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

==== 4.4.1.3 Projected Changes in Soil Moisture ====

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AR5 ( [[#Collins--2013)|Collins et al., 2013)]] mainly focused on surface (upper 10 cm) soil moisture, summarising multi-model projections of 21st century annual mean soil moisture changes as broadly decreasing in the subtropics and Mediterranean region and increasing in east Africa and central Asia across the RCPs, with the changes tending to become stronger as global warming increases. AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) draw broadly similar conclusions based on new ESMs, noting that compared to CMIP5, the CMIP6 models project more consistent drying in the Amazon basin, Siberia, westernmost North Africa and southwestern Australia. WGI ( [[#Douville--2021|Douville et al., 2021]] ) also note that soil moisture in the upper 10 cm shows more widespread drying than in the total soil column.

The CMIP6 multi-model ensemble of ESMs show varying levels of consensus on projected changes in surface soil moisture with global warming (Figure 4.15). As in CMIP5 ( [[#Cheng--2017|Cheng et al., 2017]] ), uncertainties are substantial, often associated with uncertainties in projected regional precipitation changes ( [[#4.4.1.1|Section 4.4.1.1]] ), and in most regions, both increases and decreases are projected across the ensemble. In the far north of North America and Asia, projected changes in soil moisture at 4°C global warming range from a 20–30% decrease to an increase of 30–40%. In northern mid-latitudes, projections range from a 10–20% decrease to an increase of 20–30%, except for eastern North America, where the projected changes (both increases and decreases) are less than 10%, and western Europe and the Mediterranean where there is a stronger consensus towards decreased soil moisture of up to 25%. South America, southern Africa and Asia also show a stronger consensus towards decreased soil moisture of up to 40% or more in some regions.

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'''Figure 4.15 |''' '''Projected percentage changes in annual mean total column soil moisture relative to 1981–2010 at global warming levels (GWLs) of 4°C (top), 2°C (middle) and 1''' '''.''' '''5°C (bottom) for the CMIP6 multi-model ensemble of GCMs driven by SSP5-8.5 concentrations.''' The distribution of outcomes is shown at local scales with the 5th, 50th and 95th percentile soil moisture changes in individual grid boxes. Note that these are uncertainties at individual points and are not spatially coherent, that is, they do not represent plausible global patterns of change. Results for 1.5°C, 2°C and 4°C global warming are defined as 20-year means relative to 1850–1900 and use 34, 34 and 26 ensemble members, respectively, due to some members not reaching 4°C global warming. Fewer models are shown here than in Figure 4.10 on precipitation and Figure 4.14 on ET because some do not provide soil moisture output.

Most CMIP6 models simulate direct CO 2 effects on plant transpiration, which has been shown to be a strong influence on projected future changes in soil moisture ( [[#Milly--2016|Milly and Dunne, 2016]] ). Approaches that neglect this process project greater decreases in soil moisture availability than the climate models ( [[#Roderick--2015|Roderick et al., 2015]] ; [[#Swann--2016|Swann et al., 2016]] ). Therefore, although several studies project increased global aridity and dryland expansion ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Sherwood--2014|Sherwood and Fu, 2014]] ; [[#Huang--2016a|Huang et al., 2016a]] ), these may overestimate future drying (Berg et al., 2017). Nevertheless, land surface models, including vegetation responses to CO 2, still project reduced soil moisture in many regions ( [[#Grillakis--2019|Grillakis, 2019]] ).

A critical knowledge gap concerns the relative importance of climate and CO 2 physiological effects on soil moisture, in relation to uncertainties in climate sensitivity. For a given level of global warming, the relative importance of climate effects and the direct effects of CO 2 on transpiration depend on the CO 2 concentration accompanying that level of warming (Betts and McNeall, 2018). Some CMIP6 models have very high climate sensitivities ( [[#Meehl--2020|Meehl et al., 2020]] ), which are assessed as being of low probability on the basis of other lines of evidence ( [[#Sherwood--2020|Sherwood et al., 2020]] ). This means that the CO 2 concentration accompanying specific global warming levels may be too low and lead to overly large projections of soil moisture decrease in those models.

In summary, projected soil moisture changes increase with levels of global warming ( ''high confidence'' ), although there remains substantial disagreement on specific regional changes. In the CMIP6 multi-model ensemble at 4°C global warming, decreased soil moisture of up to 40% is projected in Amazonia, southern Africa and western Europe in all models ( ''high confidence'' ). In all other regions, there is no consensus on the sign of projected soil moisture changes, and projected changes at 4°C global warming include decreases of up to 30% and increases of up to 40%. Projected changes are smaller at lower levels of global warming, with similar geographical patterns of change.

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