewers agreed that limiting global temperature rise to no more than 1.5°C would help us avoid the worst climate impacts and maintain a livable climate. Yet policies currently in place point to a 3°C temperature rise by the end of the century.

The emissions that cause climate change come from every part of the world and affect everyone, but some countries produce much more than others .The seven biggest emitters alone (China, the United States of America, India, the European Union, Indonesia, the Russian Federation, and Brazil) accounted for about half of all global greenhouse gas emissions in 2020.

Everyone must take climate action, but people and countries creating more of the problem have a greater responsibility to act first.

Photocomposition: an image of the world globe looking worried to a thermometer with raising temperatures

We face a huge challenge but already know many solutions

Many climate change solutions can deliver economic benefits while improving our lives and protecting the environment. We also have global frameworks and agreements to guide progress, such as the Sustainable Development Goals , the UN Framework Convention on Climate Change and the Paris Agreement . Three broad categories of action are: cutting emissions, adapting to climate impacts and financing required adjustments.

Switching energy systems from fossil fuels to renewables like solar or wind will reduce the emissions driving climate change. But we have to act now. While a growing number of countries is committing to net zero emissions by 2050, emissions must be cut in half by 2030 to keep warming below 1.5°C. Achieving this means huge declines in the use of coal, oil and gas: over two-thirds of today’s proven reserves of fossil fuels need to be kept in the ground by 2050 in order to prevent catastrophic levels of climate change.

Adapting to climate consequences protects people, homes, businesses, livelihoods, infrastructure and natural ecosystems. It covers current impacts and those likely in the future. Adaptation will be required everywhere, but must be prioritized now for the most vulnerable people with the fewest resources to cope with climate hazards. The rate of return can be high. Early warning systems for disasters, for instance, save lives and property, and can deliver benefits up to 10 times the initial cost.

We can pay the bill now, or pay dearly in the future

Climate action requires significant financial investments by governments and businesses. But climate inaction is vastly more expensive. One critical step is for industrialized countries to fulfil their commitment to provide $100 billion a year to developing countries so they can adapt and move towards greener economies.

To get familiar with some of the more technical terms used in connection with climate change, consult the Climate Dictionary .

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The facts on climate and energy

Climate change is a hot topic – with myths and falsehoods circulating widely. Find some essential facts here .

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See the latest climate reports from the United Nations as well as climate action facts .

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Fossil fuels are by far the largest contributor to the greenhouse gas emissions that cause climate change, which poses many risks to all forms of life on Earth. Learn more .

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Read the UN Chief’s latest statements on climate action.

What is net zero? Why is it important? Our net-zero page explains why we need steep emissions cuts now and what efforts are underway.

Renewable energy – powering a safer future

What is renewable energy and why does it matter? Learn more about why the shift to renewables is our only hope for a brighter and safer world.

How will the world foot the bill? We explain the issues and the value of financing climate action.

What is climate adaptation? Why is it so important for every country? Find out how we can protect lives and livelihoods as the climate changes.

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The Causes of Climate Change
The Causes of Climate Change
What Causes Climate Change? Human and Natural Causes
Natural causes of climate change. Some amount of climate change can be attributed to natural phenomena. Over the course of Earth's existence, volcanic eruptions, fluctuations in solar radiation ...
Causes and Effects of Climate Change Essay
There are two main causes of climate changes - natural causes and human activities. Natural causes have influenced the earth's climates such as volcanic eruptions, ocean current, the earth's orbital changes and solar variations. The eruptions of volcanoes cause a cooling effect on the earth.
Causes and Effects of Climate Change
As greenhouse gas emissions blanket the Earth, they trap the sun's heat. This leads to global warming and climate change. The world is now warming faster than at any point in recorded history ...
Causes of climate change
Causes of climate change
Causes of Climate Change
Causes of Climate Change | US EPA
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What Is Climate Change?
PDF Climate Change: Evidence & Causes 2020
It is now more certain. than ever, based on many lines of evidence, that humans are changing Earth's climate. The atmosphere and oceans have warmed, which has been accompanied by sea level rise, a strong decline in Arctic sea ice, and other climate-related changes. The impacts of climate change on people and nature are increasingly apparent.
What Is Climate Change?
Climate change is a long-term change in the average weather patterns that have come to define Earth's local, regional and global climates. These changes have a broad range of observed effects that are synonymous with the term. Changes observed in Earth's climate since the mid-20th century are driven by human activities, particularly fossil ...
Global warming
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Causes of Climate Change
Causes of Climate Change. Climate change, periodic modification of Earth 's climate brought about as a result of changes in the atmosphere as well as interactions between the atmosphere and various other geologic, chemical, biological, and geographic factors within the Earth system. The atmosphere is a dynamic fluid that is continually in motion.
Evidence & Causes of Climate Change
Climate change in 60 seconds | The Royal Society. Climate change • Climate change refers to long-term shifts in temperatures and weather patterns. Human activities have been the main driver of climate change, primarily due to the burning of fossil fuels like coal, oil and gas. Watch on.
Climate Change: Your Questions About Causes and Effects, Answered
By Christopher Flavelle. Climate change threatens Americans' access to clean drinking water in a number of ways. The most obvious is drought: Rising temperatures are reducing the snowpack that ...
What evidence exists that Earth is warming and that humans are the main
Full story. We know this warming is largely caused by human activities because the key role that carbon dioxide plays in maintaining Earth's natural greenhouse effect has been understood since the mid-1800s. Unless it is offset by some equally large cooling influence, more atmospheric carbon dioxide will lead to warmer surface temperatures.
Causes of Climate Change
Are humans causing or contributing to global warming? Climate Change & Global Warming October 29, 2020. Yes, human activity is putting carbon dioxide into the atmosphere faster than natural processes take it out. Rising carbon dioxide levels are strengthening Earth's greenhouse effect and causing global warming.
Climate change is a crisis. How did we get here?
Climate change is a crisis. How did we get here?
Causes and Effects of Climate Change
Changing weather patterns are expanding diseases, and extreme weather events increase deaths and make it difficult for health care systems to keep up. Climate change increases the factors that put ...
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The scientific consensus. Within the scientific community, there is essentially no disagreement on the causes of climate change. Multiple studies have shown that at least 97 percent of scientists agree that global warming is happening and that human activity is the primary cause. Major scientific assessments also agree.
Climate change: evidence and causes
Climate change in 60 seconds | The Royal Society. Climate change • Climate change refers to long-term shifts in temperatures and weather patterns. Human activities have been the main driver of climate change, primarily due to the burning of fossil fuels like coal, oil and gas. Watch on.
Climate change widespread, rapid, and intensifying
The evidence is clear that carbon dioxide (CO 2) is the main driver of climate change, even as other greenhouse gases and air pollutants also affect the climate. "Stabilizing the climate will require strong, rapid, and sustained reductions in greenhouse gas emissions, and reaching net zero CO 2 emissions.
Climate Change: Causes & Effects|UPSC Notes
Climate Change UPSC Notes : 1. Climate change refers to significant and lasting changes in global weather patterns, primarily driven by human activities. 2. Burning fossil fuels, deforestation, and industrial processes are major causes of climate change due to their contribution to greenhouse gas emissions. 3.
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