Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.5.1.4 Precipitation ====

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The AR5 assessed that changes in mean precipitation in a warmer world will exhibit substantial spatial variation. Also, the contrast of mean precipitation between dry and wet regions and between dry and wet seasons will increase over most of globe as temperatures increase. The general pattern of change indicates that high latitude land masses are ''likely'' to experience greater amounts of precipitation due to the increased specific humidity of the warmer troposphere as well as increased transport of water vapour from the tropics by the end of this century under the RCP8.5 scenario. Many mid-latitude and subtropical arid and semi-arid regions will ''likely'' experience less precipitation, while many moist mid-latitude regions will ''likely'' experience more precipitation by the end of this century under the RCP8.5 scenario.

Since AR5, progress has been achieved in understanding changes in patterns and rates of precipitation with GSAT rise. The projected precipitation changes can be decomposed into a part that is related to atmospheric circulation referred to as dynamical component and a part related to water vapour changes, the thermodynamic component. Based on process understanding and modelling ( [[#Fläschner--2016|Fläschner et al., 2016]] ; [[#Samset--2016|Samset et al., 2016]] ), global mean precipitation will ''very likely'' increase by 1–3% per °C of GSAT warming (Section 8.2.1). The increase in atmospheric water vapour is a robust change under global warming, the sensitivity of global precipitation change to warming is smaller (2% per °C) as compared to water vapour change (7% per °C; [[#Held--2006|Held and Soden, 2006]] ). Global energy balance places a strong constraint on the global mean precipitation ( [[#Allen--2002|Allen and Ingram, 2002]] ; [[#Pendergrass--2014|Pendergrass and Hartmann, 2014]] ; [[#Myhre--2018|Myhre et al., 2018]] ; [[#Siler--2019|Siler et al., 2019]] ). Tropospheric radiative cooling constrains global precipitation ( [[#Pendergrass--2014|Pendergrass and Hartmann, 2014]] ), leading to a slow SST-dependent response and a forcing-dependent rapid adjustment. Rapid adjustments account for large regional differences in hydrological sensitivity across multiple drivers ( [[#Samset--2016|Samset et al., 2016]] ; [[#Myhre--2017|Myhre et al., 2017]] ). The rapid regional precipitation response to increased CO <sub>2</sub> is robust across models, implying that the uncertainty in long-term changes is mainly associated with the response to SST-mediated feedbacks ( [[#Richardson--2016|Richardson et al., 2016]] ). Precipitation response to fast adjustments and slow temperature-driven responses are assessed in detail in [[IPCC:Wg1:Chapter:Chapter-8|Chapter 8]] ( [[IPCC:Wg1:Chapter:Chapter-8#8.2.1|Section 8.2.1]] ).

The thermodynamic response to global warming is associated with a ‘wet get wetter’ mechanism, with enhanced moisture flux leading to subtropical dry regions getting drier and tropical and mid-latitude wet regions getting wetter ( [[#Held--2006|Held and Soden, 2006]] ; [[#Chou--2009|Chou et al., 2009]] ). Recent studies suggest that the dry-get-drier argument does not hold, especially over subtropical land regions ( [[#Greve--2014|Greve et al., 2014]] ; [[#Feng--2015|Feng and Zhang, 2015]] ; [[#Greve--2015|Greve and Seneviratne, 2015]] ). The discrepancy may be partly arising due to differences in model climatologies and by change in the location of wet and dry regions ( [[#Polson--2017|Polson and Hegerl, 2017]] ). Over the 21st century, significant rate of precipitation change is associated with a spatial stabilization and intensification of moistening and drying patterns ( [[#Chavaillaz--2016|Chavaillaz et al., 2016]] ). In the tropics, weakening of circulation leads to a ‘wet gets drier, dry gets wetter’ pattern ( [[#Chadwick--2013|Chadwick et al., 2013]] ). Climate model agreement for precipitation change in the tropics is lower than for other regions ( [[#Knutti--2013|Knutti and Sedláček, 2013]] ; [[#McSweeney--2013|McSweeney and Jones, 2013]] ). Sources of inter-model uncertainty in regional rainfall projections arise from circulation changes ( [[#Kent--2015|Kent et al., 2015]] ; [[#Chadwick--2016|Chadwick, 2016]] ) and spatial shifts in convection and convergence, associated with SST pattern change and land–sea thermal contrast change ( [[#Kent--2015|Kent et al., 2015]] ; [[#Chadwick--2017|Chadwick et al., 2017]] ) with a secondary contribution from the response to direct CO <sub>2</sub> forcing ( [[#Chadwick--2016|Chadwick, 2016]] ). Factors governing changes in large-scale precipitation patterns are assessed in detail in Sections [[IPCC:Wg1:Chapter:Chapter-8#8.2.2|8.2.2]] and [[IPCC:Wg1:Chapter:Chapter-10#10.4.1|10.4.1]] .

Long-term multi-model mean change in seasonal precipitation (JJA and DJF) from CMIP6 models (Figure 4.24) shows substantial regional differences and seasonal contrast. Changes in seasonal precipitation under SSP1-2.6 are small compared to internal variability. Consistent with the AR5, patterns of precipitation change are ''very likely'' to increase in the high latitudes especially during local winter and over tropical oceans under SSP3-7.0 ( ''high confidence'' ). CMIP6 projections show an increase in precipitation over larger parts of the monsoon regions and decreases in many subtropical regions including the Mediterranean, southern Africa and south-west Australia ( ''medium confidence'' ). The large-scale patterns of precipitation shown in Figure 4.24 are consistent with the patterns presented in Section 8.4.1.3. Precipitation changes exhibit strong seasonal characteristics ( [[IPCC:Wg1:Chapter:Chapter-8#box-8.2|Box 8.2]] ), and, in many regions, the sign of the precipitation changes varies with season. Precipitation variability is projected to increase over a majority of global land area, as assessed in [[IPCC:Wg1:Chapter:Chapter-8|Chapter 8]] (Section 8.4.1.3.3), over a wide range of time scales in response to warming ( [[#Pendergrass--2017|Pendergrass et al., 2017]] ).

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[[File:40ed6b163e8333e1955471a23a207836 IPCC_AR6_WGI_Figure_4_24.png]]

'''Figure 4.24 |''' '''Long-term change of seasonal mean precipitation.''' Displayed are projected spatial patterns of multi-model mean change (%) in '''(top)''' December–January–February (DJF) and '''(bottom)''' June–July–August (JJA) mean precipitation in 2081–2100 relative to 1995–2014, for (left) SSP1-2.6 and (right) SSP3-7.0. The number of models used is indicated in the top right of the maps. No map overlay indicates regions where the change is robust and ''likely'' emerges from internal variability, that is, where at least 66% of the models show a change greater than the internal-variability threshold ( [[#4.2.6|Section 4.2.6]] ) and at least 80% of the models agree on the sign of change. Diagonal lines indicate regions with no change or no robust significant change, where fewer than 66% of the models show change greater than the internal-variability threshold. Crossed lines indicate areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Most of the projected changes in precipitation exhibit a sharp contrast between land and ocean (Sections 8.2.1 and 8.4.1). Temperature-driven intensification of land-mean precipitation during the 20th century has been masked by fast precipitation responses to anthropogenic sulphate and volcanic forcing ( [[#Allen--2002|Allen and Ingram, 2002]] ; [[#Richardson--2018a|Richardson et al., 2018a]] ). Based on the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), land-mean precipitation is expected to increase more rapidly with the projected decrease in sulphate forcing and continued warming, contributing to increased global mean precipitation (Table 4.3) and will be clearly observable by the mid-21st century based on RCP4.5 and RCP8.5 scenarios ( [[#Richardson--2018a|Richardson et al., 2018a]] ).

Consistent with the findings of AR5, a gradual increase in global mean precipitation is projected over the 21st century with an increase of approximately 2.9% (1.0–5.2%) under SSP1-2.6 and 4.7% (2.3–8.2%) under SSP3-7.0 during 2081–2100 relative to 1995–2014. The corresponding increase in annual mean global land precipitation is 3.3% (0–6.6%), in the SSP1-2.6 and 5.8% (0.5–9.6%) in the SSP3-7.0 (Table 4.3). CMIP6 models show greater increases in precipitation over land than either globally or over the ocean ( ''high confidence'' ).

Based on the assessment of CMIP6 models, we conclude that it is ''very likely'' that, in the long term, global mean land and global mean ocean precipitation will increase with increasing GSAT. Annual mean and global mean precipitation will ''very likely'' increase by 1–3% per °C GSAT warming. The patterns of precipitation change will exhibit substantial regional differences and seasonal contrast as GSAT increases over the 21st century ( ''high confidence'' ). Precipitation will ''very likely'' increase over high latitudes and the tropical ocean and will ''likely'' increase in large parts of the monsoon regions. However, it is ''likely'' to decrease over the subtropics, including Mediterranean, southern Africa and south-west Australia, in response to GHG-induced warming.

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