1.3.1. Attribution of climate changes to human and natural influences on the climate system Share

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together (Figure 1.9). The best estimate of the human induced contribution to warming is similar to the observed warming over this period. GHGs contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3°C over the period 1951 to 2010, with further contributions from other anthropogenic forcings, including the cooling effect of aerosols, from natural forcings, and from natural internal variability (see Figure 1.9). Together these assessed contributions are consistent with the observed warming of approximately 0.6°C to 0.7°C over this period. {WGI SPM D.3, 10.3.1}

Figure 1.9 Download Share Edit Figure 1.9 | Assessed likely ranges (whiskers) and their mid-points (bars) for warming trends over the 1951–2010 period from well-mixed greenhouse gases, other anthropogenic forcings (including the cooling effect of aerosols and the effect of land use change), combined anthropogenic forcings, natural forcings, and natural internal climate variability (which is the element of climate variability that arises spontaneously within the climate system, even in the absence of forcings). The observed surface temperature change is shown in black, with the 5%– 95% uncertainty range due to observational uncertainty. The attributed warming ranges (colours) are based on observations combined with climate model simulations, in order to estimate the contribution by an individual external forcing to the observed warming. The contribution from the combined anthropogenic forcings can be estimated with less uncertainty than the separate contributions from greenhouse gases and other anthropogenic forcings separately. This is because these two contributions are partially compensational, resulting in a signal that is better constrained by observations. {Based on Figure WGI TS.10}

It is very likely that anthropogenic influence, particularly GHGs and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961. {WGI SPM D.3, 2.4.4, 9.4.1, 10.3.1}

Over every continental region except Antarctica, anthropogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century (Figure 1.10). For Antarctica, large observational uncertainties result in low confidence that anthropogenic forcings have contributed to the observed warming averaged over available stations. In contrast, it is likely that there has been an anthropogenic contribution to the very substantial Arctic warming since the mid-20th century. Human influence has likely contributed to temperature increases in many sub-continental regions. {WGI SPM D.3, TS.4.8, 10.3.1}

Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979 (Figure 1.10). 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. {WGI SPM D.3, 10.5.1, Figure 10.16}

Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface melting of the Greenland ice sheet since 1993. Due to a low level of scientific understanding, however, there is low confidence in attributing the causes of the observed loss of mass from the Antarctic ice sheet over the past two decades. It is likely that there has been an anthropogenic contribution to observed reductions in Northern Hemisphere spring snow cover since 1970. {WGI 4.3.3, 10.5.2, 10.5.3}

It is likely that anthropogenic influences have affected the global water cycle since 1960. Anthropogenic influences have contributed to observed increases in atmospheric moisture content (medium confidence), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence) (see 1.4), and to changes in surface and subsurface ocean salinity (very likely). {WGI SPM D.3, 2.5.1, 2.6.2, 3.3.2, 3.3.3, 7.6.2, 10.3.2, 10.4.2, 10.6}

It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0–700 m) observed since the 1970s (Figure 1.10). There is evidence for human influence in some individual ocean basins. It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s. This is based on the high confidence in an anthropogenic influence on the two largest contributions to sea level rise: thermal expansion and glacier mass loss. Oceanic uptake of anthropogenic carbon dioxide has resulted in gradual acidification of ocean surface waters (high confidence). {WGI SPM D.3, 3.2.3, 3.8.2, 10.4.1, 10.4.3, 10.4.4, 10.5.2, 13.3, Box 3.2, TS.4.4, WGII 6.1.1.2, Box CC-OA}