Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.3.5 Precipitation minus evaporation =====

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The AR5 concluded that the pattern of precipitation minus evaporation (P–E) over the ocean had been enhanced since the 1950s ( ''medium confidence'' ). Saline surface waters had become saltier, while the relatively fresh surface waters had become fresher. The inferred changes in P–E were consistent with the observed increased TCWV, although uncertainties in the available products prevented identifying robust trends.

Estimating global-scale trends in P–E using direct observations alone is challenging due to limited evaporation measurements and inhomogeneities in satellite-derived precipitation and evaporation datasets ( [[#Hegerl--2015|Hegerl et al., 2015]] ; [[#López--2017|López et al., 2017]] ). Hence, the assessment of global P–E trends is generally performed using reanalyses, although changes in the observing system imply considerable uncertainty ( [[#Skliris--2014|Skliris et al., 2014]] ). Since the second half of the <sup></sup> 20th century, several reanalyses and observational datasets have shown increases in P–E over global land, although 75% of land areas exhibit no significant changes and both internal variability and observational uncertainty are substantial ( [[#Greve--2014|Greve et al., 2014]] ; [[#Robertson--2016|Robertson et al., 2016]] ). The recently released ERA5 ( [[#Hersbach--2020|Hersbach et al., 2020]] ) showed improvements in the representation of tropical precipitation, although it overestimates global precipitation trends in comparison to ERA-Interim and GPCP ( [[#Nogueira--2020|Nogueira, 2020]] ), and suffers from temporal changes in the annual balance between precipitation and evaporation ( [[#Hersbach--2020|Hersbach et al., 2020]] ). The spatial pattern of P–E trends over 1980–2019 (Figure 2.16a) are largely consistent with the trends in the GPCP v2.3 precipitation dataset (Figure 2.15f and [[#2.3.1.3.4|Section 2.3.1.3.4]] ) and agrees in sign with the trends from other reanalyses such as JRA-55 and MERRA-2 (L. [[#Yu--2020|]] [[#Yu--2020|Yu et al., 2020]] ).

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[[File:065e14ccefd8d7dafb150d10c9e869fc IPCC_AR6_WGI_Figure_2_16.png]]

'''Figure 2.''' '''16 |''' '''Changes in precipitation minus evaporation. (a)''' Trends in precipitation minus evaporation (P–E) between 1980 and 2019. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] (‘×’ marks denote non-significant trends). Time series of '''(b)''' global, '''(c)''' land-only and '''(d)''' ocean-only average annual P–E (mm day <sup>–1</sup> ). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

A variety of reanalysis products exhibit diverse temporal evolutions of P–E (Figure 2.16b–d). Globally MERRA-2, ERA20C and ERA20CM exhibit little change whereas JRA-55, ERA5 and 20CRv3 all imply long-term changes (Figure 2.16d). A potential limitation in estimating P–E from some reanalysis products is readily apparent when considering the temporal evolution of global P–E from CFSR and MERRA (Figure 2.16d) which both exhibit strong discontinuities over the global ocean in the late 1990s. Over global land as a whole, precipitation exceeds evaporation (P–E &gt;0) for all the reanalysis products (Figure 2.16c), with decreasing trends in P–E for ERA5 and JRA-55 and increasing trends for MERRA-2 and CFSR. The P–E over the global ocean is negative (evaporation exceeding precipitation) for most reanalyses (Figure 2.16d), with declining trends in ERA5 and MERRA-2 dominated by trends in evaporation ( [[#Bosilovich--2017|Bosilovich et al., 2017]] ; [[#Hersbach--2020|Hersbach et al., 2020]] ) (Figure 2.16d). The recent increase in ocean evaporation was also documented for several reanalyses ( [[#Craig--2017|Craig et al., 2017]] ) and in satellite data ( [[#Andersson--2011|Andersson et al., 2011]] ; [[#Robertson--2014|Robertson et al., 2014]] ), although with considerable differences between available estimates ( [[#Chandanpurkar--2017|Chandanpurkar et al., 2017]] ; L. [[#Yu--2020|]] [[#Yu--2020|Yu et al., 2020]] ). An alternative indirect approach to estimate P–E changes is based on near-surface ocean salinity ( [[#2.3.3.2|Section 2.3.3.2]] ), which is partially driven by the freshwater flux at the ocean surface. The near-surface salinity trends are more spatially coherent compared to those revealed by P–E estimates from reanalyses, with an intensification of the water cycle over oceans, especially in subtropical regions ( [[#Durack--2012|Durack et al., 2012]] ; [[#Skliris--2014|Skliris et al., 2014]] ; L. [[#Yu--2020|]] [[#Yu--2020|Yu et al., 2020]] ). However, the precise rate of water cycle intensification implied by salinity trends is sensitive to methodological choices (e.g., [[#Skliris--2016|Skliris et al., 2016]] ; [[#Zika--2018|Zika et al., 2018]] ).

In conclusion, observational uncertainty yields ''low confidence'' in globally averaged trends in P–E over the 20th century, with a spatial pattern dominated by precipitation changes over land and by evaporation increases over the ocean. Different reanalyses disagree on the sign of long-term changes in the global mean P–E.

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