| Greenhouse gases (GHGs) |
B.5 |
The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have all increased since 1750 due to human activity. In 2011, the concentrations of these greenhouse gases were 391ppm, 1,803ppb, and 324ppb…respectively. |
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|
A.1.1 |
Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities. Since 2011 (measurements reported in AR5), concentrations have continued to increase in the atmosphere, reaching annual averages of 410ppm for carbon dioxide (CO2), 1,866ppb for methane (CH4), and 332ppb for nitrous oxide (N2O) in 2019. |
| Greenhouse gases (GHGs) |
B.5 |
Concentrations of CO2, CH4, and N2O now substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years. |
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|
A.2.1 |
In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2m years (high confidence), and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years (very high confidence). |
| Greenhouse gases (GHGs) |
D.3 |
Greenhouse gases contributed a global mean surface warming likely to be in the range of 0.5C to 1.3C over the period 1951 to 2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of −0.6C to 0.1C. The contribution from natural forcings is likely to be in the range of −0.1C to 0.1C, and from natural internal variability is likely to be in the range of −0.1C to 0.1C. |
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A.1.3 |
It is likely that well-mixed GHGs contributed a warming of 1.0C to 2.0C, other human drivers (principally aerosols) contributed a cooling of 0.0C to 0.8C, natural drivers changed global surface temperature by –0.1C to 0.1C, and internal variability changed it by –0.2C to 0.2C. |
| Global temperatures |
B.1 |
The total increase between the average of the 1850–1900 period and the 2003–12 period is 0.78 [0.72 to 0.85] C |
SR15 A1.1 |
Observed global mean surface temperature (GMST) for the decade 2006–15 was 0.87C (likely between 0.75C and 0.99C) higher than the average over the 1850–1900 period (very high confidence). |
A.1.2 |
Global surface temperature was 1.09 [0.95 to 1.20] C higher in 2011–2020 than 1850–1900. |
| Global temperatures |
B.1 |
Each of the past three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. |
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|
A.1.2 |
Each of the last four decades has been successively warmer than any decade that preceded it since 1850. |
| Global temperatures |
B |
It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. |
SR15 A.1 |
Human activities are estimated to have caused approximately 1.0C of global warming above pre-industrial levels, with a likely range of 0.8C to 1.2C. |
A.1 |
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. |
| Ocean warming |
B.2 |
It is virtually certain that the upper ocean (0−700 metres) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. |
SROCC A.2 |
It is virtually certain that the global ocean has warmed unabated since 1970 and has taken up more than 90% of the excess heat in the climate system (high confidence). |
A.1.6 |
It is virtually certain that the global upper ocean (0–700 metres) has warmed since the 1970s. |
| Ocean warming |
D.3 |
It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0–700 metres) observed since the 1970s. There is evidence for human influence in some individual ocean basins. |
SROCC A.2.1 |
The ocean warming trend documented in the IPCC fifth assessment report (AR5) has continued. Since 1993 the rate of ocean warming and thus heat uptake has more than doubled (likely)…and is attributed to anthropogenic forcing (very likely). |
A.1.6 |
It is…extremely likely that human influence is the main driver [of ocean warming]. |
| Troposphere |
D.3 |
It is very likely that anthropogenic influence, particularly greenhouse gases and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961. |
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A.1.3 |
It is very likely that well-mixed GHGs were the main driver of tropospheric warming since 1979. |
| Precipitation over land |
B.1 |
Confidence in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium afterwards. |
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|
A.1.4 |
Globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s (medium confidence). |
| Precipitation over land |
D.3 |
Anthropogenic influences have contributed…to global-scale changes in precipitation patterns over land (medium confidence). |
SROCC A.3.6 |
Anthropogenic climate change has increased observed precipitation (medium confidence). |
A.1.4 |
It is likely that human influence contributed to the pattern of observed precipitation changes [over land] since the mid-20th century. |
| Heat extremes |
B.1 |
It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heatwaves has increased in large parts of Europe, Asia and Australia. |
SRCCL A.2.2 |
Warming has resulted in an increased frequency, intensity and duration of heat-related events, including heatwaves in most land regions (high confidence). |
A.3.1 |
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. |
| Heat extremes |
D.3 |
It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heatwaves in some locations. |
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|
A.3.1 |
[There is] high confidence that human-induced climate change is the main driver of these changes [in heat extremes]. Some recent hot extremes observed over the past decade would have been extremely unlikely to occur without human influence on the climate system. |
| Drought |
Table SPM.1 |
The frequency and intensity of drought has likely increased in the Mediterranean and West Africa [since 1950], and likely decreased in central North America and north-west Australia. [Assessment of a human contribution]: Low confidence. |
SRCCL A.2.2 |
Frequency and intensity of droughts has increased in some regions (including the Mediterranean, west Asia, many parts of South America, much of Africa, and north-eastern Asia) (medium confidence). |
A.3.2 |
Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increased land evapotranspiration (medium confidence). |
| Rainfall extremes |
B.1 |
There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency or intensity of heavy precipitation events has likely increased in North America and Europe. In other continents, confidence in changes in heavy precipitation events is at most medium. |
SRCCL A.2.2 |
There has been an increase in the intensity of heavy precipitation events at a global scale (medium confidence). |
A.3.2 |
The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence). |
| Rainfall extremes |
D.3 |
Anthropogenic influences have contributed…to intensification of heavy precipitation over land regions where data are sufficient (medium confidence). |
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|
A.3.2 |
Human-induced climate change is likely the main driver [of observed increases in heavy precipitation events]. |
| Intense tropical cyclone activity |
Table SPM.1 |
Low confidence in long-term (centennial) changes [since 1950]; Virtually certain in North Atlantic since 1970. [Assessment of a human contribution]: Low confidence. |
SROCC A.3.6 |
There is emerging evidence for an increase in annual global proportion of Category 4 or 5 tropical cyclones in recent decades (low confidence). |
A.3.4 |
It is likely that the global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward; these changes cannot be explained by internal variability alone (medium confidence). There is low confidence in long-term (multi-decadal to centennial) trends in the frequency of all-category tropical cyclones. |
| Greenland ice sheet |
D.3 |
Anthropogenic influences likely contributed to…the increased surface mass loss of the Greenland ice sheet since 1993. |
SROCC A.1.1 |
Between 2006 and 2015, the Greenland Ice Sheet lost ice mass at an average rate of 278 ± 11 Gt/yr (equivalent to 0.77 ± 0.03 mm/yr of global sea level rise), mostly due to surface melting (high confidence). |
A.1.5 |
It is very likely that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades. |
| Antarctic ice sheet |
D.3 |
Due to a low level of scientific understanding there is low confidence in attributing the causes of the observed loss of mass from the Antarctic ice sheet over the past two decades. |
SROCC A.1.1 |
In 2006–2015, the Antarctic Ice Sheet lost mass at an average rate of 155 ± 19 Gt/yr (0.43 ± 0.05 mm/yr), mostly due to rapid thinning and retreat of major outlet glaciers draining the West Antarctic Ice Sheet (very high confidence). |
A.1.5 |
There is only limited evidence, with medium agreement, of human influence on the Antarctic Ice Sheet mass loss. |
| Arctic sea ice |
B.3 |
There is medium confidence from reconstructions that over the past three decades, Arctic summer sea ice retreat was unprecedented and sea surface temperatures were anomalously high in at least the last 1,450 years. |
SROCC A.1.4 |
Between 1979 and 2018, Arctic sea ice extent has very likely decreased for all months of the year. September sea ice reductions are very likely 12.8 ± 2.3% per decade. These sea ice changes in September are likely unprecedented for at least 1,000 years. |
A.2.3 |
In 2011–20, annual average Arctic sea ice area reached its lowest level since at least 1850 (high confidence). Late summer Arctic sea ice area was smaller than at any time in at least the past 1,000 years (medium confidence). |
| Arctic sea ice |
D.3 |
Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979. |
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A.1.5 |
Human influence is very likely the main driver of…the decrease in Arctic sea ice area between 1979–88 and 2010–19 (about 40% in September and about 10% in March). |
| Antarctic sea ice |
B.3 & D.3 |
It is very likely that the annual mean Antarctic sea ice extent increased at a rate in the range of 1.2 to 1.8% per decade (range of 0.13–0.20m km2 per decade) between 1979 and 2012…There is low confidence in the scientific understanding of the small observed increase in Antarctic sea ice extent due to the incomplete and competing scientific explanations for the causes of change and low confidence in estimates of natural internal variability in that region. |
SROCC A.1.4 |
Antarctic sea ice extent overall has had no statistically significant trend (1979–2018) due to contrasting regional signals and large interannual variability (high confidence). |
A.1.5 |
There has been no significant trend in Antarctic sea ice area from 1979 to 2020 due to regionally opposing trends and large internal variability. |
| Glaciers |
B.3 |
The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of the ice sheets, was very likely 226 [91 to 361] Gt/yr over the period 1971 to 2009, and very likely 275 [140 to 410] Gt/yr over the period 1993 to 2009. |
SROCC A.1.1 |
Ice sheets and glaciers worldwide have lost mass (very high confidence)…Glaciers worldwide outside Greenland and Antarctica lost mass at an average rate of 220 ± 30 Gt/yr (equivalent to 0.61 ± 0.08 mm/yr sea level rise) in 2006–2015. |
A.2.3 |
The global nature of glacier retreat, with almost all of the world’s glaciers retreating synchronously, since the 1950s is unprecedented in at least the last 2,000 years (medium confidence). |
| Glaciers |
D.3 |
Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s. |
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A.1.5 |
Human influence is very likely the main driver of the global retreat of glaciers since the 1990s. |
| Snow cover |
D.3 |
It is likely that there has been an anthropogenic contribution to observed reductions in northern hemisphere spring snow cover since 1970. |
SROCC A.1.2 |
In nearly all high mountain areas, the depth, extent and duration of snow cover have declined over recent decades, especially at lower elevation (high confidence). |
A.1.5 |
Human influence very likely contributed to the decrease in northern hemisphere spring snow cover since 1950. |
| Sea level rise |
B.4 |
It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010, 2.0 [1.7 to 2.3] mm/yr between 1971 and 2010, and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010. |
SROCC A.3.1 |
Total GMSL rise for 1902–2015 is 0.16 metres (likely range 0.12–0.21 metres). The rate of GMSL rise for 2006–15 of 3.6 mm/yr (3.1–4.1 mm/yr, very likely range), is unprecedented over the last century (high confidence), and about 2.5 times the rate for 1901–90 of 1.4 mm/yr (0.8– 2.0 mm/yr, very likely range). |
A.1.7 |
Global mean sea level increased by 0.20 [0.15 to 0.25] metres between 1901 and 2018. The average rate of sea level rise was 1.3 [0.6 to 2.1] mm/yr between 1901 and 1971, increasing to 1.9 [0.8 to 2.9] mm/yr between 1971 and 2006, and further increasing to 3.7 [3.2 to 4.2] mm/yr between 2006 and 2018 (high confidence). |
| Sea level rise |
D.3 |
It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s. |
SROCC A.3.1 |
The dominant cause of global mean sea level rise since 1970 is anthropogenic forcing (high confidence). |
A.1.7 |
Human influence was very likely the main driver of these [observed sea level increases] since at least 1971. |
| Sea level rise |
B.4 |
Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise (high confidence). |
SROCC A.3.1 |
The sum of ice sheet and glacier contributions over the period 2006–15 is the dominant source of sea level rise (1.8 mm yr–1, very likely range 1.7–1.9 mm yr–1), exceeding the effect of thermal expansion of ocean water (1.4 mm yr–1, very likely range 1.1–1.7 mm yr–1) (very high confidence). |
A.4.3 |
Heating of the climate system has caused global mean sea level rise through ice loss on land and thermal expansion from ocean warming. Thermal expansion explained 50% of sea level rise during 1971–2018, while ice loss from glaciers contributed 22%, ice sheets 20% and changes in land water storage 8%. |
| Ocean acidification |
B.5 |
Ocean acidification is quantified by decreases in pH. The pH of ocean surface water has decreased by 0.1 since the beginning of the industrial era (high confidence), corresponding to a 26% increase in hydrogen ion concentration. |
SROCC A.2.5 |
The ocean has taken up between 20–30% (very likely) of total anthropogenic CO2 emissions since the 1980s causing further ocean acidification. Open ocean surface pH has declined by a very likely range of 0.017–0.027 pH units per decade since the late 1980s, with the decline in surface ocean pH very likely to have already emerged from background natural variability for more than 95% of the ocean surface area. |
A.2.4 |
A long-term increase in surface open ocean pH occurred over the past 50m years (high confidence), and surface open ocean pH as low as recent decades is unusual in the last 2m years (medium confidence). |
| Ocean salinity |
D.3 |
Anthropogenic influences have contributed to…changes in surface and sub-surface ocean salinity (very likely). |
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A.1.4 |
It is…extremely likely that human influence contributed to the pattern of observed changes in near-surface ocean salinity. |
| Radiative forcing |
C |
The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W/m2, and it has increased more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than that reported in AR4 for the year 2005. |
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|
A.4.1 |
Human-caused radiative forcing of 2.72 [1.96 to 3.48] W/m2 in 2019 relative to 1750 has warmed the climate system…The radiative forcing has increased by 0.43 W/m2 (19%) relative to AR5, of which 0.34 W/m2 is due to the increase in GHG concentrations since 2011. |
| Climate models |
D.1 |
Climate models have improved since the AR4. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions (very high confidence). |
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Box SPM.1.2 |
This report assesses results from climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) of the World Climate Research Programme. These models include new and better representation of physical, chemical and biological processes, as well as higher resolution, compared to climate models considered in previous IPCC assessment reports. |
