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Chapter 1: Our Globally Changing Climate

Key findings, key finding 1.

The global climate continues to change rapidly compared to the pace of the natural variations in climate that have occurred throughout Earth’s history. Trends in globally averaged temperature, sea level rise, upper-ocean heat content, land-based ice melt, arctic sea ice, depth of seasonal permafrost thaw, and other climate variables provide consistent evidence of a warming planet. These observed trends are robust and have been confirmed by multiple independent research groups around the world. ( Very high confidence )

Key Finding 2

The frequency and intensity of extreme heat and heavy precipitation events are increasing in most continental regions of the world ( very high confidence ). These trends are consistent with expected physical responses to a warming climate. Climate model studies are also consistent with these trends, although models tend to underestimate the observed trends, especially for the increase in extreme precipitation events ( very high confidence for temperature, high confidence for extreme precipitation). The frequency and intensity of extreme high temperature events are virtually certain to increase in the future as global temperature increases ( high confidence ). Extreme precipitation events will very likely continue to increase in frequency and intensity throughout most of the world ( high confidence ). Observed and projected trends for some other types of extreme events, such as floods, droughts, and severe storms, have more variable regional characteristics.

Key Finding 3

Many lines of evidence demonstrate that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Formal detection and attribution studies for the period 1951 to 2010 find that the observed global mean surface temperature warming lies in the middle of the range of likely human contributions to warming over that same period. We find no convincing evidence that natural variability can account for the amount of global warming observed over the industrial era. For the period extending over the last century, there are no convincing alternative explanations supported by the extent of the observational evidence. Solar output changes and internal variability can only contribute marginally to the observed changes in climate over the last century, and we find no convincing evidence for natural cycles in the observational record that could explain the observed changes in climate. ( Very high confidence )

Key Finding 4

Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse (heat-trapping) gases emitted globally and on the remaining uncertainty in the sensitivity of Earth’s climate to those emissions ( very high confidence ). With significant reductions in the emissions of greenhouse gases, the global annually averaged temperature rise could be limited to 3.6°F (2°C) or less. Without major reductions in these emissions, the increase in annual average global temperatures relative to preindustrial times could reach 9°F (5°C) or more by the end of this century ( high confidence ).

Key Finding 5

Natural variability, including El Niño events and other recurring patterns of ocean–atmosphere interactions, impact temperature and precipitation, especially regionally, over months to years. The global influence of natural variability, however, is limited to a small fraction of observed climate trends over decades. ( Very high confidence )

Key Finding 6

Longer-term climate records over past centuries and millennia indicate that average temperatures in recent decades over much of the world have been much higher, and have risen faster during this time period, than at any time in the past 1,700 years or more, the time period for which the global distribution of surface temperatures can be reconstructed. ( High confidence )

Confidence Level

Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Guide to this Report for more on assessments of likelihood and confidence.

1.1: Introduction

<b>Wuebbles</b>, D.J., D.R. Easterling, K. Hayhoe, T. Knutson, R.E. Kopp, J.P. Kossin, K.E. Kunkel, A.N. LeGrande, C. Mears, W.V. Sweet, P.C. Taylor, R.S. Vose, and M.F. Wehner, 2017: Our globally changing climate. In: <i>Climate Science Special Report: Fourth National Climate Assessment, Volume I</i> [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 35-72, doi: <a href="http://doi.org/10.7930/J08S4N35">10.7930/J08S4N35</a>.

Since the Third U.S. National Climate Assessment (NCA3) was published in May 2014, new observations along multiple lines of evidence have strengthened the conclusion that Earth’s climate is changing at a pace and in a pattern not explainable by natural influences. While this report focuses especially on observed and projected future changes for the United States, it is important to understand those changes in the global context (this chapter).

The world has warmed over the last 150 years, especially over the last six decades, and that warming has triggered many other changes to Earth’s climate. Evidence for a changing climate abounds, from the top of the atmosphere to the depths of the oceans. Thousands of studies conducted by tens of thousands of scientists around the world have documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers; disappearing snow cover; shrinking sea ice; rising sea level; and an increase in atmospheric water vapor. Rainfall patterns and storms are changing, and the occurrence of droughts is shifting.

Many lines of evidence demonstrate that human activities, especially emissions of greenhouse gases, are primarily responsible for the observed climate changes in the industrial era, especially over the last six decades (see attribution analysis in Ch. 3: Detection and Attribution ). Formal detection and attribution studies for the period 1951 to 2010 find that the observed global mean surface temperature warming lies in the middle of the range of likely human contributions to warming over that same period. The Intergovernmental Panel on Climate Change concluded that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. 1 Over the last century, there are no alternative explanations supported by the evidence that are either credible or that can contribute more than marginally to the observed patterns. There is no convincing evidence that natural variability can account for the amount of and the pattern of global warming observed over the industrial era. 2 , 3 , 4 , 5 Solar flux variations over the last six decades have been too small to explain the observed changes in climate. 6 , 7 , 8 There are no apparent natural cycles in the observational record that can explain the recent changes in climate (e.g., PAGES 2k Consortium 2013; 9 Marcott et al. 2013; 10 Otto-Bliesner et al. 2016 11 ). In addition, natural cycles within Earth’s climate system can only redistribute heat; they cannot be responsible for the observed increase in the overall heat content of the climate system. 12 Any explanations for the observed changes in climate must be grounded in understood physical mechanisms, appropriate in scale, and consistent in timing and direction with the long-term observed trends. Known human activities quite reasonably explain what has happened without the need for other factors. Internal variability and forcing factors other than human activities cannot explain what is happening, and there are no suggested factors, even speculative ones, that can explain the timing or magnitude and that would somehow cancel out the role of human factors. 3 , 13 The science underlying this evidence, along with the observed and projected changes in climate, is discussed in later chapters, starting with the basis for a human influence on climate in Chapter 2: Physical Drivers of Climate Change .

Throughout this report, we also analyze projections of future changes in climate. As discussed in Chapter 4 , beyond the next few decades, the magnitude of climate change depends primarily on cumulative emissions of greenhouse gases and aerosols and the sensitivity of the climate system to those emissions. Predicting how climate will change in future decades is a different scientific issue from predicting weather a few weeks from now. Local weather is short term, with limited predictability, and is determined by the complicated movement and interaction of high pressure and low pressure systems in the atmosphere; thus, it is difficult to forecast day-to-day changes beyond about two weeks into the future. Climate, on the other hand, is the statistics of weather—meaning not just average values but also the prevalence and intensity of extremes—as observed over a period of decades. Climate emerges from the interaction, over time, of rapidly changing local weather and more slowly changing regional and global influences, such as the distribution of heat in the oceans, the amount of energy reaching Earth from the sun, and the composition of the atmosphere. See Chapter 4: Projections and later chapters for more on climate projections.

Throughout this report, we include many findings that further strengthen or add to the understanding of climate change relative to those found in NCA3 and other assessments of the science. Several of these are highlighted in an “Advances Since NCA3” box at the end of this chapter.

1.2: Indicators of a Globally Changing Climate

Highly diverse types of direct measurements made on land, sea, and in the atmosphere over many decades have allowed scientists to conclude with high confidence that global mean temperature is increasing. Observational datasets for many other climate variables support the conclusion with high confidence that the global climate is changing (also see EPA 2016 14 ). 15 , 16 Figure 1.1 depicts several of the observational indicators that demonstrate trends consistent with a warming planet over the last century. Temperatures in the lower atmosphere and ocean have increased, as have near-surface humidity and sea level. Not only has ocean heat content increased dramatically (Figure 1.1), but more than 90% of the energy gained in the combined ocean–atmosphere system over recent decades has gone into the ocean. 17 , 18 Five different observational datasets show the heat content of the oceans is increasing.

Basic physics tells us that a warmer atmosphere can hold more water vapor; this is exactly what is measured from satellite data. At the same time, a warmer world means higher evaporation rates and major changes to the hydrological cycle (e.g., Kundzewicz 2008; 19 IPCC 2013 1 ), including increases in the prevalence of torrential downpours. In addition, arctic sea ice, mountain glaciers, and Northern Hemisphere spring snow cover have all decreased. The relatively small increase in Antarctic sea ice in the 15-year period from 2000 through early 2016 appears to be best explained as being due to localized natural variability (see e.g., Meehl et al. 2016a; 16 Ramsayer 2014 20 ); while possibly also related to natural variability, the 2017 Antarctic sea ice minimum reached in early March was the lowest measured since reliable records began in 1979. The vast majority of the glaciers in the world are losing mass at significant rates. The two largest ice sheets on our planet—on the land masses of Greenland and Antarctica—are shrinking.

Many other indicators of the changing climate have been determined from other observations—for example, changes in the growing season and the allergy season (see e.g., EPA 2016; 14 USGCRP 2017 21 ). In general, the indicators demonstrate continuing changes in climate since the publication of NCA3. As with temperature, independent researchers have analyzed each of these indicators and come to the same conclusion: all of these changes paint a consistent and compelling picture of a warming planet.

This image shows observations globally from nine different variables that are key indicators of a warming climate. The indicators (listed below) all show long-term trends that are consistent with global warming. In parentheses are the number of datasets shown in each graph, the length of time covered by the combined datasets and their anomaly reference period (where applicable), and the direction of the trend: land surface air temperature (4 datasets, 1850–2016 relative to 1976–2005, increase); sea surface temperature (3 datasets, 1850–2016 relative to 1976–2005, increase); sea level (4 datasets, 1880–2014 relative to 1996–2005, increase); tropospheric temperature (5 datasets, 1958–2016 relative to 1981–2005, increase); ocean heat content, upper 700m (5 datasets, 1950–2016 relative to 1996–2005, increase); specific humidity (4 datasets, 1973–2016 relative to 1980–2003, increase); Northern Hemisphere snow cover, March–April and annual (1 dataset, 1967–2016 relative to 1976–2005, decrease); arctic sea ice extent, September and annual (1 dataset, 1979–2016, decrease); glacier cumulative mass balance (1 dataset, 1980–2016, decrease). More information on the datasets can be found in the accompanying metadata. (Figure source: NOAA NCEI and CICS-NC, updated from Melillo et al. 2014; 144 Blunden and Arndt 2016 15 ).

1.3: Trends in Global Temperatures

Global annual average temperature (as calculated from instrumental records over both land and oceans; used interchangeably with global average temperature in the discussion below) has increased by more than 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960 (Figure 1.2); see Vose et al. 22 for discussion on how global annual average temperature is derived by scientists. The linear regression change over the entire period from 1901–2016 is 1.8°F (1.0°C). Global average temperature is not expected to increase smoothly over time in response to the human warming influences, because the warming trend is superimposed on natural variability associated with, for example, the El Niño/La Niña ocean-heat oscillations and the cooling effects of particles emitted by volcanic eruptions. Even so, 16 of the 17 warmest years in the instrumental record (since the late 1800s) occurred in the period from 2001 to 2016 (1998 was the exception). Global average temperature for 2016 has now surpassed 2015 by a small amount as the warmest year on record. The year 2015 far surpassed 2014 by 0.29°F (0.16°C), four times greater than the difference between 2014 and the next warmest year, 2010. 23 Three of the four warmest years on record have occurred since the analyses through 2012 were reported in NCA3.

A strong El Niño contributed to 2015’s record warmth. 15 Though an even more powerful El Niño occurred in 1998, the global temperature in that year was significantly lower (by 0.49°F [0.27°C]) than that in 2015. This suggests that human-induced warming now has a stronger influence on the occurrence of record temperatures than El Niño events. In addition, the El Niño/La Niña cycle may itself be affected by the human influence on Earth’s climate system. 3 , 24 It is the complex interaction of natural sources of variability with the continuously growing human warming influence that is now shaping Earth’s weather and, as a result, its climate.

Globally, the persistence of the warming over the past 60 years far exceeds what can be accounted for by natural variability alone. 1 That does not mean, of course, that natural sources of variability have become insignificant. They can be expected to continue to contribute a degree of “bumpiness” in the year-to-year global average temperature trajectory, as well as exert influences on the average rate of warming that can last a decade or more (see Box 1.1). 25 , 26 , 27

Top: Global annual average temperatures (as measured over both land and oceans) for 1880–2016 relative to the reference period of 1901–1960; red bars indicate temperatures above the average over 1901–1960, and blue bars indicate temperatures below the average. Global annual average temperature has increased by more than 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960. While there is a clear long-term global warming trend, some years do not show a temperature increase relative to the previous year, and some years show greater changes than others. These year-to-year fluctuations in temperature are mainly due to natural sources of variability, such as the effects of El Niños, La Niñas, and volcanic eruptions. Based on the NCEI (NOAAGlobalTemp) dataset (updated from Vose et al. 22 ) Bottom: Global average temperature averaged over decadal periods (1886–1895, 1896–1905, …, 1996–2005, except for the 11 years in the last period, 2006–2016). Horizontal label indicates midpoint year of decadal period. Every decade since 1966–1975 has been warmer than the previous decade. (Figure source: [top] adapted from NCEI 2016, 23 [bottom] NOAA NCEI and CICS-NC).

Warming during the first half of the 1900s occurred mostly in the Northern Hemisphere. 28 Recent decades have seen greater warming in response to accelerating increases in greenhouse gas concentrations, particularly at high northern latitudes, and over land as compared to the ocean (see Figure 1.3). In general, winter is warming faster than summer (especially in northern latitudes). Also, nights are warming faster than days. 29 , 30 There is also some evidence of faster warming at higher elevations. 31

Most ocean areas around Earth are warming (see Ch. 13: Ocean Changes ). Even in the absence of significant ice melt, the ocean is expected to warm more slowly given its larger heat capacity, leading to land–ocean differences in warming (as seen in Figure 1.3). As a result, the climate for land areas often responds more rapidly than the ocean areas, even though the forcing driving a change in climate occurs equally over land and the oceans. 1 A few regions, such as the North Atlantic Ocean, have experienced cooling over the last century, though these areas have warmed over recent decades. Regional climate variability is important to determining potential effects of climate change on the ocean circulation (e.g., Hurrell and Deser 2009; 32 Hoegh-Guldberg et al. 2014 33 ) as are the effects of the increasing freshwater in the North Atlantic from melting of sea and land ice. 34

Surface temperature change (in °F) for the period 1986–2015 relative to 1901–1960 from the NOAA National Centers for Environmental Information’s (NCEI) surface temperature product. For visual clarity, statistical significance is not depicted on this map. Changes are generally significant (at the 90% level) over most land and ocean areas. Changes are not significant in parts of the North Atlantic Ocean, the South Pacific Ocean, and the southeastern United States. There is insufficient data in the Arctic Ocean and Antarctica for computing long-term changes (those sections are shown in gray because no trend can be derived). The relatively coarse resolution (5.0° × 5.0°) of these maps does not capture the finer details associated with mountains, coastlines, and other small-scale effects (see Ch. 6: Temperature Changes for a focus on the United States). (Figure source: updated from Vose et al. 2012 22 ).

Multimodel simulated time series from 1900 to 2100 for the change in global annual mean surface temperature relative to 1901–1960 for a range of the Representative Concentration Pathways (RCPs; see Ch. 4: Projections for more information ). These scenarios account for the uncertainty in future emissions from human activities (as analyzed with the 20+ models from around the world used in the most recent international assessment 1 ). The mean (solid lines) and associated uncertainties (shading, showing ±2 standard deviations [5%–95%] across the distribution of individual models based on the average over 2081–2100) are given for all of the RCP scenarios as colored vertical bars. The numbers of models used to calculate the multimodel means are indicated. (Figure source: adapted from Walsh et al. 2014 201 ).

Figure 1.4 shows the projected changes in globally averaged temperature for a range of future pathways that vary from assuming strong continued dependence on fossil fuels in energy and transportation systems over the 21st century (the high scenario is Representative Concentration Pathway 8.5, or RCP8.5) to assuming major emissions reduction (the even lower scenario, RCP2.6). Chapter 4: Projections describes the future scenarios and the models of Earth’s climate system being used to quantify the impact of human choices and natural variability on future climate. These analyses also suggest that global surface temperature increases for the end of the 21st century are very likely to exceed 1.5°C (2.7°F) relative to the 1850–1900 average for all projections, with the exception of the lowest part of the uncertainty range for RCP2.6. 1 , 35 , 36 , 37

1.4: Trends in Global Precipitation

Annual averaged precipitation across global land areas exhibits a slight rise (that is not statistically significant because of a lack of data coverage early in the record) over the past century (see Figure 1.7) along with ongoing increases in atmospheric moisture levels. Interannual and interdecadal variability is clearly found in all precipitation evaluations, owing to factors such as the North Atlantic Oscillation (NAO) and ENSO—note that precipitation reconstructions are updated operationally by NOAA NCEI on a monthly basis. 57 , 58

Surface annually averaged precipitation change (in inches) for the period 1986–2015 relative to 1901–1960. The data is from long-term stations, so precipitation changes over the ocean and Antarctica cannot be evaluated. The trends are not considered to be statistically significant because of a lack of data coverage early in the record. The relatively coarse resolution (0.5° × 0.5°) of these maps does not capture the finer details associated with mountains, coastlines, and other small-scale effects. (Figure source: NOAA NCEI and CICS-NC).

The hydrological cycle and the amount of global mean precipitation is primarily controlled by the atmosphere’s energy budget and its interactions with clouds. 59 The amount of global mean precipitation also changes as a result of a mix of fast and slow atmospheric responses to the changing climate. 60 In the long term, increases in tropospheric radiative effects from increasing amounts of atmospheric CO 2 (i.e., increasing CO 2 leads to greater energy absorbed by the atmosphere and re-emitted to the surface, with the additional transport to the atmosphere coming by convection) must be balanced by increased latent heating, resulting in precipitation increases of approximately 0.55% to 0.72% per °F (1% to 3% per °C). 1 , 61 Global atmospheric water vapor should increase by about 6%–7% per °C of warming based on the Clausius–Clapeyron relationship (see Ch. 2: Physical Drivers of Climate Change ); satellite observations of changes in precipitable water over oceans have been detected at about this rate and attributed to human-caused changes in the atmosphere. 62 Similar observed changes in land-based measurements have also been attributed to the changes in climate from greenhouse gases. 63

Earlier studies suggested a climate change pattern of wet areas getting wetter and dry areas getting drier (e.g., Greve et al. 2014 64 ). While Hadley Cell expansion should lead to more drying in the subtropics, the poleward shift of storm tracks should lead to enhanced wet regions. While this high/low rainfall behavior appears to be valid over ocean areas, changes over land are more complicated. The wet versus dry pattern in observed precipitation has only been attributed for the zonal mean 65 , 66 and not regionally due to the large amount of spatial variation in precipitation changes as well as significant natural variability. The detected signal in zonal mean precipitation is largest in the Northern Hemisphere, with decreases in the subtropics and increases at high latitudes. As a result, the observed increase (about 5% since the 1950s 67 , 68 ) in annual averaged arctic precipitation have been detected and attributed to human activities. 69

1.5: Trends in Global Extreme Weather Events

A change in the frequency, duration, and/or magnitude of extreme weather events is one of the most important consequences of a warming climate. In statistical terms, a small shift in the mean of a weather variable, with or without this shift occurring in concert with a change in the shape of its probability distribution, can cause a large change in the probability of a value relative to an extreme threshold (see Figure 1.8 in IPCC 2013 1 ). 70 Examples include extreme high temperature events and heavy precipitation events. Some of the other extreme events, such as intense tropical cyclones, midlatitude cyclones, lightning, and hail and tornadoes associated with thunderstorms can occur as more isolated events and generally have more limited temporal and spatial observational datasets, making it more difficult to study their long-term trends. Detecting trends in the frequency and intensity of extreme weather events is challenging. 71 The most intense events are rare by definition, and observations may be incomplete and suffer from reporting biases. Further discussion on trends and projections of extreme events for the United States can be found in Chapters 6 – 9 and 11 .

An emerging area in the science of detection and attribution has been the attribution of extreme weather and climate events. Extreme event attribution generally addresses the question of whether climate change has altered the odds of occurrence of an extreme event like one just experienced. Attribution of extreme weather events under a changing climate is now an important and highly visible aspect of climate science. As discussed in a recent National Academy of Sciences (NAS) report, 72 the science of event attribution is rapidly advancing, including the understanding of the mechanisms that produce extreme events and the development of methods that are used for event attribution. Several other reports and papers have reviewed the topic of extreme event attribution. 73 , 74 , 75 This report briefly reviews extreme event attribution methodologies in practice ( Ch. 3: Detection and Attribution ) and provides a number of examples within the chapters on various climate phenomena (especially relating to the United States in Chapters 6 – 9 ).

Extreme Heat and Cold

The frequency of multiday heat waves and extreme high temperatures at both daytime and nighttime hours is increasing over many of the global land areas. 1 There are increasing areas of land throughout our planet experiencing an excess number of daily highs above given thresholds (for example, the 90th percentile), with an approximate doubling of the world’s land area since 1998 with 30 extreme heat days per year. 76 At the same time, frequencies of cold waves and extremely low temperatures are decreasing over the United States and much of the earth. In the United States, the number of record daily high temperatures has been about double the number of record daily low temperatures in the 2000s, 77 and much of the United States has experienced decreases of 5%–20% per decade in cold wave frequency. 1 , 75

The enhanced radiative forcing caused by greenhouse gases has a direct influence on heat extremes by shifting distributions of daily temperature. 78 Recent work indicates changes in atmospheric circulation may also play a significant role (see Ch. 5: Circulation and Variability ). For example, a recent study found that increasing anticyclonic circulations partially explain observed trends in heat events over North America and Eurasia, among other effects. 79 Observed changes in circulation may also be the result of human influences on climate, though this is still an area of active research.

Extreme Precipitation

A robust consequence of a warming climate is an increase in atmospheric water vapor, which exacerbates precipitation events under similar meteorological conditions, meaning that when rainfall occurs, the amount of rain falling in that event tends to be greater. As a result, what in the past have been considered to be extreme precipitation events are becoming more frequent. 1 , 80 , 81 , 82 On a global scale, the observational annual-maximum daily precipitation has increased by 8.5% over the last 110 years; global climate models also derive an increase in extreme precipitation globally but tend to underestimate the rate of the observed increase. 80 , 82 , 83 Extreme precipitation events are increasing in frequency globally over both wet and dry regions. 82 Although more spatially heterogeneous than heat extremes, numerous studies have found increases in precipitation extremes on many regions using a variety of methods and threshold definitions, 84 and those increases can be attributed to human-caused changes to the atmosphere. 85 , 86 Finally, extreme precipitation associated with tropical cyclones (TCs) is expected to increase in the future, 87 but current trends are not clear. 84

The impact of extreme precipitation trends on flooding globally is complex because additional factors like soil moisture and changes in land cover are important. 88 Globally, due to limited data, there is low confidence for any significant current trends in river-flooding associated with climate change, 89 but the magnitude and intensity of river flooding is projected to increase in the future. 90 More on flooding trends in the United States is in Chapter 8: Droughts, Floods, and Wildfires .

Tornadoes and Thunderstorms

Increasing air temperature and moisture increase the risk of extreme convection, and there is evidence for a global increase in severe thunderstorm conditions. 91 Strong convection, along with wind shear, represents favorable conditions for tornadoes. Thus, there is reason to expect increased tornado frequency and intensity in a warming climate. 92 Inferring current changes in tornado activity is hampered by changes in reporting standards, and trends remain highly uncertain (see Ch. 9: Extreme Storms ). 84

Winter Storms

Winter storm tracks have shifted slightly northward (by about 0.4 degrees latitude) in recent decades over the Northern Hemisphere. 93 More generally, extratropical cyclone activity is projected to change in complex ways under future climate scenarios, with increases in some regions and seasons and decreases in others. There are large model-to-model differences among CMIP5 climate models, with some models underestimating the current cyclone track density. 94 , 95

Enhanced arctic warming (arctic amplification), due in part to sea ice loss, reduces lower tropospheric meridional temperature gradients, diminishing baroclinicity (a measure of how misaligned the gradient of pressure is from the gradient of air density)—an important energy source for extratropical cyclones. At the same time, upper-level meridional temperature gradients will increase due to a warming tropical upper troposphere and a cooling high-latitude lower stratosphere. While these two effects counteract each other with respect to a projected change in midlatitude storm tracks, the simulations indicate that the magnitude of arctic amplification may modulate some aspects (e.g., jet stream position, wave extent, and blocking frequency) of the circulation in the North Atlantic region in some seasons. 96

Tropical Cyclones

Detection and attribution of trends in past tropical cyclone (TC) activity is hampered by uncertainties in the data collected prior to the satellite era and by uncertainty in the relative contributions of natural variability and anthropogenic influences. Theoretical arguments and numerical modeling simulations support an expectation that radiative forcing by greenhouse gases and anthropogenic aerosols can affect TC activity in a variety of ways, but robust formal detection and attribution for past observed changes has not yet been realized. Since the IPCC AR5, 1 there is new evidence that the locations where tropical cyclones reach their peak intensity have migrated poleward in both the Northern and Southern Hemispheres, in concert with the independently measured expansion of the tropics. 97 In the western North Pacific, this migration has substantially changed the tropical cyclone hazard exposure patterns in the region and appears to have occurred outside of the historically measured modes of regional natural variability. 98

Whether global trends in high-intensity tropical cyclones are already observable is a topic of active debate. Some research suggests positive trends, 99 , 100 but significant uncertainties remain (see Ch. 9: Extreme Storms ). 100 Other studies have suggested that aerosol pollution has masked the increase in TC intensity expected otherwise from enhanced greenhouse warming. 101 , 102

Tropical cyclone intensities are expected to increase with warming, both on average and at the high end of the scale, as the range of achievable intensities expands, so that the most intense storms will exceed the intensity of any in the historical record. 102 Some studies have projected an overall increase in tropical cyclone activity. 103 However, studies with high-resolution models are giving a different result. For example, a high-resolution dynamical downscaling study of global TC activity under the lower scenario (RCP4.5) projects an increased occurrence of the highest-intensity tropical cyclones (Saffir–Simpson Categories 4 and 5), along with a reduced overall tropical cyclone frequency, though there are considerable basin-to-basin differences. 87 Chapter 9: Extreme Storms covers more on extreme storms affecting the United States.

1.6: Global Changes in Land Processes

Changes in regional land cover have had important effects on climate, while climate change also has important effects on land cover (also see Ch. 10: Land Cover ). 1 In some cases, there are changes in land cover that are both consequences of and influences on global climate change (e.g., declines in land ice and snow cover, thawing permafrost, and insect damage to forests).

Northern Hemisphere snow cover extent has decreased, especially in spring, primarily due to earlier spring snowmelt (by about 0.2 million square miles [0.5 million square km] 104 , 105 ), and this decrease since the 1970s is at least partially driven by anthropogenic influences. 106 Snow cover reductions, especially in the Arctic region in summer, have led to reduced seasonal albedo. 107

While global-scale trends in drought are uncertain due to insufficient observations, regional trends indicate increased frequency and intensity of drought and aridification on land cover in the Mediterranean 108 , 109 and West Africa 110 , 111 and decreased frequency and intensity of droughts in central North America 112 and northwestern Australia. 110 , 111 , 113

Anthropogenic land-use changes, such as deforestation and growing cropland extent, have increased the global land surface albedo, resulting in a small cooling effect. Effects of other land-use changes, including modifications of surface roughness, latent heat flux, river runoff, and irrigation, are difficult to quantify, but may offset the direct land-use albedo changes. 114 , 115

Globally, land-use change since 1750 has been typified by deforestation, driven by the growth in intensive farming and urban development. Global land-use change is estimated to have released 190 ± 65 GtC (gigatonnes of carbon) through 2015. 116 , 117 Over the same period, cumulative fossil fuel and industrial emissions are estimated to have been 410 ± 20 GtC, yielding total anthropogenic emissions of 600 ± 70 GtC, of which cumulative land-use change emissions were about 32%. 116 , 117 Tropical deforestation is the dominant driver of land-use change emissions, estimated at 0.1–1.7 GtC per year, primarily from biomass burning. Global deforestation emissions of about 3 GtC per year are compensated by around 2 GtC per year of forest regrowth in some regions, mainly from abandoned agricultural land. 118 , 119

Natural terrestrial ecosystems are gaining carbon through uptake of CO 2 by enhanced photosynthesis due to higher CO 2 levels, increased nitrogen deposition, and longer growing seasons in mid- and high latitudes. Anthropogenic atmospheric CO 2 absorbed by land ecosystems is stored as organic matter in live biomass (leaves, stems, and roots), dead biomass (litter and woody debris), and soil carbon.

Many studies have documented a lengthening growing season, primarily due to the changing climate, 120 , 121 , 122 , 123 and elevated CO 2 is expected to further lengthen the growing season in places where the length is water limited. 124 In addition, a recent study has shown an overall increase in greening of Earth in vegetated regions, 125 while another has demonstrated evidence that the greening of Northern Hemisphere extratropical vegetation is attributable to anthropogenic forcings, particularly rising atmospheric greenhouse gas levels. 126 However, observations 127 , 128 , 129 and models 130 , 131 , 132 indicate that nutrient limitations and land availability will constrain future land carbon sinks.

Modifications to the water, carbon, and biogeochemical cycles on land result in both positive and negative feedbacks to temperature increases. 114 , 133 , 134 Snow and ice albedo feedbacks are positive, leading to increased temperatures with loss of snow and ice extent. While land ecosystems are expected to have a net positive feedback due to reduced natural sinks of CO 2 in a warmer world, anthropogenically increased nitrogen deposition may reduce the magnitude of the net feedback. 131 , 135 , 136 Increased temperature and reduced precipitation increase wildfire risk and susceptibility of terrestrial ecosystems to pests and disease, with resulting feedbacks on carbon storage. Increased temperature and precipitation, particularly at high latitudes, drives up soil decomposition, which leads to increased CO 2 and CH 4 (methane) emissions. 137 , 138 , 139 , 140 , 141 , 142 , 143 While some of these feedbacks are well known, others are not so well quantified and yet others remain unknown; the potential for surprise is discussed further in Chapter 15: Potential Surprises .

1.7: Global Changes in Sea Ice, Glaciers, and Land Ice

Since NCA3, 144 there have been significant advances in the understanding of changes in the cryosphere. Observations continue to show declines in arctic sea ice extent and thickness, Northern Hemisphere snow cover, and the volume of mountain glaciers and continental ice sheets. 1 , 145 , 146 , 147 , 148 , 149 Evidence suggests in many cases that the net loss of mass from the global cryosphere is accelerating indicating significant climate feedbacks and societal consequences. 150 , 151 , 152 , 153 , 154 , 155

Arctic sea ice areal extent, thickness, and volume have declined since 1979. 1 , 146 , 147 , 148 , 156 The annual arctic sea ice extent minimum for 2016 relative to the long-term record was the second lowest (2012 was the lowest) ( http://nsidc.org/arcticseaicenews/ ). The arctic sea ice minimum extents in 2014 and 2015 were also among the lowest on record. Annually averaged arctic sea ice extent has decreased by 3.5%–4.1% per decade since 1979 with much larger reductions in summer and fall. 1 , 146 , 148 , 157 For example, September sea ice extent decreased by 13.3% per decade between 1979 and 2016. At the same time, September multi-year sea ice has melted faster than perennial sea ice (13.5% ± 2.5% and 11.5% ± 2.1% per decade, respectively, relative to the 1979–2012 average) corresponding to 4–7.5 feet (1.3–2.3 meter) declines in winter sea ice thickness. 1 , 156 October 2016 serves as a recent example of the observed lengthening of the arctic sea ice melt season marking the slowest recorded arctic sea ice growth rate for that month. 146 , 158 , 159 The annual arctic sea ice maximum in March 2017 was the lowest maximum areal extent on record ( http://nsidc.org/arcticseaicenews/ ).

While current generation climate models project a nearly ice-free Arctic Ocean in late summer by mid-century, they still simulate weaker reductions in volume and extent than observed, suggesting that projected changes are too conservative. 1 , 147 , 160 , 161 See Chapter 11: Arctic Changes for further discussion of the implications of changes in the Arctic.

In contrast to the Arctic, sea ice extent around Antarctica has increased since 1979 by 1.2% to 1.8% per decade. 1 Strong regional differences in the sea ice growth rates are found around Antarctica but most regions (about 75%) show increases over the last 30 years. 162 The gain in antarctic sea ice is much smaller than the decrease in arctic sea ice. Changes in wind patterns, ice–ocean feedbacks, and freshwater flux have contributed to antarctic sea ice growth. 162 , 163 , 164 , 165

Since the NCA3, 144 the Gravity Recovery and Climate Experiment (GRACE) constellation (e.g., Velicogna and Wahr 2013 166 ) has provided a record of gravimetric land ice measurements, advancing knowledge of recent mass loss from the global cryosphere. These measurements indicate that mass loss from the Antarctic Ice Sheet, Greenland Ice Sheet, and mountain glaciers around the world continues accelerating in some cases. 151 , 152 , 154 , 155 , 167 , 168 The annually averaged ice mass from 37 global reference glaciers has decreased every year since 1984, a decline expected to continue even if climate were to stabilize. 1 , 153 , 169 , 170

Ice sheet dynamics in West Antarctica are characterized by land ice that transitions to coastal and marine ice sheet systems. Recent observed rapid mass loss from West Antarctica’s floating ice shelves is attributed to increased glacial discharge rates due to diminishing ice shelves from the surrounding ocean becoming warmer. 171 , 172 Recent evidence suggests that the Amundsen Sea sector is expected to disintegrate entirely 151 , 168 , 172 raising sea level by at least 1.2 meters (about 4 feet) and potentially an additional foot or more on top of current sea level rise projections during this century (see Section 1.2.7 and Ch. 12: Sea Level Rise for further details). 173 The potential for unanticipated rapid ice sheet melt and/or disintegration is discussed further in Chapter 15: Potential Surprises .

Over the last decade, the Greenland Ice Sheet mass loss has accelerated, losing 244 ± 6 Gt per year on average between January 2003 and May 2013. 1 , 155 , 174 , 175 The portion of the Greenland Ice Sheet experiencing annual melt has increased since 1980 including significant events. 1 , 176 , 177 , 178 A recent example, an unprecedented 98.6% of the Greenland Ice Sheet surface experienced melt on a single day in July 2012. 179 , 180 Encompassing this event, GRACE data indicate that Greenland lost 562 Gt of mass between April 2012 and April 2013—more than double the average annual mass loss.

In addition, permafrost temperatures and active layer thicknesses have increased across much of the Arctic (also see Ch. 11: Arctic Changes ). 1 , 181 , 182 Rising permafrost temperatures causing permafrost to thaw and become more discontinuous raises concerns about potential emissions of carbon dioxide and methane. 1 The potentially large contribution of carbon and methane emissions from permafrost and the continental shelf in the Arctic to overall warming is discussed further in Chapter 15: Potential Surprises .

1.8: Global Changes in Sea Level

Statistical analyses of tide gauge data indicate that global mean sea level has risen about 8–9 inches (20–23 cm) since 1880, with a rise rate of approximately 0.5–0.6 inches/decade from 1901 to1990 (about 12–15 mm/decade; also see Ch. 12: Sea Level Rise ). 183 , 184 However, since the early 1990s, both tide gauges and satellite altimeters have recorded a faster rate of sea level rise of about 1.2 inches/decade (approximately 3 cm/decade), 183 , 184 , 185 resulting in about 3 inches (about 8 cm) of the global rise since the early 1990s. Nearly two-thirds of the sea level rise measured since 2005 has resulted from increases in ocean mass, primarily from land-based ice melt; the remaining one-third of the rise is in response to changes in density from increasing ocean temperatures. 186

Global sea level rise and its regional variability forced by climatic and ocean circulation patterns are contributing to significant increases in annual tidal-flood frequencies, which are measured by NOAA tide gauges and associated with minor infrastructure impacts to date; along some portions of the U.S. coast, frequency of the impacts from such events appears to be accelerating (also see Ch. 12: Sea-Level Rise ). 187 , 188

Future projections show that by 2100, global mean sea level is very likely to rise by 1.6–4.3 feet (0.5–1.3 m) under the higher scenario (RCP8.5), 1.1–3.1 feet (0.35–0.95 m) under a lower scenario (RCP4.5), and 0.8–2.6 feet (0.24–0.79 m) under and even lower scenario (RCP2.6) (see Ch. 4: Projections for a description of the scenarios). 189 Sea level will not rise uniformly around the coasts of the United States and its oversea territories. Local sea level rise is likely to be greater than the global average along the U.S. Atlantic and Gulf Coasts and less than the global average in most of the Pacific Northwest. Emerging science suggests these projections may be underestimates, particularly for higher scenarios; a global mean sea level rise exceeding 8 feet (2.4 m) by 2100 cannot be excluded (see Ch. 12: Sea Level Rise ), and even higher amounts are possible as a result of marine ice sheet instability (see Ch. 15: Potential Surprises ). We have updated the global sea level rise scenarios for 2100 of Parris et al. 190 accordingly, 191 and also extended to year 2200 in Chapter 12: Sea Level Rise . The scenarios are regionalized to better match the decision context needed for local risk framing purposes.

1.9: Recent Global Changes Relative to Paleoclimates

Proxy temperatures reconstructions for the seven regions of the PAGES 2k Network. Temperature anomalies are relative to the 1961–1990 reference period. If this graph were plotted relative to 1901–1960 instead of 1961–1990, the temperature changes would 0.47°F (0.26°C) higher. Gray lines around expected-value estimates indicate uncertainty ranges as defined by each regional group (see PAGE 2k Consortium 9 and related Supplementary Information). Note that the changes in temperature over the last century tend to occur at a much faster rate than found in the previous time periods. The teal values are from the HadCRUT4 surface observation record for land and ocean for the 1800s to 2000. 202

(Figure source: adapted from PAGES 2k Consortium 2013 9 ).

Paleoclimate records demonstrate long-term natural variability in the climate and overlap the records of the last two millennia, referred to here as the “Common Era.” Before the emissions of greenhouse gases from fossil fuels and other human-related activities became a major factor over the last few centuries, the strongest drivers of climate during the last few thousand years had been volcanoes and land-use change (which has both albedo and greenhouse gas emissions effects). 192 Based on a number of proxies for temperature (for example, from tree rings, fossil pollen, corals, ocean and lake sediments, and ice cores), temperature records are available for the last 2,000 years on hemispherical and continental scales (Figures 1.8 and 1.9). 9 , 193 High-resolution temperature records for North America extend back less than half of this period, with temperatures in the early parts of the Common Era inferred from analyses of pollen and other archives. For this era, there is a general cooling trend, with a relatively rapid increase in temperature over the last 150–200 years (Figure 1.9, ). For context, global annual averaged temperatures for 1986–2015 are likely much higher, and appear to have risen at a more rapid rate during the last 3 decades, than any similar period possibly over the past 2,000 years or longer (IPCC 1 makes a similar statement, but for the last 1,400 years because of data quality issues before that time).

Changes in the temperature of the Northern Hemisphere from surface observations (in red) and from proxies (in black; uncertainty range represented by shading) relative to 1961–1990 average temperature. If this graph were plotted relative to 1901–1960 instead of 1961–1990, the temperature changes would be 0.47°F (0.26°C) higher. These analyses suggest that current temperatures are higher than seen in the Northern Hemisphere, and likely globally, in at least the last 1,700 years, and that the last decade (2006–2015) was the warmest decade on record. (Figure source: adapted from Mann et al. 2008 193 ).

Global temperatures of the magnitude observed recently (and projected for the rest of this century) are related to very different forcings than past climates, but studies of past climates suggest that such global temperatures were likely last observed during the Eemian period—the last interglacial—125,000 years ago; at that time, global temperatures were, at their peak, about 1.8°F–3.6°F (1°C­–2°C) warmer than preindustrial temperatures. 194 Coincident with these higher temperatures, sea levels during that period were about 16–30 feet (6–9 meters) higher than modern levels 195 , 196 (for further discussion on sea levels in the past, see Ch. 12: Sea Level Rise ).

Modeling studies suggest that the Eemian period warming can be explained in part by the hemispheric changes in solar insolation from orbital forcing as a result of cyclic changes in the shape of Earth’s orbit around the sun (e.g., Kaspar et al. 2005 197 ), even though greenhouse gas concentrations were similar to preindustrial levels. Equilibrium climate with modern greenhouse gas concentrations (about 400 ppm CO 2 ) most recently occurred 3 million years ago during the Pliocene. During the warmest parts of this period, global temperatures were 5.4°F–7.2°F (3°C–4°C) higher than today, and sea levels were about 82 feet (25 meters) higher. 198

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Isn't there a lot of disagreement among climate scientists about global warming?

No. By a large majority, climate scientists agree that average global temperature today is warmer than in pre-industrial times and that human activity is the most significant factor. 

Today, there is no real disagreement among climate experts that humans are the primary cause of recent global warming. NOAA Climate.gov cartoon by Emily Greenhalgh. 

Consensus of experts

The United States' foremost scientific agencies and organizations have recognized global warming as a human-caused problem that should be addressed. The U.S. Global Change Research Program has published a series of scientific reports documenting the causes and impacts of global climate change. NOAA , NASA , the National Science Foundation , the National Research Council , and the Environmental Protection Agency have all published reports and fact sheets stating that Earth is warming mainly due to the increase in human-produced heat-trapping gases.

On their climate home page , the National Academies of Sciences, Engineering, and Medicines says, "Scientists have known for some time, from multiple lines of evidence, that humans are changing Earth’s climate, primarily through greenhouse gas emissions," and that "Climate change is increasingly affecting people’s lives." 

Soot from fires and air pollution contributes to global warming, and its impacts may be especially strong in the Arctic, where it darkens the snow and ice—as shown in this photo—and accelerates melting. Despite some uncertainty about just how much influence soot and other aerosol particles have played in climate change in the past century, there's little debate among climate scientists that the primary driver of recent global warming is carbon dioxide emissions. Photo from NOAA Ocean Today .

The American Meteorological Society (AMS) issued this position statement : "Scientific evidence indicates that the leading cause of climate change in the most recent half century is the anthropogenic increase in the concentration of atmospheric greenhouse gases, including carbon dioxide (CO 2 ), chlorofluorocarbons, methane, tropospheric ozone, and nitrous oxide." (Adopted April 15, 2019)

The American Geophysical Union (AGU) issued this position statement : "Human activities are changing Earth's climate, causing increasingly disruptive societal and ecological impacts. Such impacts are creating hardships and suffering now, and they will continue to do so into the future—in ways expected as well as potentially unforeseen. To limit these impacts, the world's nations have agreed to hold the increase in global average temperature to well below 2°C (3.6°F) above pre-industrial levels. To achieve this goal, global society must promptly reduce its greenhouse gas emissions." (Reaffirmed in November 2019)

The American Association for the Advancement of Science (AAAS) What We Know site states: "Based on the evidence, about 97 percent of climate scientists agree that human-caused climate change is happening."

Consensus of evidence

These scientific organizations have not issued statements in a void; they echo the findings of individual papers published in refereed scientific journals. The Institute for Scientific Information (ISI) maintains a database of over 8,500 peer-reviewed science journals, and multiple studies of this database show evidence of overwhelming agreement among climate scientists. In 2004, science historian Naomi Oreskes published the results of her examination of the ISI database in the journal Science . She reviewed 928 abstracts published between 1993 and 2003 related to human activities warming the Earth's surface, and stated, "Remarkably, none of the papers disagreed with the consensus position."

This finding hasn't changed with time. In 2016, a review paper summarized the results of several independent studies on peer-reviewed research related to climate. The authors found results consistent with a 97-percent consensus that human activity is causing climate change. A 2021 paper found a greater than 99-percent consensus.

Probably the most definitive assessments of global climate science come from the Intergovernmental Panel on Climate Change (IPCC). Founded by the United Nations in 1988, the IPCC releases periodic reports, and each major release includes three volumes: one on the science, one on impacts, and one on mitigation. Each volume is authored by a separate team of experts, who reviews, evaluates, and summarizes relevant research published since the prior report. Each IPCC report undergoes several iterations of expert and government review. The 2021 IPCC report, for instance, received and responded to more than 78,000 expert and government review comments.

Every five years, the Intergovernmental Panel on Climate Change convenes hundreds of international scientists and government representatives to review and assess peer-reviewed research on climate science. In each cycle, the panel publishes three key reports: one on the basic science , one on impacts , and one on mitigation .

The IPCC does not involve just a few scientists, or even just dozens of scientists. An IPCC press release explains: "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."

Governments and climate experts across the globe nominate scientists for IPCC authorship, and the IPCC works to find a mix of authors, from developed and developing countries, among men and women, and among authors who are experienced with the IPCC and new to the process. Published in 2021, the Sixth Assessment Report was assembled by 751 experts from more than 60 countries (31 coordinating authors, 167 lead authors, 36 review editors, and 517 contributing authors). Collectively, the authors cited more than 14,000 scientific papers. In other words, the IPCC reports themselves are a comprehensive, consensus statement on the state climate science.

In the headline statements from the Sixth Assessment report's Summary for Policymakers, the IPCC concluded:

It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred. The scale of recent changes across the climate system as a whole – and the present state of many aspects of the climate system – are unprecedented over many centuries to many thousands of years. Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since [our last report].

Cook, J., D. Nuccitelli, S.A. Green, M. Richardson, B. Winkler, R. Painting, R. Way, P. Jacobs, and A. Skuce (2013). Quantifying the consensus on anthropogenic global warming in the scientific literature. Environmental Research Letters , 8, 024024. https://doi.org/10.1088/1748-9326/8/2/024024 .

Cook, J., Oreskes, N., Doran, P.T., Anderegg, W.R.L., Verheggen, B., Mailbach, E.W., Carlton, J.S., Lewandowsky, S., Skuce, A.G., Green, S.A., Nuccitelli, D., Jacobs, P., Richardson, M., Winkler, B., Painting, R., Rice, K. (2016). Consensus on consensus: A synthesis of consensus estimates on human-caused global warming. Environmental Research Letters , 11, 048002. https://doi.org/10.1088/1748-9326/11/4/048002 .

Doran, P., and M.K. Zimmerman (2009): Examining the Scientific Consensus on Climate Change. Eos , 90(3), 22–23.

IPCC. (2013). Factsheet: How does the IPCC select its authors? Accessed January 3, 2020.

IPCC. (2014). Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva Switzerland. Accessed January 22, 2020.

IPCC. (2021). Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I 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, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32, doi:10.1017/9781009157896.001.

IPCC Sixth Assessment Report. (2021). https://www.ipcc.ch/report/ar6/wg1/

Lynas, M., Houlton, B.Z., Perry, S. (2021). Greater than 99% consensus on human caused climate change in the peer-reviewed scientific literature. Environmental Research Letters , 16, 114005. https://doi.org/10.1088/1748-9326/ac2966 .

Oreskes, N. (2004). The Scientific Consensus on Climate Change. Science , 306, 1686. https://doi.org/10.1126/science.1103618 .

Oreskes, N. (2018). The scientific consensus on climate change: How do we know we're not wrong? Climate Modelling , pp. 31–64. https://doi.org/10.1007/978-3-319-65058-6_2 .

Sherwood, S. (2011, May 10). Trust us, we're climate scientists: The case for the IPCC . The Conversation .

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Health effects of climate change: an overview of systematic reviews

Rhea j rocque, caroline beaudoin, ruth ndjaboue, laura cameron, louann poirier-bergeron, rose-alice poulin-rheault, catherine fallon, andrea c tricco, holly o witteman.

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Correspondence to Dr Rhea J Rocque; [email protected]

Corresponding author.

Series information

Original research

Received 2020 Oct 27; Accepted 2021 May 28; Collection date 2021.

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See:  http://creativecommons.org/licenses/by-nc/4.0/ .

We aimed to develop a systematic synthesis of systematic reviews of health impacts of climate change, by synthesising studies’ characteristics, climate impacts, health outcomes and key findings.

We conducted an overview of systematic reviews of health impacts of climate change. We registered our review in PROSPERO (CRD42019145972). No ethical approval was required since we used secondary data. Additional data are not available.

Data sources

On 22 June 2019, we searched Medline, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Embase, Cochrane and Web of Science.

Eligibility criteria

We included systematic reviews that explored at least one health impact of climate change.

Data extraction and synthesis

We organised systematic reviews according to their key characteristics, including geographical regions, year of publication and authors’ affiliations. We mapped the climate effects and health outcomes being studied and synthesised major findings. We used a modified version of A MeaSurement Tool to Assess systematic Reviews-2 (AMSTAR-2) to assess the quality of studies.

We included 94 systematic reviews. Most were published after 2015 and approximately one-fifth contained meta-analyses. Reviews synthesised evidence about five categories of climate impacts; the two most common were meteorological and extreme weather events. Reviews covered 10 health outcome categories; the 3 most common were (1) infectious diseases, (2) mortality and (3) respiratory, cardiovascular or neurological outcomes. Most reviews suggested a deleterious impact of climate change on multiple adverse health outcomes, although the majority also called for more research.

Conclusions

Most systematic reviews suggest that climate change is associated with worse human health. This study provides a comprehensive higher order summary of research on health impacts of climate change. Study limitations include possible missed relevant reviews, no meta-meta-analyses, and no assessment of overlap. Future research could explore the potential explanations between these associations to propose adaptation and mitigation strategies and could include broader sociopsychological health impacts of climate change.

Keywords: public health, social medicine, public health

Strengths and limitations of this study.

A strength of this study is that it provides the first broad overview of previous systematic reviews exploring the health impacts of climate change. By targeting systematic reviews, we achieve a higher order summary of findings than what would have been possible by consulting individual original studies.

By synthesising findings across all included studies and according to the combination of climate impact and health outcome, we offer a clear, detailed and unique summary of the current state of evidence and knowledge gaps about how climate change may influence human health.

A limitation of this study is that we were unable to access some full texts and therefore some studies were excluded, even though we deemed them potentially relevant after title and abstract inspection.

Another limitation is that we could not conduct meta-meta-analyses of findings across reviews, due to the heterogeneity of the included systematic reviews and the relatively small proportion of studies reporting meta-analytic findings.

Finally, the date of the systematic search is a limitation, as we conducted the search in June 2019.

Introduction

The environmental consequences of climate change such as sea-level rise, increasing temperatures, more extreme weather events, increased droughts, flooding and wildfires are impacting human health and lives. 1 2 Previous studies and reviews have documented the multiple health impacts of climate change, including an increase in infectious diseases, respiratory disorders, heat-related morbidity and mortality, undernutrition due to food insecurity, and adverse health outcomes ensuing from increased sociopolitical tension and conflicts. 2–5 Indeed, the most recent Lancet Countdown report, 2 which investigates 43 indicators of the relationship between climate change and human health, arrived at their most worrisome findings since the beginning of their on-going annual work. This report underlines that the health impacts of climate change continue to worsen and are being felt on every continent, although they are having a disproportionate and unequal impact on populations. 2 Authors caution that these health impacts will continue to worsen unless we see an immediate international response to limiting climate change.

To guide future research and action to mitigate and adapt to the health impacts of climate change and its environmental consequences, we need a complete and thorough overview of the research already conducted regarding the health impacts of climate change. Although the number of original studies researching the health impacts of climate change has greatly increased in the recent decade, 2 these do not allow for an in-depth overview of the current literature on the topic. Systematic reviews, on the other hand, allow a higher order overview of the literature. Although previous systematic reviews have been conducted on the health impacts of climate change, these tend to focus on specific climate effects (eg, impact of wildfires on health), 6 7 health impacts (eg, occupational health outcomes), 8 9 countries, 10–12 or are no longer up to date, 13 14 thus limiting our global understanding of what is currently known about the multiple health impacts of climate change across the world.

In this study, we aimed to develop such a complete overview by synthesising systematic reviews of health impacts of climate change. This higher order overview of the literature will allow us to better prepare for the worsening health impacts of climate change, by identifying and describing the diversity and range of health impacts studied, as well as by identifying gaps in previous research. Our research objectives were to synthesise studies’ characteristics such as geographical regions, years of publication, and authors’ affiliations, to map the climate impacts, health outcomes, and combinations of these that have been studied, and to synthesise key findings.

We applied the Cochrane method for overviews of reviews. 15 This method is designed to systematically map the themes of studies on a topic and synthesise findings to achieve a broader overview of the available literature on the topic.

Research questions

Our research questions were the following: (1) What is known about the relationship between climate change and health, as shown in previous systematic reviews? (2) What are the characteristics of these studies? We registered our plan (CRD42019145972 16 ) in PROSPERO, an international prospective register of systematic reviews and followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 17 to report our findings, as a reporting guideline for overviews is still in development. 18

Search strategy and selection criteria

To identify relevant studies, we used a systematic search strategy. There were two inclusion criteria. We included studies in this review if they (1) were systematic reviews of original research and (2) reported at least one health impact as it related (directly or indirectly) to climate change.

We defined a systematic review, based on Cochrane’s definition, as a review of the literature in which one ‘attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a specific research question [by] us[ing] explicit, systematic methods that are selected with a view aimed at minimizing bias, to produce more reliable findings to inform decision making’. 19 We included systematic reviews of original research, with or without meta-analyses. We excluded narrative reviews, non-systematic literature reviews and systematic reviews of materials that were not original research (eg, systematic reviews of guidelines.)

We based our definition of health impacts on the WHO’s definition of health as, ‘a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity’. 20 Therefore, health impacts included, among others, morbidity, mortality, new conditions, worsening/improving conditions, injuries and psychological well-being. Included studies could refer to climate change or global warming directly or indirectly, for instance, by synthesising the direct or indirect health effects of temperature rises or of natural conditions/disasters made more likely by climate change (eg, floods, wildfires, temperature variability, droughts.) Although climate change and global warming are not equivalent terms, in an effort to avoid missing relevant literature, we included studies using either term. We included systematic reviews whose main focus was not the health impacts of climate change, providing they reported at least one result regarding health effects related to climate change (or consequences of climate change.) We excluded studies if they did not report at least one health effect of climate change. For instance, we excluded studies which reported on existing measures of health impacts of climate change (and not the health impact itself) and studies which reported on certain health impacts without a mention of climate change, global warming or environmental consequences made more likely by climate change.

On 22 June 2019, we retrieved systematic reviews regarding the health effects of climate change by searching from inception the electronic databases Medline, CINAHL, Embase, Cochrane, Web of Science using a structured search (see online supplemental appendix 1 for final search strategy developed by a librarian.) We did not apply language restrictions. After removing duplicates, we imported references into Covidence. 21

bmjopen-2020-046333supp001.pdf (42.4KB, pdf)

Screening process and data extraction

To select studies, two trained analysts first screened independently titles and abstracts to eliminate articles that did not meet our inclusion criteria. Next, the two analysts independently screened the full text of each article. A senior analyst resolved any conflict or disagreement.

Next, we decided on key information that needed to be extracted from studies. We extracted the first author’s name, year of publication, number of studies included, time frame (in years) of the studies included in the article, first author’s institution’s country affiliation, whether the systematic review included a meta-analysis, geographical focus, population focus, the climate impact(s) and the health outcome(s) as well as the main findings and limitations of each systematic review.

Two or more trained analysts (RR, CB, RN, LC, LPB, RAPR) independently extracted data, using Covidence and spreadsheet software (Google Sheets). An additional trained analyst from the group or senior research team member resolved disagreements between individual judgments.

Coding and data mapping

To summarise findings from previous reviews, we first mapped articles according to climate impacts and health outcomes. To develop the categories of climate impacts and health outcomes, two researchers (RR and LC) consulted the titles and abstracts of each article. We started by identifying categories directly based on our data and finalised our categories by consulting previous conceptual frameworks of climate impacts and health outcomes. 1 22 23 The same two researchers independently coded each article according to their climate impact and health outcome. We then compared coding and resolved disagreements through discussion.

Next, using spreadsheet software, we created a matrix to map articles according to their combination of climate impacts and health outcomes. Each health outcome occupied one row, whereas climate impacts each occupied one column. We placed each article in the matrix according to the combination(s) of their climate impact(s) and health outcome(s). For instance, if we coded an article as ‘extreme weather’ for climate and ‘mental health’ for health impact, we noted the reference of this article in the cell at the intersection of these two codes. We calculated frequencies for each cell to identify frequent combinations and gaps in literature. Because one study could investigate more than one climate impact and health outcome, the frequency counts for each category could exceed the number of studies included in this review.

Finally, we re-read the Results and Discussion sections of each article to summarise findings of the studies. We first wrote an individual summary for each study, then we collated the summaries of all studies exploring the same combination of categories to develop an overall summary of findings for each combination of categories.

Quality assessment

We used a modified version of AMSTAR-2 to assess the quality of the included systematic reviews ( online supplemental appendix 2 ). The purpose of this assessment was to evaluate the quality of the included studies as a whole to get a sense of the overall quality of evidence in this field. Therefore, individual quality scores were not compiled for each article, but scores were aggregated according to items. Since AMSTAR-2 was developed for syntheses of systematic reviews of randomised controlled trials, working with a team member with expertise in knowledge synthesis (AT), we adapted it to suit a research context that is not amenable to randomised controlled trials. For instance, we changed assessing and accounting for risk of bias in studies’ included randomised controlled trials to assessing and accounting for limitations in studies’ included articles. Complete modifications are presented in online supplemental appendix 2 .

bmjopen-2020-046333supp002.pdf (22.3KB, pdf)

Patient and public involvement

Patients and members of the public were not involved in this study.

Articles identified

As shown in the PRISMA diagram in figure 1 , from an initial set of 2619 references, we retained 94 for inclusion. More precisely, following screening of titles and abstracts, 146 studies remained for full-text inspection. During full-text inspection, we excluded 52 studies, as they did not report a direct health effect of climate change (n=17), did not relate to climate change (n=15), were not systematic reviews (n=10), or we could not retrieve the full text (n=10).

The flow chart for included articles in this review.

Study descriptions

A detailed table of all articles and their characteristics can be found in online supplemental appendix 3 . Publication years ranged from 2007 to 2019 (year of data extraction), with the great majority of included articles (n=69; 73%) published since 2015 ( figure 2 ). A median of 30 studies had been included in the systematic reviews (mean=60; SD=49; range 7–722). Approximately one-fifth of the systematic reviews included meta-analyses of their included studies (n=18; 19%). The majority of included systematic reviews’ first authors had affiliations in high-income countries, with the largest representations by continent in Europe (n=30) and Australia (n=24) ( figure 3 ). Countries of origin by continents include (from highest to lowest frequency, then by alphabetical order): Europe (30); UK (9), Germany (6), Italy (4), Sweden (4), Denmark (2), France (2), Georgia (1), Greece (1) and Finland (1); Australia (24); Asia (21); China (11), Iran (4), India (1), Jordan (1), Korea (1), Nepal (1), Philippines (1), Taiwan (1); North America (16); USA (15), Canada (1); Africa (2); Ethiopia (1), Ghana (1), and South America (1); Brazil (1).

Number of included systematic reviews by year of publication.

Number of publications according to geographical affiliation of the first author.

bmjopen-2020-046333supp003.pdf (185.2KB, pdf)

Regarding the geographical focus of systematic reviews, most of the included studies (n=68; 72%) had a global focus or no specified geographical limitations and therefore included studies published anywhere in the world. The remaining systematic reviews either targeted certain countries (n=12) (1 for each Australia, Germany, Iran, India, Ethiopia, Malaysia, Nepal, New Zealand and 2 reviews focused on China and the USA), continents (n=5) (3 focused on Europe and 2 on Asia), or regions according to geographical location (n=6) (1 focused on Sub-Saharan Africa, 1 on Eastern Mediterranean countries, 1 on Tropical countries, and 3 focused on the Arctic), or according to the country’s level of income (n=3) (2 on low to middle income countries, 1 on high income countries).

Regarding specific populations of interest, most of the systematic reviews did not define a specific population of interest (n=69; 73%). For the studies that specified a population of interest (n=25; 26.6%), the most frequent populations were children (n=7) and workers (n=6), followed by vulnerable or susceptible populations more generally (n=4), the elderly (n=3), pregnant people (n=2), people with disabilities or chronic illnesses (n=2) and rural populations (n=1).

We assessed studies for quality according to our revised AMSTAR-2. Complete scores for each article and each item are available in online supplemental appendix 4 . Out of 94 systematic reviews, the most commonly fully satisfied criterion was #1 (Population, Intervention, Comparator, Outcome (PICO) components) with 81/94 (86%) of included systematic reviews fully satisfying this criterion. The next most commonly satisfied criteria were #16 (potential sources of conflict of interest reported) (78/94=83% fully), #13 (account for limitations in individual studies) (70/94=75% fully and 2/94=2% partially), #7 (explain both inclusion and exclusion criteria) (64/94=68% fully and 19/94=20% partially), #8 (description of included studies in adequate detail) (36/94=38% fully and 41/94=44% partially), and #4 (use of a comprehensive literature search strategy) (0/94=0% fully and 80/94=85% partially). For criteria #11, #12, and #15, which only applied to reviews including meta-analyses, 17/18 (94%) fully satisfied criterion #11 (use of an appropriate methods for statistical combination of results), 12/18 (67%) fully satisfied criterion #12 (assessment of the potential impact of Risk of Bias (RoB) in individual studies) (1/18=6% partially), and 11/18 (61%) fully satisfied criterion #15 (an adequate investigation of publication bias, small study bias).

bmjopen-2020-046333supp004.pdf (75KB, pdf)

Climate impacts and health outcomes

Regarding climate impacts, we identified 5 mutually exclusive categories, with 13 publications targeting more than one category of climate impacts: (1) meteorological (n=71 papers) (eg, temperature, heat waves, humidity, precipitation, sunlight, wind, air pressure), (2) extreme weather (n=24) (eg, water-related, floods, cyclones, hurricanes, drought), (3) air quality (n=7) (eg, air pollution and wildfire smoke exposure), (4) general (n=5), and (5) other (n=3). Although heat waves could be considered an extreme weather event, papers investigating heat waves’ impact on health were classified in the meteorological impact category, since some of these studies treated them with high temperature. ‘General’ climate impacts included articles that did not specify climate change impacts but stated general climate change as their focus. ‘Other’ climate impacts included studies investigating other effects indirectly related to climate change (eg, impact of environmental contaminants) or general environmental risk factors (eg, environmental hazards, sanitation and access to clean water.)

We identified 10 categories to describe the health outcomes studied by the systematic reviews, and 29 publications targeted more than one category of health outcomes: (1) infectious diseases (n=41 papers) (vector borne, food borne and water borne), (2) mortality (n=32), (3) respiratory, cardiovascular and neurological (n=23), (4) healthcare systems (n=16), 5) mental health (n=13), (6) pregnancy and birth (n=11), 7) nutritional (n=9), (8) skin diseases and allergies (n=8), (9) occupational health and injuries (n=6) and (10) other health outcomes (n=17) (eg, sleep, arthritis, disability-adjusted life years, non-occupational injuries, etc)

Figure 4 depicts the combinations of climate impact and health outcome for each study, with online supplemental appendix 5 offering further details. The five most common combinations are studies investigating the (1) meteorological impacts on infectious diseases (n=35), (2) mortality (n=24) and (3) respiratory, cardiovascular and neurological outcomes (n=17), (4) extreme weather events’ impacts on infectious diseases (n=14), and (5) meteorological impacts on health systems (n=11).

Summary of the combination of climate impact and health outcome (frequencies). The total frequency for one category of health outcome could exceed the number of publications included in this health outcome, since one publication could explore the health impact according to more than one climate factor (eg, one publication could explore both the impact of extreme weather events and temperature on mental health).

bmjopen-2020-046333supp005.pdf (32.8KB, pdf)

For studies investigating meteorological impacts on health, the three most common health outcomes studied were impacts on (1) infectious diseases (n=35), (2) mortality (n=24) and (3) respiratory, cardiovascular and neurological outcomes (n=17). Extreme weather event studies most commonly reported health outcomes related to (1) infectious diseases (n=14), (2) mental health outcomes (n=9) and (3) nutritional outcomes (n=6) and other health outcomes (eg, injuries, sleep) (n=6). Studies focused on the impact of air quality were less frequent and explored mostly health outcomes linked to (1) respiratory, cardiovascular and neurological outcomes (n=6), (2) mortality (n=5) and (3) pregnancy and birth outcomes (n=3).

Summary of findings

Most reviews suggest a deleterious impact of climate change on multiple adverse health outcomes, with some associations being explored and/or supported with consistent findings more often than others. Some reviews also report conflicting findings or an absence of association between the climate impact and health outcome studied (see table 1 for a detailed summary of findings according to health outcomes).

Summary of findings from systematic reviews according to health outcome and climate impact

Reviews that covered multiple climate impacts are listed in each relevant category.

Notable findings of health outcomes according to climate impact include the following. For meteorological factors (n=71), temperature and humidity are the variables most often studied and report the most consistent associations with infectious diseases and respiratory, cardiovascular, and neurological outcomes. Temperature is also consistently associated with mortality and healthcare service use. Some associations are less frequently studied, but remain consistent, including the association between some meteorological factors (eg, temperature and heat) and some adverse mental health outcomes (eg, hospital admissions for mental health reasons, suicide, exacerbation of previous mental health conditions), and the association between heat and adverse occupational outcomes and some adverse birth outcomes. Temperature is also associated with adverse nutritional outcomes (likely via crop production and food insecurity) and temperature and humidity are associated with some skin diseases and allergies. Some health outcomes are less frequently studied, but studies suggest an association between temperature and diabetes, impaired sleep, cataracts, heat stress, heat exhaustion and renal diseases.

Extreme weather events (n=24) are consistently associated with mortality, some mental health outcomes (eg, distress, anxiety, depression) and adverse nutritional outcomes (likely via crop production and food insecurity). Some associations are explored less frequently, but these studies suggest an association between drought and respiratory and cardiovascular outcomes (likely via air quality), between extreme weather events and an increased use of healthcare services and some adverse birth outcomes (likely due to indirect causes, such as experiencing stress). Some health outcomes are less frequently studied, but studies suggest an association between extreme weather events and injuries, impaired sleep, oesophageal cancer and exacerbation of chronic illnesses. There are limited and conflicting findings for the association between extreme weather events and infectious diseases, as well as for certain mental health outcomes (eg, suicide and substance abuse). At times, different types of extreme weather events (eg, drought vs flood) led to conflicting findings for some health outcomes (eg, mental health outcomes, infectious diseases), but for other health outcomes, the association was consistent independently of the extreme weather event studied (eg, mortality, healthcare service use and nutritional outcomes).

The impact of air quality on health (n=7) was less frequently studied, but the few studies exploring this association report consistent findings regarding an association with respiratory-specific mortality, adverse respiratory outcomes and an increase in healthcare service use. There is limited evidence regarding the association between air quality and cardiovascular outcomes, limited and inconsistent evidence between wildfire smoke exposure and adverse birth outcomes, and no association is found between exposure to wildfire smoke and increase in use of health services for mental health reasons. Only one review explored the impact of wildfire smoke exposure on ophthalmic outcomes, and it suggests that it may be associated with eye irritation and cataracts.

Reviews which stated climate change as their general focus and did not specify the climate impact(s) under study were less frequent (n=5), but they suggest an association between climate change and pollen allergies in Europe, increased use of healthcare services, obesity, skin diseases and allergies and an association with disability-adjusted life years. Reviews investigating the impact of other climate-related factors (n=3) show inconsistent findings concerning the association between environmental pollutant and adverse birth outcomes, and two reviews suggest an association between environmental risk factors and pollutants and childhood stunting and occupational diseases.

Most reviews concluded by calling for more research, noting the limitations observed among the studies included in their reviews, as well as limitations in their reviews themselves. These limitations included, among others, some systematic reviews having a small number of publications, 24 25 language restrictions such as including only papers in English, 26 27 arriving at conflicting evidence, 28 difficulty concluding a strong association due to the heterogeneity in methods and measurements or the limited equipment and access to quality data in certain contexts, 24 29–31 and most studies included were conducted in high-income countries. 32 33

Previous authors also discussed the important challenge related to exploring the relationship between climate change and health. Not only is it difficult to explore the potential causal relationship between climate change and health, mostly due to methodological challenges, but there are also a wide variety of complex causal factors that may interact to determine health outcomes. Therefore, the possible causal mechanisms underlying these associations were at times still unknown or uncertain and the impacts of some climate factors were different according to geographical location and specificities of the context. Nonetheless, some reviews offered potential explanations for the climate-health association, with the climate factor at times, having a direct impact on health (eg, flooding causing injuries, heat causing dehydration) and in other cases, having an indirect impact (eg, flooding causing stress which in turn may cause adverse birth outcomes, heat causing difficulty concentrating leading to occupational injuries.)

Principal results

In this overview of systematic reviews, we aimed to develop a synthesis of systematic reviews of health impacts of climate change by mapping the characteristics and findings of studies exploring the relationship between climate change and health. We identified four key findings.

First, meteorological impacts, mostly related to temperature and humidity, were the most common impacts studied by included publications, which aligns with findings from a previous scoping review on the health impacts of climate change in the Philippines. 10 Indeed, meteorological factors’ impact on all health outcomes identified in this review are explored, although some health outcomes are more rarely explored (eg, mental health and nutritional outcomes). Although this may not be surprising given that a key implication of climate change is the long-term meteorological impact of temperature rise, this finding suggests we also need to undertake research focused on other climate impacts on health, including potential direct and indirect effects of temperature rise, such as the impact of droughts and wildfire smoke. This will allow us to better prepare for the health crises that arise from these ever-increasing climate-related impacts. For instance, the impacts of extreme weather events and air quality on certain health outcomes are not explored (eg, skin diseases and allergies, occupational health) or only rarely explored (eg, pregnancy outcomes).

Second, systematic reviews primarily focus on physical health outcomes, such as infectious diseases, mortality, and respiratory, cardiovascular and neurological outcomes, which also aligns with the country-specific previous scoping review. 10 Regarding mortality, we support Campbell and colleagues’ 34 suggestion that we should expand our focus to include other types of health outcomes. This will provide better support for mitigation policies and allow us to adapt to the full range of threats of climate change.

Moreover, it is unclear whether the distribution of frequencies of health outcomes reflects the actual burden of health impacts of climate change. The most commonly studied health outcomes do not necessarily reflect the definition of health presented by the WHO as, ‘a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity’. 20 This suggests that future studies should investigate in greater depth the impacts of climate change on mental and broader social well-being. Indeed, some reviews suggested that climate change impacts psychological and social well-being, via broader consequences, such as political instability, health system capacity, migration, and crime, 3 4 35 36 thus illustrating how our personal health is determined not only by biological and environmental factors but also by social and health systems. The importance of expanding our scope of health in this field is also recognised in the most recent Lancet report, which states that future reports will include a new mental health indicator. 2

Interestingly, the reviews that explored the mental health impacts of climate change were focused mostly on the direct and immediate impacts of experiencing extreme weather events. However, psychologists are also warning about the long-term indirect mental health impacts of climate change, which are becoming more prevalent for children and adults alike (eg, eco-anxiety, climate depression). 37 38 Even people who do not experience direct climate impacts, such as extreme weather events, report experiencing distressing emotions when thinking of the destruction of our environment or when worrying about one’s uncertain future and the lack of actions being taken. To foster emotional resilience in the face of climate change, these mental health impacts of climate change need to be further explored. Humanity’s ability to adapt to and mitigate climate change ultimately depends on our emotional capacity to face this threat.

Third, there is a notable geographical difference in the country affiliations of first authors, with three quarters of systematic reviews having been led by first authors affiliated to institutions in Europe, Australia, or North America, which aligns with the findings of the most recent Lancet report. 2 While perhaps unsurprising given the inequalities in research funding and institutions concentrated in Western countries, this is of critical importance given the significant health impacts that are currently faced (and will remain) in other parts of the world. Research funding organisations should seek to provide more resources to authors in low-income to middle-income countries to ensure their expertise and perspectives are better represented in the literature.

Fourth, overall, most reviews suggest an association between climate change and the deterioration of health in various ways, illustrating the interdependence of our health and well-being with the well-being of our environment. This interdependence may be direct (eg, heat’s impact on dehydration and exhaustion) or indirect (eg, via behaviour change due to heat.) The most frequently explored and consistently supported associations include an association between temperature and humidity with infectious diseases, mortality and adverse respiratory, cardiovascular and neurological outcomes. Other less frequently studied but consistent associations include associations between climate impacts and increased use of healthcare services, some adverse mental health outcomes, adverse nutritional outcomes and adverse occupational health outcomes. These associations support key findings of the most recent Lancet report, in which authors report, among others, increasing heat exposure being associated with increasing morbidities and mortality, climate change leading to food insecurity and undernutrition, and to an increase in infectious disease transmission. 2

That said, a number of reviews included in this study reported limited, conflicting and/or an absence of evidence regarding the association between the climate impact and health outcome. For instance, there was conflicting or limited evidence concerning the association between extreme weather events and infectious diseases, cardiorespiratory outcomes and some mental health outcomes and the association between air quality and cardiovascular-specific mortality and adverse birth outcomes. These conflicting and limited findings highlight the need for further research. These associations are complex and there exist important methodological challenges inherent to exploring the causal relationship between climate change and health outcomes. This relationship may at times be indirect and likely determined by multiple interacting factors.

The climate-health link has been the target of more research in recent years and it is also receiving increasing attention from the public and in both public health and climate communication literature. 2 39–41 However, the health framing of climate change information is still underused in climate communications, and researchers suggest we should be doing more to make the link between human health and climate change more explicit to increase engagement with the climate crisis. 2 41–43 The health framing of climate communication also has implications for healthcare professionals 44 and policy-makers, as these actors could play a key part in climate communication, adaptation and mitigation. 41 42 45 These key stakeholders’ perspectives on the climate-health link, as well as their perceived role in climate adaptation and mitigation could be explored, 46 since research suggests that health professionals are important voices in climate communications 44 and especially since, ultimately, these adverse health outcomes will engender pressure on and cost to our health systems and health workers.

Strengths and limitations

To the best of our knowledge, the current study provides the first broad overview of previous systematic reviews exploring the health impacts of climate change. Our review has three main strengths. First, by targeting systematic reviews, we achieve a higher order summary of findings than what would have been possible by consulting individual original studies. Second, by synthesising findings across all included studies and according to the combination of climate impact and health outcome, we offer a clear, detailed and unique summary of the current state of evidence and knowledge gaps about how climate change may influence human health. This summary may be of use to researchers, policy-makers and communities. Third, we included studies published in all languages about any climate impact and any health outcome. In doing so, we provide a comprehensive and robust overview.

Our work has four main limitations. First, we were unable to access some full texts and therefore some studies were excluded, even though we deemed them potentially relevant after title and abstract inspection. Other potentially relevant systematic reviews may be missing due to unseen flaws in our systematic search. Second, due to the heterogeneity of the included systematic reviews and the relatively small proportion of studies reporting meta-analytic findings, we could not conduct meta-meta-analyses of findings across reviews. Future research is needed to quantify the climate and health links described in this review, as well as to investigate the causal relationship and other interacting factors. Third, due to limited resources, we did not assess overlap between the included reviews concerning the studies they included. Frequencies and findings should be interpreted with potential overlap in mind. Fourth, we conducted the systematic search of the literature in June 2019, and it is therefore likely that some recent systematic reviews are not included in this study.

Overall, most systematic reviews of the health impacts of climate change suggest an association between climate change and the deterioration of health in multiple ways, generally in the direction that climate change is associated with adverse human health outcomes. This is worrisome since these outcomes are predicted to rise in the near future, due to the rise in temperature and increase in climate-change-related events such as extreme weather events and worsened air quality. Most studies included in this review focused on meteorological impacts of climate change on adverse physical health outcomes. Future studies could fill knowledge gaps by exploring other climate-related impacts and broader psychosocial health outcomes. Moreover, studies on health impacts of climate change have mostly been conducted by first authors affiliated with institutions in high-income countries. This inequity needs to be addressed, considering that the impacts of climate change are and will continue to predominantly impact lower income countries. Finally, although most reviews also recommend more research to better understand and quantify these associations, to adapt to and mitigate climate change’s impacts on health, it will also be important to unpack the ‘what, how, and where’ of these effects. Health effects of climate change are unlikely to be distributed equally or randomly through populations. It will be important to mitigate the changing climate’s potential to exacerbate health inequities.

Supplementary Material

Acknowledgments.

The authors gratefully acknowledge the contributions of Selma Chipenda Dansokho, as research associate, and Thierry Provencher, as research assistant, to this project, and of Frederic Bergeron, for assistance with search strategy, screening and selection of articles for the systematic review.

Twitter: @RutNdjab, @ATricco, @hwitteman

Contributors: RN, CF, ACT, HOW contributed to the design of the study. CB, RN, LPB, RAPR and HOW contributed to the systematic search of the literature and selection of studies. RR, HOW, LC conducted data analysis and interpretation. RR and HOW drafted the first version of the article with early revision by CB, LC and RN. All authors critically revised the article and approved the final version for submission for publication. RR and HOW had full access to all the data in the study and had final responsibility for the decision to submit for publication.

Funding: This study was funded by the Canadian Institutes of Health Research (CIHR) FDN-148426. The CIHR had no role in determining the study design, the plans for data collection or analysis, the decision to publish, nor the preparation of this manuscript. ACT is funded by a Tier 2 Canada Research Chair in Knowledge Synthesis. HOW is funded by a Tier 2 Canada Research Chair in Human-Centred Digital Health.

Competing interests: None declared.

Provenance and peer review: Not commissioned; externally peer reviewed.

Supplemental material: This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

Data availability statement

Data sharing not applicable as no datasets generated and/or analysed for this study. All data relevant to the study are included in the article or uploaded as supplementary information. Additional data are not available.

Ethics statements

Patient consent for publication.

Not required.

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Scientific Consensus: Earth's Climate Is Warming

Temperature data showing rapid warming in the past few decades, the latest data going up to 2022. According to NASA, 2016 and 2020 are tied for the warmest year since 1880, continuing a long-term trend of rising global temperatures. On top of that, the nine most recent years have been the hottest. Credit: NASA's Goddard Institute for Space Studies

It’s important to remember that scientists always focus on the evidence, not on opinions. Scientific evidence continues to show that human activities ( primarily the human burning of fossil fuels ) have warmed Earth’s surface and its ocean basins, which in turn have continued to impact Earth’s climate . This is based on over a century of scientific evidence forming the structural backbone of today's civilization.

NASA Global Climate Change presents the state of scientific knowledge about climate change while highlighting the role NASA plays in better understanding our home planet. This effort includes citing multiple peer-reviewed studies from research groups across the world, 1 illustrating the accuracy and consensus of research results (in this case, the scientific consensus on climate change) consistent with NASA’s scientific research portfolio.

With that said, multiple studies published in peer-reviewed scientific journals 1 show that climate-warming trends over the past century are extremely likely due to human activities. In addition, most of the leading scientific organizations worldwide have issued public statements endorsing this position. The following is a partial list of these organizations, along with links to their published statements and a selection of related resources.

AMERICAN SCIENTIFIC SOCIETIES

Statement on climate change from 18 scientific associations.

"Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research demonstrates that the greenhouse gases emitted by human activities are the primary driver." (2009) 2

"Based on well-established evidence, about 97% of climate scientists have concluded that human-caused climate change is happening." (2014) 3

"The Earth’s climate is changing in response to increasing concentrations of greenhouse gases (GHGs) and particulate matter in the atmosphere, largely as the result of human activities." (2016-2019) 4

"Based on extensive scientific evidence, 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. There is no alterative explanation supported by convincing evidence." (2019) 5

"Our AMA ... supports the findings of the Intergovernmental Panel on Climate Change’s fourth assessment report and concurs with the scientific consensus that the Earth is undergoing adverse global climate change and that anthropogenic contributions are significant." (2019) 6

"Research has found a human influence on the climate of the past several decades ... The IPCC (2013), USGCRP (2017), and USGCRP (2018) indicate that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-twentieth century." (2019) 7

"Earth's changing climate is a critical issue and poses the risk of significant environmental, social and economic disruptions around the globe. While natural sources of climate variability are significant, multiple lines of evidence indicate that human influences have had an increasingly dominant effect on global climate warming observed since the mid-twentieth century." (2015) 8

"The Geological Society of America (GSA) concurs with assessments by the National Academies of Science (2005), the National Research Council (2011), the Intergovernmental Panel on Climate Change (IPCC, 2013) and the U.S. Global Change Research Program (Melillo et al., 2014) that global climate has warmed in response to increasing concentrations of carbon dioxide (CO2) and other greenhouse gases ... Human activities (mainly greenhouse-gas emissions) are the dominant cause of the rapid warming since the middle 1900s (IPCC, 2013)." (2015) 9

SCIENCE ACADEMIES

International academies: joint statement.

"Climate change is real. There will always be uncertainty in understanding a system as complex as the world’s climate. However there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001)." (2005, 11 international science academies) 10

"Scientists have known for some time, from multiple lines of evidence, that humans are changing Earth’s climate, primarily through greenhouse gas emissions." 11

U.S. GOVERNMENT AGENCIES

"Earth’s climate is now changing faster than at any point in the history of modern civilization, primarily as a result of human activities." (2018, 13 U.S. government departments and agencies) 12

INTERGOVERNMENTAL BODIES

“It is unequivocal that the increase of CO 2 , methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the principal driver of many changes observed across the atmosphere, ocean, cryosphere, and biosphere. “Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact.” 13-17

OTHER RESOURCES

List of worldwide scientific organizations.

The following page lists the nearly 200 worldwide scientific organizations that hold the position that climate change has been caused by human action. http://www.opr.ca.gov/facts/list-of-scientific-organizations.html

U.S. Agencies

The following page contains information on what federal agencies are doing to adapt to climate change. https://www.c2es.org/site/assets/uploads/2012/02/climate-change-adaptation-what-federal-agencies-are-doing.pdf

Technically, a “consensus” is a general agreement of opinion, but the scientific method steers us away from this to an objective framework. In science, facts or observations are explained by a hypothesis (a statement of a possible explanation for some natural phenomenon), which can then be tested and retested until it is refuted (or disproved).

As scientists gather more observations, they will build off one explanation and add details to complete the picture. Eventually, a group of hypotheses might be integrated and generalized into a scientific theory, a scientifically acceptable general principle or body of principles offered to explain phenomena.

References​

• K. Myers, et al, "Consensus revisited: quantifying scientific agreement on climate change and climate expertise among Earth scientists 10 years later", Environmental Research Letters Vol.16 No. 10, 104030 (20 October 2021); DOI:10.1088/1748-9326/ac2774 M. Lynas, et al, " Greater than 99% consensus on human caused climate change in the peer-reviewed scientific literature ", Environmental Research Letters Vol.16 No. 11, 114005 (19 October 2021); DOI:10.1088/1748-9326/ac2966 J. Cook et al., "Consensus on consensus: a synthesis of consensus estimates on human-caused global warming", Environmental Research Letters Vol. 11 No. 4, (13 April 2016); DOI:10.1088/1748-9326/11/4/048002 J. Cook et al., "Quantifying the consensus on anthropogenic global warming in the scientific literature", Environmental Research Letters Vol. 8 No. 2, (15 May 2013); DOI:10.1088/1748-9326/8/2/024024 W. R. L. Anderegg, “Expert Credibility in Climate Change”, Proceedings of the National Academy of Sciences Vol. 107 No. 27, 12107-12109 (21 June 2010); DOI: 10.1073/pnas.1003187107 P. T. Doran & M. K. Zimmerman, "Examining the Scientific Consensus on Climate Change", Eos Transactions American Geophysical Union Vol. 90 Issue 3 (2009), 22; DOI: 10.1029/2009EO030002 N. Oreskes, “Beyond the Ivory Tower: The Scientific Consensus on Climate Change”, Science Vol. 306 no. 5702, p. 1686 (3 December 2004); DOI: 10.1126/science.1103618
• Statement on climate change from 18 scientific associations (2009)
• AAAS Board Statement on Climate Change (2014)
• ACS Public Policy Statement: Climate Change (2016-2019)
• Society Must Address the Growing Climate Crisis Now (2019)​
• Global Climate Change and Human Health (2019)
• Climate Change: An Information Statement of the American Meteorological Society (2019)
• American Physical Society (2021)
• GSA Position Statement on Climate Change (2015)
• Joint science academies' statement: Global response to climate change (2005)
• Climate at the National Academies
• Fourth National Climate Assessment: Volume II (2018)
• IPCC Fifth Assessment Report, Summary for Policymakers, SPM 1.1 (2014)
• IPCC Fifth Assessment Report, Summary for Policymakers, SPM 1 (2014)
• IPCC Sixth Assessment Report, Working Group 1 (2021)
• IPCC Sixth Assessment Report, Working Group 2 (2022)
• IPCC Sixth Assessment Report, Working Group 3 (2022)

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10 Big Findings from the 2023 IPCC Report on Climate Change

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March 20 marked the release of the final installment of the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6) , an eight-year long undertaking from the world’s most authoritative scientific body on climate change. Drawing on the findings of 234 scientists on the  physical science of climate change , 270 scientists on  impacts, adaptation and vulnerability to climate change , and 278 scientists on  climate change mitigation , this  IPCC synthesis report  provides the most comprehensive, best available scientific assessment of climate change.

It also makes for grim reading. Across nearly 8,000 pages, the AR6 details the devastating consequences of rising greenhouse gas (GHG) emissions around the world — the destruction of homes, the loss of livelihoods and the fragmentation of communities, for example — as well as the increasingly dangerous and irreversible risks should we fail to change course.

But the IPCC also offers hope, highlighting pathways to avoid these intensifying risks. It identifies readily available, and in some cases, highly cost-effective actions that can be undertaken now to reduce GHG emissions, scale up carbon removal and build resilience. While the window to address the climate crisis is rapidly closing, the IPCC affirms that we can still secure a safe, livable future.

Here are 10 key findings you need to know:

1. Human-induced global warming of 1.1 degrees C has spurred changes to the Earth’s climate that are unprecedented in recent human history.

Already, with 1.1 degrees C (2 degrees F) of global temperature rise, changes to the climate system that are unparalleled over centuries to millennia are now occurring in every region of the world, from rising sea levels to more extreme weather events to rapidly disappearing sea ice.

Additional warming will increase the magnitude of these changes. Every 0.5 degree C (0.9 degrees F) of global temperature rise, for example, will cause clearly discernible increases in the frequency and severity of heat extremes, heavy rainfall events and regional droughts. Similarly, heatwaves that, on average, arose once every 10 years in a climate with little human influence will likely occur 4.1 times more frequently with 1.5 degrees C (2.7 degrees F) of warming, 5.6 times with 2 degrees C (3.6 degrees F) and 9.4 times with 4 degrees C (7.2 degrees F) — and the intensity of these heatwaves will also increase by 1.9 degrees C (3.4 degrees F), 2.6 degrees C (4.7 degrees F) and 5.1 degrees C (9.2 degrees F) respectively.

Rising global temperatures also heighten the probability of reaching dangerous tipping points in the climate system that, once crossed, can trigger self-amplifying feedbacks that further increase global warming, such as thawing permafrost or massive forest dieback. Setting such reinforcing feedbacks in motion can also lead to other substantial, abrupt and irreversible changes to the climate system. Should warming reach between 2 degrees C (3.6 degrees F) and 3 degrees C (5.4 degrees F), for example, the West Antarctic and Greenland ice sheets could melt almost completely and irreversibly over many thousands of years, causing sea levels to rise by several meters.

2. Climate impacts on people and ecosystems are more widespread and severe than expected, and future risks will escalate rapidly with every fraction of a degree of warming.

Described as an “an atlas of human suffering and a damning indictment of failed climate leadership” by United Nations Secretary-General António Guterres, one of AR6’s most alarming conclusions is that adverse climate impacts are already more far-reaching and extreme than anticipated. About half of the global population currently contends with severe water scarcity for at least one month per year, while higher temperatures are enabling the spread of vector-borne diseases, such as malaria, West Nile virus and Lyme disease. Climate change has also slowed improvements in agricultural productivity in middle and low latitudes, with crop productivity growth shrinking by a third in Africa since 1961. And since 2008, extreme floods and storms have forced over 20 million people from their homes every year.

Every fraction of a degree of warming will intensify these threats, and even limiting global temperature rise to 1.5 degree C is not safe for all. At this level of warming, for example, 950 million people across the world’s drylands will experience water stress, heat stress and desertification, while the share of the global population exposed to flooding will rise by 24%.

Similarly, overshooting 1.5 degrees C (2.7 degrees F), even temporarily, will lead to much more severe, oftentimes irreversible impacts, from local species extinctions to the complete drowning of salt marshes to loss of human lives from increased heat stress. Limiting the magnitude and duration of overshooting 1.5 degrees C (2.7 degrees F), then, will prove critical in ensuring a safe, livable future, as will holding warming to as close to 1.5 degrees C (2.7 degrees F) or below as possible. Even if this temperature limit is exceeded by the end of the century, the imperative to rapidly curb GHG emissions to avoid higher levels of warming and associated impacts remains unchanged.

3. Adaptation measures can effectively build resilience, but more finance is needed to scale solutions.

Climate policies in at least 170 countries now consider adaptation, but in many nations, these efforts have yet to progress from planning to implementation. Measures to build resilience are still largely small-scale, reactive and incremental, with most focusing on immediate impacts or near-term risks. This disparity between today’s levels of adaptation and those required persists in large part due to limited finance. According to the IPCC, developing countries alone will need $127 billion per year by 2030 and $295 billion per year by 2050 to adapt to climate change. But funds for adaptation reached just $23 billion to $46 billion from 2017 to 2018, accounting for only 4% to 8% of tracked climate finance.

The good news is that the IPCC finds that, with sufficient support, proven and readily available adaptation solutions can build resilience to climate risks and, in many cases, simultaneously deliver broader sustainable development benefits.

Ecosystem-based adaptation, for example, can help communities adapt to impacts that are already devastating their lives and livelihoods, while also safeguarding biodiversity, improving health outcomes, bolstering food security, delivering economic benefits and enhancing carbon sequestration. Many ecosystem-based adaptation measures — including the protection, restoration and sustainable management of ecosystems, as well as more sustainable agricultural practices like integrating trees into farmlands and increasing crop diversity — can be implemented at relatively low costs today. Meaningful collaboration with Indigenous Peoples and local communities is critical to the success of this approach, as is ensuring that ecosystem-based adaptation strategies are designed to account for how future global temperature rise will impact ecosystems.

4. Some climate impacts are already so severe they cannot be adapted to, leading to losses and damages.

Around the world, highly vulnerable people and ecosystems are already struggling to adapt to climate change impacts. For some, these limits are “soft” — effective adaptation measures exist, but economic, political and social obstacles constrain implementation, such as lack of technical support or inadequate funding that does not reach the communities where it’s needed most. But in other regions, people and ecosystems already face or are fast approaching “hard” limits to adaptation, where climate impacts from 1.1 degrees C (2 degrees F) of global warming are becoming so frequent and severe that no existing adaptation strategies can fully avoid losses and damages. Coastal communities in the tropics, for example, have seen entire coral reef systems that once supported their livelihoods and food security experience widespread mortality, while rising sea levels have forced other low-lying neighborhoods to move to higher ground and abandon cultural sites. 

Whether grappling with soft or hard limits to adaptation, the result for vulnerable communities is oftentimes irreversible and devastating. Such losses and damages will only escalate as the world warms. Beyond 1.5 degrees C (2.7 degrees F) of global temperature rise, for example, regions reliant on snow and glacial melt will likely experience water shortages to which they cannot adapt. At 2 degrees C (3.6 degrees F), the risk of concurrent maize production failures across important growing regions will rise dramatically. And above 3 degrees C (5.4 degrees F), dangerously high summertime heat will threaten the health of communities in parts of southern Europe.

Urgent action is needed to avert, minimize and address these losses and damages. At COP27, countries took a critical step forward by agreeing to establish funding arrangements for loss and damage, including a dedicated fund. While this represents  a historic breakthrough  in the climate negotiations, countries must now figure out the details of what these funding arrangements, as well as the new fund , will look like in practice — and it’s these details that will ultimately determine the adequacy, accessibility, additionality and predictability of these financial flows to those experiencing loss and damage.

5. Global GHG emissions peak before 2025 in 1.5 degrees C-aligned pathways.

The IPCC finds that there is a more than 50% chance that global temperature rise will reach or surpass 1.5 degrees C (2.7 degrees F) between 2021 and 2040 across studied scenarios, and under a high-emissions pathway, specifically, the world may hit this threshold even sooner — between 2018 and 2037. Global temperature rise in such a carbon-intensive scenario could also increase to 3.3 degrees C to 5.7 degrees C (5.9 degrees F to 10.3 degrees F) by 2100. To put this projected amount of warming into perspective, the last time global temperatures exceeded 2.5 degrees C (4.5 degrees F) above pre-industrial levels was more than 3 million years ago.

Changing course to limit global warming to 1.5 degrees C (2.7 degrees F) — with no or limited overshoot — will instead require deep GHG emissions reductions in the near-term. In modelled pathways that limit global warming to this goal, GHG emissions peak immediately and before 2025 at the latest. They then drop rapidly, declining 43% by 2030 and 60% by 2035, relative to 2019 levels.

While there are some bright spots — the annual growth rate of GHG emissions slowed from an average of 2.1% per year between 2000 and 2009 to 1.3% per year between 2010 and 2019, for example — global progress in mitigating climate change remains woefully off track. GHG emissions have climbed steadily over the past decade, reaching 59 gigatonnes of carbon dioxide equivalent (GtCO2e) in 2019 — approximately 12% higher than in 2010 and 54% greater than in 1990.

Even if countries achieved their climate pledges (also known as nationally determined contributions or NDCs),  WRI research  finds that they would reduce GHG emissions by just 7% from 2019 levels by 2030, in contrast to the 43% associated with limiting temperature rise to 1.5 degrees C (2.7 degrees F). And while handful of countries have submitted  new or enhanced NDCs  since the IPCC’s cut-off date,  more recent analysis  that takes these submissions into account finds that these commitments collectively still fall short of closing this emissions gap.

6. The world must rapidly shift away from burning fossil fuels — the number one cause of the climate crisis.

In pathways limiting warming to 1.5 degrees C (2.7 degrees F) with no or limited overshoot just a net 510 GtCO2 can be emitted before carbon dioxide emissions reach net zero in the early 2050s. Yet future carbon dioxide emissions from existing and planned fossil fuel infrastructure alone could surpass that limit by 340 GtCO2, reaching 850 GtCO2.

A mix of strategies can help avoid  locking in  these emissions, including retiring existing fossil fuel infrastructure, canceling new projects, retrofitting fossil-fueled power plants with carbon capture and storage (CCS) technologies and scaling up renewable energy sources like solar and wind (which are now cheaper than fossil fuels in many regions).

In pathways that limit warming to 1.5 degrees C (2.7 degrees F) — with no or limited overshoot — for example, global use of coal falls by 95% by 2050, oil declines by about 60% and gas by about 45%. These figures assume significant use of abatement technologies like CCS, and without them, these same pathways show much steeper declines by mid-century. Global use of coal without CCS, for example, is virtually phased out by 2050.

Although coal-fired power plants are starting to be retired across Europe and the United States, some multilateral development banks continue to invest in new coal capacity. Failure to change course risks stranding assets worth trillions of dollars.

7. We also need urgent, systemwide transformations to secure a net-zero, climate-resilient future.

While fossil fuels are the number one source of GHG emissions, deep emission cuts are necessary across all of society to combat the climate crisis. Power generation, buildings, industry, and transport are responsible for close to 80% of global emissions while agriculture, forestry and other land uses account for the remainder.

Take the  transport system , for instance. Drastically cutting emissions will require urban planning that minimizes the need for travel, as well as the build-out of shared, public and nonmotorized transport, such as rapid transit and bicycling in cities. Such a transformation will also entail increasing the supply of electric passenger vehicles, commercial vehicles and buses, coupled with wide-scale installation of rapid-charging infrastructure, investments in zero-carbon fuels for shipping and aviation and more.

Policy measures that make these changes less disruptive can help accelerate needed transitions, such as subsidizing zero-carbon technologies and taxing high-emissions technologies like fossil-fueled cars. Infrastructure design — like reallocating street space for sidewalks or bike lanes — can help people transition to lower-emissions lifestyles. It is important to note there are many co-benefits that accompany these transformations, too. Minimizing the number of passenger vehicles on the road, in this example, reduces harmful local air pollution and cuts traffic-related crashes and deaths.

Explore Systems Change Lab

Systems Change Lab  monitors, learns from and mobilizes action to achieve the far-reaching transformational shifts needed to limit global warming to 1.5 degrees C, halt biodiversity loss and build a just and equitable economy.

Transformative adaptation measures, too, are critical for securing a more prosperous future. The IPCC emphasizes the importance of ensuring that adaptation measures drive systemic change, cut across sectors and are distributed equitably across at-risk regions. The good news is that there are oftentimes strong synergies between transformational mitigation and adaptation. For example, in the global food system, climate-smart agriculture practices like shifting to  agroforestry  can improve resilience to climate impacts, while simultaneously advancing mitigation.  

8. Carbon removal is now essential to limit global temperature rise to 1.5 degrees C.

Deep decarbonization across all systems while building resilience won’t be enough to achieve global climate goals, though. The IPCC finds that all pathways that limit warming to 1.5 degrees C (2.7 degrees F) — with no or limited overshoot — depend on some quantity of  carbon removal . These approaches encompass both natural solutions, such as sequestering and storing carbon in trees and soil, as well as more nascent technologies that pull carbon dioxide directly from the air.

Hover over each carbon removal approach to learn more:

Note: This figure includes carbon removal approaches mentioned in countries' long-term climate strategies as well as other leading proposed approaches. The natural/biotic vs. technological/abiotic categorization shown here is illustrative rather than definitive and will vary depending on how approaches are applied, particularly for carbon removal approaches in the ocean.

The amount of carbon removal required depends on how quickly we reduce GHG emissions across other systems and the extent to which climate targets are overshot, with estimates ranging from between 5 GtCO2 to 16 GtCO2 per year needed by mid-century.

All carbon removal approaches have merits and drawbacks. Reforestation, for instance, represents a readily available, relatively low-cost strategy that, when implemented appropriately, can deliver a wide range of benefits to communities. Yet the carbon stored within these ecosystems is also vulnerable to disturbances like wildfires, which may increase in frequency and severity with additional warming. And, while technologies like bioenergy with carbon capture and storage (BECCS) may offer a more permanent solution, such approaches also risk displacing croplands, and in doing so, threatening food security. Responsibly researching, developing and deploying emerging carbon removal technologies, alongside existing natural approaches, will therefore require careful understanding of each solution’s unique benefits, costs and risks.

9. Climate finance for both mitigation and adaptation must increase dramatically this decade.

The IPCC finds that public and private finance flows for fossil fuels today far surpass those directed toward climate mitigation and adaptation. Thus, while annual public and private climate finance has risen by upwards of 60% since the IPCC’s Fifth Assessment Report, much more is still required to achieve global climate change goals. For instance, climate finance will need to increase between 3 and 6 times by 2030 to achieve mitigation goals, alone.

This gap is widest in developing countries, particularly those already struggling with debt, poor credit ratings and economic burdens from the COVID-19 pandemic. Recent mitigation investments, for example, need to increase by at least sixfold in Southeast Asia and developing countries in the Pacific, fivefold in Africa and fourteenfold in the Middle East by 2030 to hold warming below 2 degrees C (3.6 degrees F). And across sectors, this shortfall is most pronounced for agriculture, forestry and other land use, where recent financial flows are 10 to 31 times below what is required to achieve the Paris Agreement’s goals.

Finance for adaptation, as well as loss and damage, will also need to rise dramatically. Developing countries, for example, will need $127 billion per year by 2030 and $295 billion per year by 2050. While AR6 does not assess countries’ needs for finance to avert, minimize and address losses and damages,  recent estimates  suggest that they will be substantial in the coming decades. Current funds for both fall well below estimated needs, with the highest estimates of adaptation finance totaling under $50 billion per year.

10. Climate change — as well as our collective efforts to adapt to and mitigate it — will exacerbate inequity should we fail to ensure a just transition.  

Households with incomes in the top 10%, including a relatively large share in developed countries, emit upwards of 45% of the world's GHGs, while those families earning in the bottom 50% account for 15% at most. Yet the effects of climate change already — and will continue to — hit poorer, historically marginalized communities the hardest.

Today, between 3.3 billion and 3.6 billion people live in countries that are highly vulnerable to climate impacts, with global hotspots concentrated in the Arctic, Central and South America, Small Island Developing states, South Asia and much of sub-Saharan Africa. Across many countries in these regions, conflict, existing inequalities and development challenges (e.g., poverty and limited access to basic services like clean water) not only heighten sensitivity to climate hazards, but also limit communities’ capacity to adapt.  Mortality from storms, floods and droughts, for instance, was 15 times higher in countries with high vulnerability to climate change than in those with very low vulnerability from 2010 to 2020.

At the same time, efforts to mitigate climate change also risk disruptive changes and exacerbating inequity. Retiring coal-fired power plants, for instance, may displace workers, harm local economies and reconfigure the social fabric of communities, while inappropriately implemented efforts to halt deforestation could heighten poverty and intensify food insecurity. And certain climate policies, such as  carbon taxes  that raise the cost of emissions-intensive goods like gasoline, can also prove to be regressive, absent of efforts to recycle the revenues raised from these taxes back into programs that benefit low-income communities.

Fortunately, the IPCC identifies a range of measures that can support a just transition and help ensure that no one is left behind as the world moves toward a net-zero-emissions, climate-resilient future. Reconfiguring social protection programs (e.g., cash transfers, public works programs and social safety nets) to include adaptation, for example, can reduce communities’ vulnerability to a wide range of future climate impacts, while strengthening justice and equity. Such programs are particularly effective when paired with efforts to expand access to infrastructure and basic services.

Similarly, policymakers can design mitigation strategies to better distribute the costs and benefits of reducing GHG emissions. Governments can pair efforts to phase out coal-fired electricity generation, for instance, with subsidized job retraining programs that support workers in developing the skills needed to secure new, high-quality jobs. Or, in another example, officials can couple policy interventions dedicated to expanding access to public transit with interventions to improve access to nearby, affordable housing.

Across both mitigation and adaptation measures, inclusive, transparent and participatory decision-making processes will play a central role in ensuring a just transition. More specifically, these forums can help cultivate public trust, deepen public support for transformative climate action and avoid unintended consequences.

Looking Ahead

The IPCC’s AR6 makes clear that risks of inaction on climate are immense and the way ahead requires change at a scale not seen before. However, this report also serves as a reminder that we have never had more information about the gravity of the climate emergency and its cascading impacts — or about what needs to be done to reduce intensifying risks.

Limiting global temperature rise to 1.5 degrees C (2.7 degrees F) is still possible, but only if we act immediately. As the IPCC makes clear, the world needs to peak GHG emissions before 2025 at the very latest, nearly halve GHG emissions by 2030 and reach net-zero CO2 emissions around mid-century, while also ensuring a just and equitable transition. We’ll also need an all-hands-on-deck approach to guarantee that communities experiencing increasingly harmful impacts of the climate crisis have the resources they need to adapt to this new world. Governments, the private sector, civil society and individuals must all step up to keep the future we desire in sight. A narrow window of opportunity is still open, but there’s not one second to waste.

Note: In addition to showcasing findings from the IPCC’s AR6 Synthesis Report, this article also draws on previous articles detailing the IPCC’s findings on  the physical science of climate change ,  impacts, adaption and vulnerability ,  and  climate change mitigation .

Relevant Work

6 takeaways from the 2022 ipcc climate change mitigation report, 6 big findings from the ipcc 2022 report on climate impacts, adaptation and vulnerability, 5 big findings from the ipcc’s 2021 climate report, 8 things you need to know about the ipcc 1.5˚c report.

Join us on March 23 for a high-level webinar featuring IPCC authors, government representatives and leading carbon removal experts to discuss how carbon removal is a critical tool in our toolbox to address the climate crisis.

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