Key findings of AR6 Summary for Policymakers

by the World Health Organization (WHO) Climate & Health Program staff

The human fingerprint of climate change

The science is unequivocal: the heating of the planet is caused by human activities. Anthropogenic climate change has caused warmer temperatures, rising rainfall and storms, the retreat of glaciers, sea level rise, the shifting of climate zones, among others:
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. [A.1]
Each of the last four decades has been successively warmer than any decade that preceded it since 1850. [A.1.2]

Climate Impacts

We are entering a period of unprecedented change:
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. [A.2]

Climate impacts are here and happening now. The rise in extreme weather events, such as heatwaves, is caused by climate change:
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 AR5. [A.3]
It is virtually certain that hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe, with high confidence that human-induced climate change is the main driver14 of these changes. Some recent hot extremes observed over the past decade would have been extremely unlikely to occur without human influence on the climate system. [A.3.1]

Climate change is already affecting every region in the world today:
Climate change is already affecting every inhabited region across the globe with human influence contributing to many observed changes in weather and climate extremes [Figure SPM.3]

Every fraction of a degree matters. Every bit of warming makes our planet more unsafe:
Many changes in the climate system become larger in direct relation to increasing global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, proportion of intense tropical cyclones as well as reductions in Arctic sea ice, snow cover and permafrost. [B.2]

With every additional increment of global warming, changes in extremes continue to become larger. For example, every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves (very likely), and heavy precipitation (high confidence), as well as agricultural and ecological droughts 30 in some regions (high confidence). [B.2.2]

There will be an increasing occurrence of some extreme events unprecedented in the observational record with additional global warming, even at 1.5°C of global warming. Projected percentage changes in frequency are higher for rarer events (high confidence). [B.2.2]
The Arctic is projected to experience the highest increase in the temperature of the coldest days, at about 3 times the rate of global warming (high confidence). [B.2.3]
At the global scale, extreme daily precipitation events are projected to intensify by about 7% for each 1°C of global warming (high confidence)[B.2.4]
The Arctic is likely to be practically sea ice free in September31 at least once before 2050 under the five illustrative scenarios considered in this report [B.2.5]
Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry Events. [B.3]

Every region will increasingly experience more severe climate impacts, including on human health (= climate-impact drivers, or CIDs):
All regions39 are projected to experience further increases in hot climatic impact-drivers (CIDs) and decreases in cold CIDs (high confidence). … These changes would be larger at 2°C global warming or above than at 1.5°C (high confidence). For example, extreme heat thresholds relevant to agriculture and health are projected to be exceeded more frequently at higher global warming levels (high confidence). [C.2.1]

Strongly reducing emissions now can avoid the worst climate impacts, including extreme heat and flooding:
…By the end of the century, scenarios with very low and low GHG emissions would strongly limit the change of several CIDs, such as the increase in the frequency of extreme sea level events, heavy precipitation and pluvial flooding, and exceedance of dangerous heat thresholds, while limiting the number of regions where such exceedances occur, relative to higher GHG emissions scenarios (high confidence)… [D. 2.4]

Cities are particularly vulnerable to climate impacts, with the urban heat island effect and increased flooding projected to get worse:
Cities intensify human-induced warming locally, and further urbanization together with more frequent hot extremes will increase the severity of heatwaves (very high confidence). Urbanization also increases mean and heavy precipitation over and/or downwind of cities (medium confidence) and resulting runoff intensity (high confidence). In coastal cities, the combination of more frequent extreme sea level events (due to sea level rise and storm surge) and extreme rainfall/riverflow events will make flooding more probable (high confidence). [C.2.6]

Tipping points & irreversible changes

The changes we have made, and are making, to our planet cannot be undone. They have already locked in (certain) irreversible changes.
Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level. [B.5]

Past GHG emissions since 1750 have committed the global ocean to future warming (high confidence). Over the rest of the 21st century, likely ocean warming ranges from 2–4 (SSP1-2.6) to 4–8 times (SSP5-8.5) the 1971–2018 change. Based on multiple lines of evidence, upper ocean stratification (virtually certain), ocean acidification (virtually certain) and ocean deoxygenation (high confidence) will continue to increase in the 21st century, at rates dependent on future emissions. Changes are irreversible on centennial to millennial time scales in global ocean temperature (very high confidence), deep ocean acidification (very high confidence) and deoxygenation (medium confidence). [B.5.1]

Mountain and polar glaciers are committed to continue melting for decades or centuries (very high confidence). Loss of permafrost carbon following permafrost thaw is irreversible at centennial timescales (high confidence). Continued ice loss over the 21st century is virtually certain for the Greenland Ice Sheet and likely for the Antarctic Ice Sheet. [B.5.2]

Tipping points are increasingly likely (= low-likelihood, high impact outcomes):
There is limited evidence for low-likelihood, high-impact outcomes(resulting from ice sheet instability processes characterized by deep uncertainty and in some cases involving tipping points) that would strongly increase ice loss from the Antarctic Ice Sheet for centuries under high GHG emissions scenarios34. [B.5.2]

Low-likelihood, high-impact outcomes22 could occur at global and regional scales even for global warming within the very likely range for a given GHG emissions scenario. The probability of low-likelihood, high impact outcomes increases with higher global warming levels (high confidence). Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out (high confidence). [C.3.2]

Several extreme events are increasingly happening at the same time(=compound extreme events):
Human influence has likely increased the chance of compound extreme events since the 1950s. This includes increases in the frequency of concurrent heatwaves and droughts on the global scale (high confidence); fire weather in some regions of all inhabited continents (medium confidence); and compound flooding in some locations (medium confidence). [A.3.5]
Compound extreme events are the combination of multiple drivers and/or hazards that contribute to societal or environmental risk. Examples are concurrent heatwaves and droughts, compound flooding (e.g., a storm surge in combination with extreme rainfall and/or river flow), compound fire weather conditions (i.e., a combination of hot, dry, and windy conditions), or concurrent extremes at different locations. [footnote 18]

With increasing global warming, we will likely experience events that are more extreme than anything we have experienced in recorded history:
If global warming increases, some compound extreme events18 with low likelihood in past and current climate will become more frequent, and there will be a higher likelihood that events with increased intensities, durations and/or spatial extents unprecedented in the observational record will occur (high confidence). [C.3.3]
The Atlantic Meridional Overturning Circulation is very likely to weaken over the 21st century for all emission scenarios. [C.3.4]

The different futures that lie ahead

The report provides a new generation of climate models (=Shared Socioeconomic Pathways or SSPs) with different scenarios from very high to very low GHG emissions by 2050 or 2100, and depending on different socio-economic scenarios and potential mitigation measures. [see SPM.1]

Unless transformational action is taken now, our emissions will cause the world to warm beyond 2°C ( = the Paris Agreement goal). Even under the most ambitious scenarios, the world is projected to (temporarily) breach the 1.5°C threshold:
Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered. Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades. [B.1]

The last time global surface temperature was sustained at or above 2.5°C higher than 1850–1900 was over 3 million years ago (medium confidence).[B.1.1]

Crossing the 2°C global warming level in the mid-term period (2041–2060) is very likely to occur under the very high GHG emissions scenario (SSP5-8.5), likely to occur under the high GHG emissions scenario (SSP3-7.0), and more likely than not to occur in the intermediate GHG emissions scenario (SSP2-4.5)26. [B.1.2]

Furthermore, for the very low GHG emissions scenario (SSP1-1.9), it is more likely than not that global surface temperature would decline back to below 1.5°C toward the end of the 21st century, with a temporary overshoot of no more than 0.1°C above 1.5°C global warming. [B.1.3]

There are limits to how much carbon can be absorbed by land and the ocean.
Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere. [B.4]

The magnitude of feedbacks between climate change and the carbon cycle becomes larger but also more uncertain in high CO2 emissions scenarios (very high confidence). … Additional ecosystem responses to warming not yet fully included in climate models, such as CO2 and CH4 fluxes from wetlands, permafrost thaw and wildfires, would further increase concentrations of these gases in the atmosphere (high confidence). [B.4.3]

A doubling of CO2 concentrations compared to pre-industrial levels will likely lead to 3°C of warming (= equilibrium climate sensitivity):
Improved knowledge of climate processes, paleoclimate evidence and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3°C with a narrower range compared to AR5. [A.4]

The new climate models and carbon budget now also take into account the effects of air pollution (and air pollution control measures) on future emissions:
Emissions vary between scenarios depending on socio-economic assumptions, levels of climate change mitigation and, for aerosols and non-methane ozone precursors, air pollution controls. Alternative assumptions may result in similar emissions and climate responses, but the socio-economic assumptions and the feasibility or likelihood of individual scenarios is not part of the assessment.[Box SPM.1.1]

Since AR5, estimates of remaining carbon budgets have been improved by a new methodology first presented in SR1.5, updated evidence, and the integration of results from multiple lines of evidence. A comprehensive range of possible future air pollution controls in scenarios is used to consistently assess the effects of various assumptions on projections of climate and air pollution. [Section D]

Global Carbon Budget

The new report provides an update to the carbon budget (although this estimate is slightly smaller than that in the IPCC Special Report on 1.5 in 2018, and slightly larger than the 2013 IPCC report, estimates of the carbon budget in the new generation of models have not changed very significantly):
Over the period 1850–2019, a total of 2390 ± 240 (likely range) GtCO2 of anthropogenic CO2 was emitted. Remaining carbon budgets have been estimated for several global temperature limits and various levels of probability, based on the estimated value of TCRE and its uncertainty, estimates of historical warming, variations in projected warming from non-CO2 emissions, climate system feedbacks such as emissions from thawing permafrost, and the global surface temperature change after global anthropogenic CO2 emissions reach net zero. [D.1.2]

Several factors that determine estimates of the remaining carbon budget have been re-assessed, and updates to these factors since SR1.5 are small. When adjusted for emissions since previous reports, estimates of remaining carbon budgets are therefore of similar magnitude compared to SR1.5 but larger compared to AR5 due to methodological improvements44. [D.1.3]

Reaching net-zero emissions

Reaching net-zero emissions would halt temperature rise relatively quickly, and is the only way to halt and reverse the current rise in surface temperatures.

From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. [D.1.]

Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C… This relationship implies that reaching net zero anthropogenic CO2 emissions is a requirement to stabilize human-induced global temperature increase at any level, but that limiting global temperature increase to a specific level would imply limiting cumulative CO2 emissions to within a carbon budget [D.1.1]

…Emissions pathways that reach and sustain net zero GHG emissions defined by the 100-year global warming potential are projected to result in a decline in surface temperature after an earlier peak (high confidence). [D.1.8]

Removing CO2 from the atmosphere is an important part of reaching net-zero emissions, but it can also have far-reaching consequences for water availability, food security and biodiversity:
Anthropogenic CO2 removal (CDR) has the potential to remove CO2 from the atmosphere and durably store it in reservoirs (high confidence). … CDR methods can have potentially wide-ranging effects on biogeochemical cycles and climate, which can either weaken or strengthen the potential of these methods to remove CO2 and reduce warming, and can also influence water availability and quality, food production and biodiversity45 (high confidence). [D.1.4]

Even if net-zero is reached, other climate impacts would still continue to play out for decades to millennia:
If global net negative CO2 emissions were to be achieved and be sustained, the global CO2-induced surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia (high confidence). For instance, it would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions (high confidence). [D.1.6]

Air Pollution

Mitigation of greenhouse gasses leads to rapid air quality improvements, but specific, separate targets to reduce air pollutants are needed:
Scenarios with low or very low GHG emissions (SSP1-1.9 and SSP1-2.6) lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality, relative to high and very high GHG emissions scenarios (SSP3-7.0 or SSP5-8.5). [D2]

..Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality. [D.1]

Reductions in GHG emissions also lead to air quality improvements. However, in the near term49, even in scenarios with strong reduction of GHGs, as in the low and very low GHG emission scenarios (SSP1-2.6 and SSP1-1.9), these improvements are not sufficient in many polluted regions to achieve air quality guidelines specified by the World Health Organization (high confidence). Scenarios with targeted reductions of air pollutant emissions lead to more rapid improvements in air quality within years compared to reductions in GHG emissions only, but from 2040, further improvements are projected in scenarios that combine efforts to reduce air pollutants as well as GHG emissions with the magnitude of the benefit varying between regions (high confidence). [D.2.2]

Air pollutants partly mask some of the warming that is already locked in (i.e. aerosols have a slight cooling effect on the atmosphere). A reduction of air pollution (=aerosols) could therefore lead to short-term warming, but this could be counterbalanced by enacting air pollution control measures and reductions in methane at the same time:
In the five illustrative scenarios, simultaneous changes in CH4, aerosol and ozone precursor emissions, that also contribute to air pollution, lead to a net global surface warming in the near and long-term (high confidence). In the long term, this net warming is lower in scenarios assuming air pollution controls combined with strong and sustained CH4 emission reductions (high confidence). … Because of the short lifetime of both CH4 and aerosols, these climate effects partially counterbalance each other and reductions in CH4 emissions also contribute to improved air quality by reducing global surface ozone (high confidence). [D.1.7]

The Covid-19 measures have led to a temporary drop in emissions and air pollution in 2020, but did not have a long-term impact on emissions:
Emissions reductions in 2020 associated with measures to reduce the spread of COVID-19 led to temporary but detectible effects on air pollution (high confidence), and an associated small, temporary increase in total radiative forcing, primarily due to reductions in cooling caused by aerosols arising from human activities (medium confidence). Global and regional climate responses to this temporary forcing are, however, undetectable above natural variability (high confidence). Atmospheric CO2 concentrations continued to rise in 2020, with no detectable decrease in the observed CO2 growth rate (medium confidence)48. [D.2.1]