Editing IPCC:AR6/WGI/Chapter-3 (section)

=== 3.8.1 Multivariate Attribution of Climate Change ===

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The AR5 concluded that human influence on the climate system is clear ( [[#IPCC--2013|IPCC, 2013]] ), based on observed increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and physical understanding of the climate system. The AR5 also assessed that it was ''virtually certain'' that internal variability alone could not account for observed warming since 1951 ( [[#Bindoff--2013|Bindoff et al., 2013]] ). Evidence has grown since AR5 that observed changes since the 1950s in many parts of the climate system are attributable to anthropogenic influence. So far, this chapter has focused on examining individual aspects of the climate system in separate sections. The results presented in Sections 3.3 to 3.7 substantially strengthen our assessment of the role of human influence on climate since pre-industrial times. In this section we look across the whole climate system to assess to what extent a physically consistent picture of human induced change emerges across the climate system (Figure 3.41).

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[[File:8f0a3be3cf07076861aedf6d227c1844 IPCC_AR6_WGI_Figure_3_41.png]]

Figure 3.41 | '''Summary figure showing simulated and observed changes in key large-scale indicators of climate change across the climate system, for continental, ocean basin and larger scales.''' Black lines show observations, brown lines and shading show the multi-model mean and 5th–95th percentile ranges for CMIP6 historical simulations including anthropogenic and natural forcing, and blue lines and shading show corresponding ensemble means and 5th–95th percentile ranges for CMIP6 natural-only simulations. Temperature time series are as in Figure 3.9, but with smoothing using a low pass filter. Precipitation time series are as in Figure 3.15 and ocean heat content as in Figure 3.26. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

The observed global surface air temperature warming of 0.9°C to 1.2°C in 2010–2019 is much greater than can be explained by internal variability ( ''likely'' –0.2°C to +0.2°C) or natural forcings ( ''likely'' –0.1°C to +0.1°C) alone, but consistent with the assessed anthropogenic warming ( ''likely'' 0.8°C to 1.3°C; [[#3.3.1.1|Section 3.3.1.1]] ). It is ''very likely'' that human influence is the main driver of warming over land [[#3.3.1.1|Section 3.3.1.1]] ). Moreover, the atmosphere as a whole has warmed (Table 7.1), and it is ''very likely'' that human-induced greenhouse gas increases were the main driver of tropospheric warming since 1979 ( [[#3.3.1.2|Section 3.3.1.2]] ). It is ''virtually certain'' that greenhouse gas forcing was the main driver of the observed changes in hot and cold extremes over land at the global scale (Cross-Chapter Box 3.2).

As might be expected from a warming atmosphere, moisture in the troposphere has increased and precipitation patterns have changed. Human influence has ''likely'' contributed to the observed changes in humidity and precipitation ( [[#3.3.2|Section 3.3.2]] ). It is ''likely'' that human influence, in particular due to greenhouse gas forcing, is the main driver of the observed intensification of heavy precipitation in global land regions during recent decades (Cross-Chapter Box 3.2). The pattern of ocean salinity changes indicate that fresh regions are becoming fresher and that salty regions are becoming saltier as a result of changes in ocean-atmosphere fluxes through evaporation and precipitation ( ''high confidence'' ) making it ''extremely likely'' that human influence has contributed to observed near-surface and subsurface salinity changes since the mid-20th century ( [[#3.5.2.2|Section 3.5.2.2]] ). Taken together, this evidence indicates a human influence on the water cycle.

It is ''very likely'' that human influence was the main driver of Arctic sea ice loss since the late 1970s ( [[#3.4.1.1|Section 3.4.1.1]] ), and ''very likely'' that it contributed to the observed reductions in Northern Hemisphere springtime snow cover since 1950 ( [[#3.4.2|Section 3.4.2]] ). Human influence was ''very likely'' the main driver of the recent global, near-universal retreat of glaciers and it is ''very likely'' that it contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades ( [[#3.4.3.2.1|Section 3.4.3.2.1]] ). It is ''extremely likely'' that human influence was the main driver of the ocean heat content increase observed since the 1970s ( [[#3.5.1.3|Section 3.5.1.3]] ), and ''very likely'' that human influence was the main driver of the observed GMSL rise since at least 1970 ( [[#3.5.3.2|Section 3.5.3.2]] ).

Combining the evidence from across the climate system (Sections 3.3–3.7) increases the level of confidence in the attribution of observed climate change to human influence and reduces the uncertainties associated with assessments based on a single variable. From this combined evidence, it is unequivocal that human influence has warmed the atmosphere, ocean and land components of the global climate system, taken together.

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