| Global temperature |
E.1 |
Relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is projected to likely exceed 1.5C for RCP4.5, RCP6.0 and RCP8.5 (high confidence). Warming is likely to exceed 2C for RCP6.0 and RCP8.5 (high confidence), more likely than not to exceed 2C for RCP4.5 (high confidence), but unlikely to exceed 2C for RCP2.6 (medium confidence). Warming is unlikely to exceed 4C for RCP2.6, RCP4.5 and RCP6.0 (high confidence) and is about as likely as not to exceed 4C for RCP8.5 (medium confidence). |
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B.1.1 |
Compared to 1850–1900, global surface temperature averaged over 2081–2100 is very likely to be higher by 1.0C to 1.8C under the very low GHG emissions scenario considered (SSP1-1.9), by 2.1C to 3.5C in the intermediate scenario (SSP2-4.5) and by 3.3C to 5.7C under the very high GHG emissions scenario (SSP5-8.5). |
| Ocean warming |
E.1 |
The global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation. |
SROCC B.2.1 |
The ocean will continue to warm throughout the 21st century (virtually certain). By 2100, the top 2,000m of the ocean are projected to take up 5–7 times more heat under RCP8.5 (or 2–4 times more under RCP2.6) than the observed accumulated ocean heat uptake since 1970 (very likely). |
B.5.1 |
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. |
| Arctic temperature |
E.1 |
The Arctic region will warm more rapidly than the global mean (very high confidence). |
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B.2.1 |
It is virtually certain that the Arctic will continue to warm more than global surface temperature, with high confidence above two times the rate of global warming. |
| Heat extremes |
E.1 |
It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales as global mean temperatures increase. It is very likely that heat waves will occur with a higher frequency and duration. |
SR15 B.1.2 |
Temperature extremes on land are projected to warm more than GMST (high confidence): extreme hot days in mid-latitudes warm by up to about 3C at global warming of 1.5C and about 4C at 2C, and extreme cold nights in high latitudes warm by up to about 4.5C at 1.5C and about 6C at 2C (high confidence). The number of hot days is projected to increase in most land regions, with highest increases in the tropics (high confidence). |
B.2.2 |
Every additional 0.5C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves (very likely). |
| Water cycle |
E.2 |
Changes in the global water cycle in response to the warming over the 21st century will not be uniform. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions. |
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B.3.2 |
There is strengthened evidence since AR5 that the global water cycle will continue to intensify as global temperatures rise (high confidence), with precipitation and surface water flows projected to become more variable over most land regions within seasons (high confidence) and from year to year (medium confidence). |
| El Niño & the water cycle |
E.2 |
There is high confidence that the El Niño-Southern Oscillation (ENSO) will remain the dominant mode of interannual variability in the tropical Pacific, with global effects in the 21st century. Due to the increase in moisture availability, ENSO-related precipitation variability on regional scales will likely intensify. Natural variations of the amplitude and spatial pattern of ENSO are large and thus confidence in any specific projected change in ENSO and related regional phenomena for the 21st century remains low. |
SROCC B.2.6 |
Extreme El Niño and La Niña events are projected to likely increase in frequency in the 21st century and to likely intensify existing hazards, with drier or wetter responses in several regions across the globe. Extreme El Niño events are projected to occur about as twice as often under both RCP2.6 and RCP8.5 in the 21st century when compared to the 20th century (medium confidence). |
B.3.2 |
It is very likely that rainfall variability related to the El Niño–Southern Oscillation is projected to be amplified by the second half of the 21st century in the SSP2-4.5, SSP3-7.0 and SSP5-8.5 scenarios. |
| Drought |
SPM.1 |
Increases in intensity and/or duration of drought: Low confidence [for the early 21st century]; and likely (medium confidence) on a regional to global scale [for the late 21st century]. |
SR15 B.1.3 |
Risks from droughts and precipitation deficits are projected to be higher at 2C compared to 1.5C of global warming in some regions (medium confidence). |
B.2.2 |
Discernible changes in intensity and frequency of meteorological droughts, with more regions showing increases than decreases, are seen in some regions for every additional 0.5C of global warming (medium confidence). Increases in frequency and intensity of hydrological droughts become larger with increasing global warming in some regions (medium confidence). |
| Rainfall extremes |
E.2 |
Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases. |
SR15 B.1.3 |
Risks from heavy precipitation events are projected to be higher at 2C compared to 1.5C of global warming in several northern hemisphere high-latitude and/or high-elevation regions, eastern Asia and eastern North America (medium confidence). Heavy precipitation associated with tropical cyclones is projected to be higher at 2C compared to 1.5C global warming (medium confidence). There is generally low confidence in projected changes in heavy precipitation at 2C compared to 1.5C in other regions. |
B.2.4 |
It is very likely that heavy precipitation events will intensify and become more frequent in most regions with additional global warming. At the global scale, extreme daily precipitation events are projected to intensify by about 7% for each 1C of global warming (high confidence). |
| Monsoons |
E.2 |
Globally, it is likely that the area encompassed by monsoon systems will increase over the 21st century. While monsoon winds are likely to weaken, monsoon precipitation is likely to intensify due to the increase in atmospheric moisture. |
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B.3.3 |
Monsoon precipitation is projected to increase in the mid- to long term at global scale, particularly over south and south-east Asia, east Asia and west Africa apart from the far west Sahel (high confidence). The monsoon season is projected to have a delayed onset over North and South America and West Africa (high confidence) and a delayed retreat over west Africa (medium confidence). |
| Intense tropical cyclone activity |
SPM.1 |
Increases in intense tropical cyclone activity: Low confidence [for the early 21st century]; and likely (medium confidence) on a regional to global scale [for the late 21st century]. |
SROCC B.3.6 |
The average intensity of tropical cyclones, the proportion of Category 4 and 5 tropical cyclones and the associated average precipitation rates are projected to increase for a 2C global temperature rise above any baseline period (medium confidence). |
B.2.4 |
The proportion of intense tropical cyclones (categories 4-5) and peak wind speeds of the most intense tropical cyclones are projected to increase at the global scale with increasing global warming (high confidence). |
| Arctic sea ice |
E.5 |
A nearly ice-free Arctic Ocean in September before mid-century is likely for RCP8.5 (medium confidence). A projection of when the Arctic might become nearly ice-free in September in the 21st century cannot be made with confidence for the other scenarios. |
SROCC B.1.7 |
Arctic sea ice loss is projected to continue through mid-century, with differences thereafter depending on the magnitude of global warming: for stabilised global warming of 1.5C the annual probability of a sea ice-free September by the end of century is approximately 1%, which rises to 10–35% for stabilised global warming of 2C (high confidence). |
B.2.5 |
The Arctic is likely to be practically sea ice-free in September at least once before 2050 under the five illustrative scenarios considered in this report, with more frequent occurrences for higher warming levels. |
| Antarctic sea ice |
E.5 |
In the Antarctic, a decrease in sea ice extent and volume is projected with low confidence for the end of the 21st century as global mean surface temperature rises. |
SROCC B.1.7 |
There is low confidence in projections for Antarctic sea ice. |
B.2.5 |
There is low confidence in the projected decrease of Antarctic sea ice. |
| Glaciers |
E.5 |
By the end of the 21st century, the global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6, and by 35 to 85% for RCP8.5 (medium confidence). |
SROCC B.1.1 |
Projected glacier mass reductions between 2015 and 2100 (excluding the ice sheets) range from 18 ± 7% (likely range) for RCP2.6 to 36 ± 11% (likely range) for RCP8.5. |
B.5.2 |
Mountain and polar glaciers are committed to continue melting for decades or centuries (very high confidence). |
| Greenland ice sheet |
E.6 |
The increase in surface melting of the Greenland ice sheet will exceed the increase in snowfall, leading to a positive contribution [to sea level rise] from changes in surface mass balance to future sea level (high confidence). |
SROCC B.1.2 |
In 2100, the Greenland Ice Sheet’s projected contribution to GMSL rise is 0.07 metres (0.04–0.12 metres, likely range) under RCP2.6, and 0.15 metres (0.08–0.27 metres, likely range) under RCP8.5. |
B.5.2 |
Continued ice loss over the 21st century is virtually certain for the Greenland Ice Sheet…There is high confidence that total ice loss from the Greenland Ice Sheet will increase with cumulative emissions. |
| Antarctic ice sheet |
E.6 |
While surface melting will remain small, an increase in snowfall on the Antarctic ice sheet is expected (medium confidence), resulting in a negative contribution to future sea level from changes in surface mass balance. |
SROCC B.1.2 |
In 2100, the Antarctic Ice Sheet is projected to contribute 0.04 metres (0.01–0.11 metres, likely range) under RCP2.6, and 0.12 metres (0.03–0.28 metres, likely range) under RCP8.5. |
B.5.2 |
Continued ice loss over the 21st century is…likely for the Antarctic Ice Sheet. |
| Antarctic ice sheet |
E.8 |
Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment. |
SROCC A.3.3 |
Acceleration of ice flow and retreat in Antarctica, which has the potential to lead to sea level rise of several metres within a few centuries, is observed in the Amundsen Sea Embayment of West Antarctica and in Wilkes Land, East Antarctica (very high confidence). These changes may be the onset of an irreversible ice sheet instability. Uncertainty related to the onset of ice sheet instability arises from limited observations, inadequate model representation of ice sheet processes, and limited understanding of the complex interactions between the atmosphere, ocean and the ice sheet. |
C.3.2 |
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). |
| Sea level rise |
E.6 |
Global mean sea level rise for 2081–2100 relative to 1986–2005 will likely be in the ranges of 0.26 to 0.55 metres for RCP2.6, 0.32 to 0.63 metres for RCP4.5, 0.33 to 0.63 metres for RCP6.0, and 0.45 to 0.82 metres for RCP8.5 (medium confidence). |
SROCC B.3.1 |
The global mean sea level (GMSL) rise under RCP2.6 is projected to be 0.39 metres (0.26–0.53 metres, likely range) for the period 2081–2100, and 0.43 metres (0.29–0.59 metres, likely range) in 2100 with respect to 1986–2005. For RCP8.5, the corresponding GMSL rise is 0.71 metres (0.51–0.92 metres, likely range) for 2081–2100 and 0.84 metres (0.61–1.10 metres, likely range) in 2100. |
B.5.3 |
It is virtually certain that global mean sea level will continue to rise over the 21st century. Relative to 1995-2014, the likely global mean sea level rise by 2100 is 0.28-0.55 metres under the very low GHG emissions scenario (SSP1-1.9), 0.32-0.62 metres under the low GHG emissions scenario (SSP1-2.6), 0.44-0.76 metres under the intermediate GHG emissions scenario (SSP2-4.5), and 0.63-1.01 metres under the very high GHG emissions scenario (SSP5-8.5). |
| Sea level rise |
E.6 |
Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. However, there is medium confidence that this additional contribution would not exceed several tenths of a metre of sea level rise during the 21st century. |
SROCC B.3.3 |
Processes controlling the timing of future ice-shelf loss and the extent of ice sheet instabilities could increase Antarctica’s contribution to sea level rise to values substantially higher than the likely range on century and longer time-scales (low confidence). Considering the consequences of sea level rise that a collapse of parts of the Antarctic Ice Sheet entails, this high impact risk merits attention. |
B.5.2 |
There is limited evidence for low-likelihood, high-impact outcomes (resulting from ice sheet instability processes characterised 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 scenarios. |
| Sea level rise |
E.8 |
It is virtually certain that global mean sea level rise will continue beyond 2100, with sea level rise due to thermal expansion to continue for many centuries. |
SROCC B.3.3 |
Model studies indicate multi-metre rise in sea level by 2300 (2.3–5.4 metres for RCP8.5 and 0.6–1.07 metres under RCP2.6) (low confidence), indicating the importance of reduced emissions for limiting sea level rise. |
B.5.4 |
In the longer term, sea level is committed to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years (high confidence). |
| Atlantic meridional overturning circulation (AMOC) |
E.4 |
It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century…It is likely that there will be some decline in the AMOC by about 2050, but there may be some decades when the AMOC increases due to large natural internal variability. |
SROCC B.2.7 |
The AMOC is projected to weaken in the 21st century under all RCPs (very likely). |
C.3.4 |
The Atlantic Meridional Overturning Circulation is very likely to weaken over the 21st century for all emission scenarios. While there is high confidence in the 21st century decline, there is only low confidence in the magnitude of the trend. |
| Atlantic meridional overturning circulation (AMOC) |
E.4 |
It is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century for the scenarios considered. |
SROCC B.2.7 |
A collapse is very unlikely (medium confidence). Based on CMIP5 projections, by 2300, an AMOC collapse is about as likely as not for high emissions scenarios and very unlikely for lower ones (medium confidence). |
C.3.4 |
There is medium confidence that there will not be an abrupt collapse before 2100. |
| Ocean acidification |
E.7 |
Earth System Models project a global increase in ocean acidification for all RCP scenarios. The corresponding decrease in surface ocean pH by the end of 21st century is in the range of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0, and 0.30 to 0.32 for RCP8.5. |
SROCC B.2.3 |
Continued carbon uptake by the ocean by 2100 is virtually certain to exacerbate ocean acidification. Open ocean surface pH is projected to decrease by around 0.3 pH units by 2081–2100, relative to 2006–15, under RCP8.5 (virtually certain). |
B.5.1 |
Based on multiple lines of evidence…ocean acidification (virtually certain)…will continue to increase in the 21st century, at rates dependent on future emissions. |
| Permafrost |
E.5 |
It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases. By the end of the 21st century, the area of permafrost near the surface (upper 3.5 m) is projected to decrease by between 37% (RCP2.6) to 81% (RCP8.5) for the model average (medium confidence). |
SROCC B.1.4 |
Widespread permafrost thaw is projected for this century (very high confidence) and beyond. By 2100, projected near-surface (within 3–4 metres) permafrost area shows a decrease of 24 ± 16% (likely range) for RCP2.6 and 69 ± 20% (likely range) for RCP8.5. |
B.2.5 |
Additional warming is projected to further amplify permafrost thawing (high confidence). |
| Permafrost |
E.7 |
The release of CO2 or CH4 to the atmosphere from thawing permafrost carbon stocks over the 21st century is assessed to be in the range of 50 to 250GtC for RCP8.5 (low confidence). |
SROCC B.1.4 |
The RCP8.5 scenario leads to the cumulative release of tens to hundreds of billions of tonnes (GtC) of permafrost carbon as CO2 and methane to the atmosphere by 2100 with the potential to exacerbate climate change (medium confidence). Lower emissions scenarios dampen the response of carbon emissions from the permafrost region (high confidence). |
B.5.2 |
Loss of permafrost carbon following permafrost thaw is irreversible at centennial timescales (high confidence). |
| Carbon sinks |
E.7 |
Based on Earth System Models, there is high confidence that the feedback between climate and the carbon cycle is positive in the 21st century; that is, climate change will partially offset increases in land and ocean carbon sinks caused by rising atmospheric CO2. As a result more of the emitted anthropogenic CO2 will remain in the atmosphere. A positive feedback between climate and the carbon cycle on century to millennial time scales is supported by palaeoclimate observations and modelling. |
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B.4.1 |
While natural land and ocean carbon sinks are projected to take up, in absolute terms, a progressively larger amount of CO2 under higher compared to lower CO2 emissions scenarios, they become less effective, that is, the proportion of emissions taken up by land and ocean decrease with increasing cumulative CO2 emissions. This is projected to result in a higher proportion of emitted CO2 remaining in the atmosphere (high confidence). |
| Carbon sinks |
E.7 |
Ocean uptake of anthropogenic CO2 will continue under all four RCPs through to 2100, with higher uptake for higher concentration pathways (very high confidence). The future evolution of the land carbon uptake is less certain. A majority of models projects a continued land carbon uptake under all RCPs, but some models simulate a land carbon loss due to the combined effect of climate change and land use change. |
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B.4.2 |
Based on model projections, under the intermediate scenario that stabilises atmospheric CO2 concentrations this century (SSP2-4.5), the rates of CO2 taken up by the land and oceans are projected to decrease in the second half of the 21st century (high confidence). Under the very low and low GHG emissions scenarios (SSP1-1.9, SSP1-2.6), where CO2 concentrations peak and decline during the 21st century, land and oceans begin to take up less carbon in response to declining atmospheric CO2 concentrations (high confidence) and turn into a weak net source by 2100 under SSP1-1.9 (medium confidence). It is very unlikely that the combined global land and ocean sink will turn into a source by 2100 under scenarios without net negative emissions (SSP2-4.5, SSP3-7.0, SSP5-8.5). |
| CO2-warming relationship |
E.8 |
Cumulative total emissions of CO2 and global mean surface temperature response are approximately linearly related. |
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D.1.1 |
This report reaffirms with high confidence the AR5 finding that there is a near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause. |
| Transient climate response to cumulative carbon emissions (TCRE) |
D.2 |
TCRE is likely in the range of 0.8C to 2.5C per 1,000GtC and applies for cumulative emissions up to about 2,000GtC until the time temperatures peak. |
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D.1.1 |
Each 1,000GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27C to 0.63C increase in global surface temperature with a best estimate of 0.45C. This is a narrower range compared to AR5 and SR15. [In terms of carbon, rather than CO2, this is equivalent to a likely range of 1.0C to 2.3C per 1,000GtC, with a best estimate of 1.65C.] |
| Equilibrium climate sensitivity (ECS) |
D.2 |
Equilibrium climate sensitivity is likely in the range 1.5C to 4.5C (high confidence), extremely unlikely less than 1C (high confidence), and very unlikely greater than 6C (medium confidence). |
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A.4.4 |
Based on multiple lines of evidence, the very likely range of equilibrium climate sensitivity is between 2C (high confidence) and 5C (medium confidence). The AR6 assessed best estimate is 3C with a likely range of 2.5C to 4C (high confidence), compared to 1.5C to 4.5C in AR5, which did not provide a best estimate. |
| Mitigation |
E.8 |
A lower warming target, or a higher likelihood of remaining below a specific warming target, will require lower cumulative CO2 emissions. Accounting for warming effects of increases in non-CO2 greenhouse gases, reductions in aerosols, or the release of greenhouse gases from permafrost will also lower the cumulative CO2 emissions for a specific warming target. |
SR15 C.1 |
In model pathways with no or limited overshoot of 1.5C, global net anthropogenic CO2 emissions decline by about 45% from 2010 levels by 2030 (40–60% interquartile range), reaching net-zero around 2050 (2045–55 interquartile range). For limiting global warming to below 2C, CO2 emissions are projected to decline by about 25% by 2030 in most pathways (10–30% interquartile range) and reach net-zero around 2070 (2065–80 interquartile range). Non-CO2 emissions in pathways that limit global warming to 1.5C show deep reductions that are similar to those in pathways limiting warming to 2C (high confidence). |
D.1 |
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. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality. |
| Mitigation |
E.8 |
A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. |
SR15 A.2.2 |
Reaching and sustaining net-zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal time scales (high confidence). |
D.1.6 |
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). |
| Carbon dioxide removal (CDR) |
E.8 |
Carbon Dioxide Removal (CDR) methods have biogeochemical and technological limitations to their potential on a global scale. There is insufficient knowledge to quantify how much CO2 emissions could be partially offset by CDR on a century timescale. |
SR15 C.3 |
All pathways that limit global warming to 1.5C with limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100–1,000GtCO2 over the 21st century. CDR would be used to compensate for residual emissions and, in most cases, achieve net-negative emissions to return global warming to 1.5C following a peak (high confidence). CDR deployment of several hundreds of GtCO2 is subject to multiple feasibility and sustainability constraints (high confidence). |
D.1.4 |
Deliberate removal of CO2 from the atmosphere could compensate for some residual emissions to reach net-zero CO2 or greenhouse gas emissions or, if implemented at a large scale, generate net negative emissions. Anthropogenic CO2 removal (CDR) has the potential to remove CO2 from the atmosphere and durably store it in reservoirs (high confidence). CDR aims to compensate for residual emissions to reach net-zero CO2 or net-zero GHG emissions or, if implemented at a scale where anthropogenic removals exceed anthropogenic emissions, to lower surface temperature. |
| Carbon dioxide removal (CDR) |
E.8 |
CDR and Solar Radiation Management (SRM) methods carry side effects and long-term consequences on a global scale. |
SR15 C.3.4 |
Most current and potential CDR measures could have significant impacts on land, energy, water or nutrients if deployed at large scale (high confidence). Afforestation and bioenergy may compete with other land uses and may have significant impacts on agricultural and food systems, biodiversity, and other ecosystem functions and services (high confidence). Effective governance is needed to limit such trade-offs and ensure permanence of carbon removal in terrestrial, geological and ocean reservoirs (high confidence). |
D.1.4 |
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 biodiversity (high confidence). |
