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

==== 8.3.1.3 Precipitation Amount, Frequency and Intensity ====

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This section assesses observed changes in precipitation at global and regional scales. Note that changes in precipitation seasonality are assessed in Box 8.2 and that changes in regional monsoons are assessed in section 8.3.2.4 where observed changes in both circulation and rainfall are considered. Further assessment of regional changes in precipitation is presented in Chapters 10, 12 and Atlas, while extreme precipitation is presented in Chapter 11.

The AR5 concluded that it is ''likely'' there has been an overall increase in annual mean precipitation amount over mid-latitude land areas in the NH, with ''low confidence'' since 1901, but ''medium confidence'' after 1951. There is further evidence of a faster increase since the 1980s ( ''medium confidence'' ) (Sections 2.3.1.3.4 and 3.3.2.2). Precipitation has increased from 1950 to 2018 over mid-high latitude Eurasia, most of North America, south-eastern South America, and north-western Australia, while it has decreased over most of Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Pacific coasts of Canada, as simulated by the CMIP5 multi-ensemble mean ( [[#Dai--2021|Dai, 2021]] ). Since AR5, there have been updates of several precipitation datasets, including satellite estimates, reanalysis and merged products ( [[#Adler--2017|Adler et al., 2017]] ; [[#Roca--2019|Roca, 2019]] ). However, observational uncertainties remain an issue for assessing regional trends in seasonal or annual mean precipitation amount (Hegerl et al. , 2015; Maidment et al. , 2015; Sarojini et al. , 2016; Beck et al. , 2017) , as well as the convective and stratiform types of precipitation (e.g., [[#Ye--2017|Ye et al., 2017]] ). Precipitation trends at regional scales are dominated by internal variability across much of the world ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). Regional changes in precipitation amounts can also be obscured by contrasting responses to GHG compared with aerosol forcings ( [[#Wu--2013|Wu et al., 2013]] ; [[#Hegerl--2015|Hegerl et al., 2015]] ; [[#Xie--2016|Xie et al., 2016]] ; [[#Zhao--2019|Zhao and Suzuki, 2019]] ; [[#Zhao--2020|Zhao et al., 2020]] ) and changes in precipitation intensity versus frequency ( [[#Shang--2019|Shang et al., 2019]] ).

Global and regional changes in precipitation frequency and intensity have been observed over recent decades. An analysis of 1875 rain gauge records worldwide over the period 1961–2018 indicates that there has been a general increase in the probability of precipitation exceeding 50 mm day <sup>–1</sup> , mostly due to an overall boost in rain intensity ( [[#Benestad--2019|Benestad et al., 2019]] ). Such changes in precipitation intensity and frequency have not been formally attributed to human activities, but are consistent with the heating effect of increasing CO <sub>2</sub> levels on the distribution of daily precipitation rates ( [[#8.2.3.2|Section 8.2.3.2]] ) and with a distinct overall intensification of heavy precipitation events found in both observations and CMIP5 models, though with an underestimated magnitude ( [[#Fischer--2014|Fischer and Knutti, 2014]] ). Beyond amplified precipitation extremes ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] ), CMIP5 models also indicate that anthropogenic forcings have increased temporal variability of annual precipitation amount over land from 1950 to 2005, which is most pronounced in annual mean daily precipitation intensity ( [[#Konapala--2017|Konapala et al., 2017]] ).

Anthropogenic aerosols can alter precipitation intensities both through radiative and microphysical effects (Box 8.1 and [[#8.5.1.1.2|Section 8.5.1.1.2]] ). Precipitation suppression through aerosol microphysical effects has been observed in shallow cloud regimes over South America and the south-eastern Atlantic, associated with local biomass burning ( [[#Andreae--2004|Andreae et al., 2004]] ; [[#Costantino--2010|Costantino and Bréon, 2010]] ), and in industrial regions in Australia ( [[#Rosenfeld--2000|Rosenfeld, 2000]] ; [[#Hewson--2013|Hewson et al., 2013]] ; [[#Heinzeller--2016|Heinzeller et al., 2016]] ). In contrast, precipitation intensification through aerosol microphysical effects in deep convective clouds is seen in many regions such as the Amazon, southern USA, India, and Korea. This is associated with anthropogenic aerosols from cities ( [[#Hewson--2013|Hewson et al., 2013]] ; [[#Fan--2018|Fan et al., 2018]] ; [[#Lee--2018|S.S. Lee et al., 2018]] ; [[#Sarangi--2018|Sarangi et al., 2018]] ).

In the tropics, increases in precipitation amount are observed in convergence zones and decreases in the descending branches of the atmospheric circulation since 1979 ( [[#Chou--2013|Chou et al., 2013]] ; [[#Liu--2013|Liu and Allan, 2013]] ; [[#Gu--2016|Gu et al., 2016]] ; [[#Polson--2016|Polson et al., 2016]] ; [[#Polson--2017|Polson and Hegerl, 2017]] ), consistent with increased moisture transports with warming ( [[#Gimeno--2020|Gimeno et al., 2020]] ). Over tropical land areas, there is substantial variability in the ‘wet convergent regimes get wetter’ and ‘dry divergent regimes get drier’ pattern of trends observed since 1950 that are modulated by decadal changes in ENSO ( [[#Liu--2013|Liu and Allan, 2013]] ; [[#Gu--2018|Gu and Adler, 2018]] ). CMIP6 models indicate an increased contrast between wet and dry regions in the tropics and subtropics (Figure 8.7; [[#Schurer--2020|Schurer et al., 2020]] ). This provides further evidence that rainfall has increased in wet regimes, and slightly decreased in dry regimes over the period 1988 – 2019 (Figure 3.14). This greater contrast is primarily attributable to greenhouse gas forcings, although the observed trends are statistically larger than the model responses ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.3|Section 3.3.2.3]] ).

Over the African continent, there are distinct precipitation trends observed in multiple datasets since the 1980s (Figure 8.7; [[#Maidment--2015|Maidment et al., 2015]] ; P. [[#Nguyen--2018|]] [[#Nguyen--2018|Nguyen et al., 2018]] ). Increases in intense convective storms affecting the Sahel have been attributed to increased land – ocean temperature gradients ( [[#Taylor--2017|Taylor et al., 2017]] ), enhanced by intense heating of the Sahara ( [[#Dong--2015|Dong and Sutton, 2015]] ) rather than thermodynamics ( [[#8.2.2|Section 8.2.2]] ). Changes in Sahel rainfall, with reduced precipitation amounts from the 1960s to the 1980s and a subsequent recovery, are assessed in Sections 8.3.2.4.3 and 10.4.2.1. In eastern Africa, decreasing precipitation amount (−2 to −7 % per decade for 1983 – 2010) was reported for the March to May ‘long rains’ season ( [[#Lyon--2012|Lyon and Dewitt, 2012]] ; [[#Viste--2013|Viste et al., 2013]] ; [[#Liebmann--2014|Liebmann et al., 2014]] ; [[#Maidment--2015|Maidment et al., 2015]] ; [[#Rowell--2015|Rowell et al., 2015]] ) and evidence of a recovery since, with internal variability playing a large role in these decadal changes ( [[#Wainwright--2019|Wainwright et al., 2019]] ). In contrast, the second ‘short rains’ season in eastern Africa (October to December) does not exhibit significant precipitation trends ( [[#Rowell--2015|Rowell et al., 2015]] ). Increases in annual southern African rainfall of 6 – 7% per decade during 1983 – 2010 are linked with the Pacific Decadal Oscillation (PDO; [[#Maidment--2015|Maidment et al., 2015]] ).

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[[File:7ceff4aed13183efc0490dbc9fa605ac IPCC_AR6_WGI_Figure_8_7.png]]

'''Figure 8.7 | Linear trends in annual mean precipitation (mm day''' <sup>–1</sup> '''per decade) for''' '''1901–1984''' '''(left) and''' '''1985–2014''' '''(right):''' '''(a, e) observational dataset, and the CMIP6 multi-model ensemble mean historical simulations driven by: (b, f) all radiative forcings; (c, g) GHG-only radiative forcings; (d, h) aerosol-only radiative forcings experiment.''' Colour shades without grey cross correspond to the regions exceeding 10% significant level. Grey crosses correspond to the regions not reaching the 10% statistically significant level. Nine CMIP6-DAMIP models have been used having at least three members. The ensemble mean is weighted per each model on the available and used members. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

( [[#8.3.1.6|Section 8.3.1.6]] assesses changes in precipitation over the Mediterranean region and its connection with drought and aridity.

Rainfall increases have been observed over northern Australia since the 1950s, with most of the increases occurring in the north-west ( [[#Dey--2019a|Dey et al., 2019a]] , [[#Dey--2019b|b]] ; [[#Dai--2021|Dai, 2021]] ) and decreases observed in the north-east ( [[#Li--2012|]] [[#Li--2012|J. Li et al., 2012]] ) since the 1970s. In contrast, there has been a decline in rainfall over southern Australia related to changes in the intensification and position of the subtropical ridge (CSIRO and BoM, 2015) and anthropogenic effects ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). The drying trend over south-west Australia is most pronounced during May to July, where rainfall has declined by 20% below the 1900–1969 average since 1970 and by about 28% since 2000 (BoM and CSIRO, 2020).

Over South America, there is observational and paleoclimate evidence of declining precipitation amount during the past 50 years over the Altiplano and central Chile, primarily explained by the PDO but with at least 25% of the decline attributed to anthropogenic influence ( [[#Morales--2012|Morales et al., 2012]] ; [[#Neukom--2015|Neukom et al., 2015]] ; [[#Boisier--2016|Boisier et al., 2016]] ; [[#Seager--2019b|Seager et al., 2019b]] ; [[#Garreaud--2020|Garreaud et al., 2020]] ). In contrast, a significant rainfall increase has been detected over the Peruvian–Bolivian Altiplano (from observational data and satellite-based estimations) since the 1980s (Figure 8.7; [[#Imfeld--2020|Imfeld et al., 2020]] ; [[#Segura--2020|Segura et al., 2020]] ). Long-term (1902 – 2005) precipitation data indicate positive trends over south-eastern South America and negative trends over the southern Andes, with at least a partial contribution from anthropogenic forcing ( [[#Gonzalez--2014|Gonzalez et al., 2014]] ; [[#Vera--2015|Vera and Díaz, 2015]] ; [[#Díaz--2017|Díaz and Vera, 2017]] ; [[#Boisier--2018|Boisier et al., 2018]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ; see further assessment in [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.2|Section 10.4.2.2]] and Atlas.7.2.2). The Peruvian Amazon has exhibited significant rainfall decreases during the dry season since 1980 ( [[#Lavado--2013|Lavado et al., 2013]] ; [[#Ronchail--2018|Ronchail et al., 2018]] ). Increases in wet season rainfall in the northern and central Amazon since the 1980s and decreases during the dry season in the southern Amazon ( [[#Barreiro--2014|Barreiro et al., 2014]] ; [[#Gloor--2015|Gloor et al., 2015]] ; [[#Martín-Gómez--2016|Martín-Gómez and Barreiro, 2016]] ; [[#Espinoza--2018|J.C. Espinoza et al., 2018]] ; [[#Wang--2018|X.Y. Wang et al., 2018]] ; [[#Haghtalab--2020|Haghtalab et al., 2020]] ) are not explained by radiative forcing based on CMIP6 experiments (Figure 8.7) and trends are insignificant over longer periods since 1930 ( [[#Kumar--2013|Kumar et al., 2013]] ) or more recently, since 1973 ( [[#Almeida--2017|Almeida et al., 2017]] ). See ( [[#8.3.2.4.5|Section 8.3.2.4.5]] for monsoon-related changes. For the tropical Andes region, trends in annual precipitation show heterogenous patterns, ranging between –4% per decade and +4% per decade in the northern and southern tropical Andes for a 30-year period at the end of the 20th century, although increases during 1965 – 1984 and decreases since 1984 have been registered in Bolivia ( [[#Carmona--2014|Carmona and Poveda, 2014]] ; [[#Pabón-Caicedo--2020|Pabón-Caicedo et al., 2020]] ).

Over China, annual precipitation totals changed little from 1973 to 2016, but precipitation intensity significantly increased at a rate of 0.12 mm day <sup>–1</sup> per decade, while the number of days with precipitation exceeding 0.1 mm day <sup>–1</sup> significantly decreased at a rate of 0.9 days per decade ( [[#Shang--2019|Shang et al., 2019]] ). There is consistency in trend estimates during 1998 – 2015 over mainland China among satellite-based products and station data, which show increased precipitation amounts in autumn and winter and decreases in summer ( [[#Chen--2018|Chen and Gao, 2018]] ), consistent with a decreased intensity of East Asian monsoon precipitation ( [[#Lin--2014|Lin et al., 2014]] ; [[#Deng--2018|Deng et al., 2018]] ). Further assessment of precipitation changes over the South and South East Asian and the East Asian monsoon regions is presented in [[#8.3.2.4|Section 8.3.2.4]] . An increasing trend in the frequency of heavy rainfall occurrences at the expense of low and moderate rainfall occurrences is found over central India ( [[#Krishnan--2016|Krishnan et al., 2016]] ; [[#Roxy--2017|Roxy et al., 2017]] ) and over eastern China with the latter due to increasing high aerosol levels ( [[#Qian--2009|Qian et al., 2009]] ; [[#Guo--2017|J. Guo et al., 2017]] ; [[#Xu--2017|Xu et al., 2017]] ; [[#Day--2018|Day et al., 2018]] ), consistent with the effects of absorbing aerosol on stability and convective inhibition (Box 8.1).

Observed precipitation records since the early 1900s show increases in precipitation totals over central and north-eastern North America that are attributable to anthropogenic warming but larger in magnitude than found in CMIP5 simulations ( [[#Knutson--2018|Knutson and Zeng, 2018]] ; [[#Guo--2019|Guo et al., 2019]] ). Decreases in precipitation amount over the central and south-western USA and increases over the north-central USA during 1983 – 2015 ( [[#Cui--2017|Cui et al., 2017]] ; P. [[#Nguyen--2018|]] [[#Nguyen--2018|Nguyen et al., 2018]] ), are not clearly associated with forced responses in CMIP6 simulations (Figure 8.7; see also [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.3|Section 10.4.2.3]] ). Over Europe, precipitation trends since 1979 do not show coherence across datasets ( [[#Zolina--2014|Zolina et al., 2014]] ; P. [[#Nguyen--2018|]] [[#Nguyen--2018|Nguyen et al., 2018]] ). Longer records since 1910 show increases for much of Scandinavia, north-western Russia, and parts of north-western Europe/United Kingdom and Iceland ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). Records since 1930 show increases of annual preciptation amount over western Russia (see also Atlas.8.2). Widespread increases in daily precipitation intensity appear clearly over regions with a high density of rain gauges, such as Europe and North America over the 1951 – 2014 period ( [[#Alexander--2016|Alexander, 2016]] ). Observations during 1966 – 2016 over northern Eurasia show increases in the contribution of heavy convective showers to total precipitation by 1 – 2% on average (with local trends of up to 5%) for all seasons except for winter ( [[#Chernokulsky--2019|Chernokulsky et al., 2019]] ). Increases in convective precipitation intensity have been identified, particularly on sub-daily time scales, using a range of modelling and observational data ( [[#Berg--2013|Berg et al., 2013]] ; [[#Kanemaru--2017|Kanemaru et al., 2017]] ; [[#Pfahl--2017|Pfahl et al., 2017]] ).

Snowfall is an important component of precipitation in high-latitude and mountain watersheds. Reanalysis data indicate significant reductions in annual mean potential snowfall areas over NH land by 0.52 million km <sup>2</sup> per decade, with the largest decline over the Alps, with snow water equivalent reductions of about 20 mm per decade ( [[#Tamang--2020|Tamang et al., 2020]] ). In the Tibetan Plateau, region-wide winter snowfall has increased but summer snowfall has decreased during the 1960 – 2014 period ( [[#Deng--2017|Deng et al., 2017]] ). State-of-the-art model simulations indicate reduced mean annual snowfall in the Arctic, despite the strong precipitation increase, mainly in summer and autum, when temperatures are close to the melting point ( [[#Bintanja--2017|Bintanja and Andry, 2017]] ).

In summary, regional changes in precipitation amounts can be obscured by the contrasting responses to GHG and aerosol forcings across much of the 20th century and can thus be dominated by internal variability at decadal to multi-decadal time scales ( ''high confidence'' ). There is, however, a detectable increase in northern high-latitude annual precipitation over land which has been primarily driven by human-induced global warming ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2|Section 3.3.2]] ). Human influence has strengthened the zonal mean precipitation contrast between the wet tropics and dry subtropics since the 1980s ( ''medium confidence'' ), although regional studies suggest a more complex precipitation response to evolving anthropogenic forcings. There is ''high confidence'' that daily mean precipitation intensities have increased since the mid-20th century in a majority of land regions with available observations and it is ''likely'' that such an increase is mainly due to GHG forcing (see [[IPCC:Wg1:Chapter:Chapter-11#11.4|Section 11.4]] ). [[#8.3.2.4|Section 8.3.2.4]] assesses monsoon precipitation changes in detail.

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