Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.3.1.1 Global Water Cycle Intensity and P–E Over Land and Oceans ====

<div id="h3-11-siblings" class="h3-siblings"></div>

The human influence on the global water cycle is often summarized as an intensification ( [[#Huntington--2006|Huntington, 2006]] ; [[#DeAngelis--2015|DeAngelis et al., 2015]] ; W. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] b) or an overall strengthening which has been observed since at least 1980 ( ''high confidence'' ) (see Chapter 2). There is, however, no unique definition of the global water cycle intensity ( [[#Trenberth--2011|Trenberth, 2011]] ; [[#Ficklin--2019|Ficklin et al., 2019]] ; [[#Sprenger--2019|Sprenger et al., 2019]] ). One simple metric is the global and annual mean amount of precipitation. Although an increase in global precipitation is consistent with physical expectations ( [[#8.2.1|Section 8.2.1]] ), it has not yet been detected and attributed to human activities given large observational uncertainties and low signal-to-noise ratio ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.2|Section 3.3.2.2]] ). Other metrics are more suitable to detect and attribute changes in the global water cycle, including the ''likely'' increase in global land precipitation since 1950 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4|Section 2.3.1.4]] ) which is ''likely'' due to a human influence ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.3|Section 3.3.2.3]] ).

The flux of freshwater between the ocean and atmosphere is determined by the difference between precipitation and evaporation (P–E). Evaporation is measured in very few locations across the global ocean, so that directly assessing P–E over the ocean is very challenging and relies on indirect reanalysis estimates ( [[#Robertson--2020|Robertson et al., 2020]] ). The AR5 presented ''robust evidence'' of an amplified oceanic pattern in P–E since the 1960s from both regional and global surface and subsurface salinity measurements and reanalyses. This pattern is consistent with our theoretical understanding of human-induced changes in the water cycle, leading to the conclusion that these changes are ''very likely'' the result of anthropogenic forcings ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.2.2|Section 9.2.2.2]] ).

In contrast, AR5 did not provide a conclusive assessment of observed changes in P–E over land. Continental P–E estimated from reanalyses and data-driven land surface models indicate that interannual variations are linked to ENSO ( [[#Robertson--2014|Robertson et al., 2014]] , 2020). Increasing trends in P–E since 1979 based on land models are not statistically significant. Observations and models show evidence that P–E increases in the wet parts and decreases in the dry parts of tropical circulation systems, which shift in location seasonally and from year to year, with increases in seasonality since 1979 (see Box 8.2; [[#Chou--2013|Chou et al., 2013]] ; [[#Liu--2013|Liu and Allan, 2013]] ; [[#Fu--2014|Fu and Feng, 2014]] ).

In summary, a low signal-to-noise ratio, observational uncertainties and current data assimilation techniques limit the assessment of recent global trends in P–E over both land and ocean. It is ''likely'' that the global land P–E variations observed since the late 1970s were dominated by internal variability, mostly linked to ENSO teleconnections ( ''medium confidence'' ). In contrast, the attribution of changes in sea surface salinity ( [[IPCC:Wg1:Chapter:Chapter-3#3.5.2.2|Section 3.5.2.2]] ) suggests that it is ''extremely likely'' that human influence has contributed to the regional changes in P–E observed over the global ocean since the mid-20th century.

<div id="8.3.1.2" class="h3-container"></div>

<span id="water-vapour-and-its-transport"></span>