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Climate Extremes
There has been a strengthening of the evidence for human influ- ence on temperature extremes since the AR4 and IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) reports. It is very likely that anthropogenic forcing has contributed to the observed changes in the frequency and intensity of daily temperature extremes on the global scale since the mid-20th century. Attribution of changes in temperature extremes to anthropogenic influence is robustly seen in independent analyses using different methods and different data sets. It is likely that human influence has substantially increased the prob- ability of occurrence of heatwaves in some locations. {10.6.1, 10.6.2, Table 10.1}
In land regions where observational coverage is sufficient for assessment, there is medium confidence that anthropogen- ic forcing has contributed to a global-scale intensification of heavy precipitation over the second half of the 20th century. There is low confidence in attributing changes in drought over global land areas since the mid-20th century to human influence owing to observational uncertainties and difficulties in distinguishing decad- al-scale variability in drought from long-term trends. {10.6.1, Table 10.1}
There is low confidence in attribution of changes in tropical cyclone activity to human influence owing to insufficient obser- vational evidence, lack of physical understanding of the links between anthropogenic drivers of climate and tropical cyclone activity and the low level of agreement between studies as to the relative importance of internal variability, and anthropo- genic and natural forcings. This assessment is consistent with that of SREX. {10.6.1, Table 10.1}
Atmospheric Circulation
It is likely that human influence has altered sea level pressure patterns globally. Detectable anthropogenic influence on changes in sea level pressure patterns is found in several studies. Changes in atmospheric circulation are important for local climate change since they could lead to greater or smaller changes in climate in a particular region than elsewhere. There is medium confidence that stratospheric ozone depletion has contributed to the observed poleward shift of the southern Hadley Cell border during austral summer. There are large uncertainties in the magnitude of this poleward shift. It is likely that stratospheric ozone depletion has contributed to the positive trend in the Southern Annular Mode seen in austral summer since the mid- 20th century which corresponds to sea level pressure reductions over the high latitudes and an increase in the subtropics. There is medium confidence that GHGs have also played a role in these trends of the southern Hadley Cell border and the Southern Annular Mode in Austral summer. {10.3.3, Table 10.1}
A Millennia to Multi-Century Perspective
Taking a longer term perspective shows the substantial role played by anthropogenic and natural forcings in driving climate variability on hemispheric scales prior to the twentieth century. It is very unlikely that NH temperature variations from 1400 to 1850 can be explained by internal variability alone. There is medium confi- dence that external forcing contributed to NH temperature variability from 850 to 1400 and that external forcing contributed to European temperature variations over the last five centuries. {10.7.2, 10.7.5, Table 10.1}
Climate System Properties
The extended record of observed climate change has allowed a better characterization of the basic properties of the climate system that have implications for future warming. New evidence from 21st century observations and stronger evidence from a wider range of studies have strengthened the constraint on the transient climate response (TCR) which is estimated with high confidence to be likely between 1°C and 2.5°C and extremely unlikely to be greater than 3°C. The Transient Climate Response to Cumulative CO2 Emissions (TCRE) is estimated with high confidence to be likely between 0.8°C and 2.5°C per 1000 PgC for cumulative CO2 emissions less than about 2000 PgC until the time at which temperatures peak. Estimates of the Equilibrium Climate Sensitivity (ECS) based on multiple and partly independent lines of evidence from observed climate change indicate that there is high confidence that ECS is extremely unlikely to be less than 1°C and medium confidence that the ECS is likely to be between 1.5°C and 4.5°C and very unlikely greater than 6°C. These assessments are consistent with the overall assessment in Chapter 12, where the inclusion of additional lines of evidence increases confidence in the assessed likely range for ECS. {10.8.1, 10.8.2, 10.8.4, Box 12.2}
Combination of Evidence
Human influence has been detected in the major assessed com- ponents of the climate system. Taken together, the combined evidence increases the level of confidence in the attribution of observed climate change, and reduces the uncertainties associ- ated with assessment based on a single climate variable. From this combined evidence it is virtually certain that human influ- ence has warmed the global climate system. Anthropogenic influ- ence has been identified in changes in temperature near the surface of the Earth, in the atmosphere and in the oceans, as well as changes in the cryosphere, the water cycle and some extremes. There is strong evidence that excludes solar forcing, volcanoes and internal variability as the strongest drivers of warming since 1950. {10.9.2, Table 10.1}
