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

===== 2.3.1.3.4 Global precipitation =====

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The AR5 concluded that there was ''low confidence'' in precipitation change averaged over global land areas prior to 1950, and ''medium confidence'' thereafter with no significant global trends. There was a ''likely'' overall increase in precipitation in the well-sampled NH mid-latitudes, with ''high confidence'' after 1951.

In situ precipitation records over land extend back for centuries in a few locations, and to the early to mid-20th century quasi-globally. Datasets differ in their input data, completeness of records, period covered, and the gridding procedures applied, which, given spatial clustering and the small spatial scales of precipitation, results in differences in global and regional estimates of precipitation changes (Q. [[#Sun--2018|]] [[#Sun--2018|Sun et al., 2018]] ; [[#Nogueira--2020|Nogueira, 2020]] ). The spatial variability of observed long-term trends (1901–2019) based on GPCC V2020 and CRU TS 4.04 (Figure 2.15a,b) indicates significant increases in precipitation mainly over eastern North America, northern Eurasia, southern South America and north-western Australia. Decreases are strongest across tropical western and equatorial Africa and southern Asia. The temporal evolution of global annual land precipitation anomalies exhibits little consistency between GPCC V2020, CRU TS 4.04 and GHCNv4 datasets, especially prior to 1950, that is associated with limitations in data coverage (Figure 2.15c; [[#Wu--2013|Wu et al., 2013]] ; [[#Shen--2014|Shen et al., 2014]] ; [[#Gu--2015|Gu and Adler, 2015]] ). These disagreements between datasets prior to the 1950s result in differences in trend estimates over global land (Table 2.6). A qualitative consistency in decadal and interdecadal variations between the products is only observed since the 1950s, with primarily positive land precipitation anomalies during the 1950s, 1970s and during 2000 to 2019 (Figure 2.15c).

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'''Table 2.''' '''6 |''' '''Globally averaged trend estimates over land and 90% confidence''' '''intervals for annual precipitation for each time series in Figure 2.15c over three periods all ending in 2019.''' Trends are calculated using OLS regression with significance assessed after [[#Santer--2008|Santer et al. (2008)]] . Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

{| class="wikitable"
|-
| rowspan="2"| '''Dataset'''

| colspan="3"| '''Trends in annual precipitation (mm yr''' <sup>–1</sup> '''per decade)'''

|-
| '''1901–2019'''

| '''1960–2019'''

| '''1980–2019'''

|-
| GPCCv2020

| 1.01 <sup>a</sup> ± 0.99

| 1.67 ± 3.23

| 5.60 ± 6.38

|-
| CRU TS 4.04

| 0.57 ± 2.08

| 0.17 ± 3.12

| 5.75 <sup>a</sup> ± 5.09

|-
| GHCNv4

| 3.19 <sup>a</sup> ± 1.48

| 5.03 <sup>a</sup> ± 4.87

| 11.06 <sup>a</sup> ± 9.17

|-
| GPCPv2.3

|
| 5.41 <sup>a</sup> ± 5.20

|}

<sup>a</sup> Trend values significant at the 10% level.

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[[File:c2addba488a0d6d769c525d313a70c08 IPCC_AR6_WGI_Figure_2_15.png]]

'''Figure 2.15''' '''|''' '''Changes in observed precipitation. (a, b)''' Spatial variability of observed precipitation trends over land for 1901–2019 for two global in-situ products. 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). '''(c)''' Annual time series and decadal means from 1891 to date relative to a 1981–2010 climatology (note that different products commence at distinct times). '''(d, e)''' as (a, b), but for the periods starting in 1980. '''(f)''' is for the same period for the globally complete merged GPCP v2.3 product. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Several satellite-based precipitation datasets improve the representation of the spatio-temporal changes since the late 20th century. Some of these are based exclusively on satellite data (e.g., CMORPH, [[#Joyce--2004|Joyce et al., 2004]] ; GSMaP, [[#Okamoto--2005|Okamoto et al., 2005]] ), with others being combinations of in situ observations, reanalyses and satellite retrievals (e.g., CMAP, [[#Xie--1997|Xie and Arkin, 1997]] ; TRMM 3B43 V7, [[#Huffman--2007|Huffman et al., 2007]] ; PERSIANN-CDR, [[#Ashouri--2015|Ashouri et al., 2015]] ; CHIRPS, [[#Funk--2015|Funk et al., 2015]] ; GPCP V2.3, [[#Adler--2018|Adler et al., 2018]] ). These can be affected by systematic and random uncertainties due to inhomogeneities in the satellite-derived precipitation and station data and the uncertainties of blending algorithms ( [[#Hegerl--2015|Hegerl et al., 2015]] ; Q. [[#Sun--2018|]] [[#Sun--2018|Sun et al., 2018]] ). The spatial coverage of these products is near-global, with available estimations formally covering 60°S–60°N with decreasing quality from low to high latitudes, depending on the sensors and algorithms used ( [[#Hu--2019|Hu et al., 2019]] ). A detailed description of the most relevant satellite products is provided in section 10.2.1.1.

Recent trends (1980–2019) for GPCC V2020, CRU TS 4.04 and GPCP V2.3 show significant increases in land precipitation over tropical Africa, the eastern portions of Europe and North America, central Asia and the Maritime Continent (Figure 2.14d–f). Significant decreases are observed over central South America, western North America, northern Africa and the Middle East. A detailed assessment of the recent regional precipitation trends using the same datasets can be found in the Atlas. Global trends for 1980–2019 show a general increase in annual precipitation over land, which is particularly marked for CRU TS 4.04 and GHCNv4 (Table 2.6). These changes have been accompanied by a strengthening of precipitation seasonality over tropical land areas, although with broad spread between different satellite-based (GPCP, MSWEP_V1.2, PERSIANN-CDR) and in situ gridded datasets (GPCC, CRU TS; [[#Chou--2013|Chou et al., 2013]] ; [[#Li--2016|Li et al., 2016]] ; [[#Tan--2020|Tan et al., 2020]] ). Increasing trends since 1980, in contrast to longer-term declining trends since 1901, are particularly evident over much of Africa, while more widespread negative trends were observed over much of southern South America in the more recent period ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] 7.2; [[#Knutson--2018|Knutson and Zeng, 2018]] ). A faster recent increase in precipitation over global land is inferred comparing the precipitation trends over 1960–2019 with 1980–2019 (Table 2.6). Over the global ocean, the comparison between precipitation datasets is compromised by the different measurement periods, as well as the spatial coverage of the available products ( [[#Adler--2017|Adler et al., 2017]] ; [[#Nguyen--2018|Nguyen et al., 2018]] ; [[#Jaber--2020|Jaber and Abu-Allaban, 2020]] ; [[#Nogueira--2020|Nogueira, 2020]] ), limiting the ability to assess the sign and magnitude of precipitation trends. The GPCPv2.3 database ( [[#Adler--2017|Adler et al., 2017]] , 2018) exhibits an increase of 2.94 mm yr <sup>–1</sup> per decade over 1980–2019, principally due to the trends over the Indian ocean and in the tropical western Pacific (Figure 2.15f). The regional patterns of recent trends are consistent with the documented increase in precipitation over tropical wet regions and the decrease over dry areas, estimated through GPCP v2.2 data ( [[#Liu--2013|Liu and Allan, 2013]] ; [[#Trammell--2015|Trammell et al., 2015]] ; [[#Kao--2017|Kao et al., 2017]] ; [[#Polson--2017|Polson and Hegerl, 2017]] ).

In summary, globally averaged land precipitation has ''likely'' increased since the middle of the <sup></sup> 20th century ( ''medium confidence'' ), with ''low confidence'' in trends prior to 1950. A faster increase in global land precipitation was observed since the 1980s ( ''medium confidence'' ), with large interannual variability and regional heterogeneity. Over the global ocean there is ''low confidence'' in the estimates of precipitation trends, linked to uncertainties in satellite retrievals, merging procedures and limited in situ observations.

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