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

==== 4.2.1.3 Observed and Estimated Past Changes in Soil Moisture and Aridity ====

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AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) find that a global trend in soil moisture is detectable in a reanalysis and is attributable to GHG forcing, and conclude that it is ''very likely'' that anthropogenic climate change affected global patterns of soil moisture over the 20th century.

Changes in soil moisture and land surface aridity are due to changes in the relative balance of precipitation and ET. Soil moisture is also affected by irrigation. Regional trends derived from satellite remote sensing products show increases and decreases in annual surface soil moisture of up to 20% or more between the late 1970s and late 2010s (Figure 4.4). For example, using the ESA CCI SM v03.2 COMBINED products ( [[#van%20der%20Schalie--2021|van der Schalie et al., 2021]] ), approximately 0.9 billion people live in regions with decreasing surface soil moisture, and 2.1 billion people live in regions with increasing surface soil moisture (Figure 4.4, b). However, there are disagreements between data sets on the direction of change in some regions ( [[#Seneviratne--2010|Seneviratne et al., 2010]] ; [[#Feng--2015|Feng and Zhang, 2015]] ; [[#Feng--2016|Feng, 2016]] ), so quantification is subject to ''low confidence'' .

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[[File:8628d0bedc684eb085010919d5df685c IPCC_AR6_WGII_Figure_4_004.png]]

'''Figure 4.4 |''' '''Global patterns of changes in surface soil moisture and people in regions with significant changes.'''

'''(a)''' Percentage changes in annual mean surface soil moisture (0–5 cm) for 1978–2018 from satellite remote sensing: the “COMBINED” product of European Space Agency Climate Change Initiative Soil Moisture (ESA CCI SM v03.2), which blends data products from two microwave instruments, a scatterometer measuring radar backscattering and a radiometer measuring brightness temperature ( [[#van%20der%20Schalie--2021|van der Schalie et al., 2021]] ).

'''(b)''' The population density in 0.25° grid boxes with trends of significantly increasing and decreasing soil moisture from (a). Stippling indicates where changes are not significant.

Analysis of changes in P–ET estimates for 1948–2005 ( [[#Greve--2014|Greve et al., 2014]] ) suggests that geographical variations in soil moisture trends are more complex than the ‘wet get wetter, dry get drier’ (WGWDGD) paradigm. This is also supported by remote sensing data, with ESA CCI data for 1979–2013 showing only 15% of land following the WGWDGD paradigm for soil moisture ( [[#Feng--2015|Feng and Zhang, 2015]] ). Defining arid, humid and transitional areas according to precipitation and temperature regimes, all three classes of regions see more widespread trends of declining soil moisture than increasing soil moisture ( [[#Feng--2015|Feng and Zhang, 2015]] ). In the ESA CCI product, increasing soil moisture trends are mainly seen in humid or transitional areas and are rare in arid regions (Table 4.2)

'''Table 4.2 |''' Proportions of arid, transitional and humid areas with drying and wetting trends in surface soil moisture from remote sensing, 1979–2013 ( [[#Feng--2015|Feng and Zhang, 2015]] ).

{| class="wikitable"
|-
! Areas

! % of the area with a drying trend

! % of the area with a wetting trend

|-
| Arid

| 38.4

| 2.9

|-
| Transitional

| 13.0

| 10.5

|-
| Humid

| 16.3

| 8.1

|}

Reconstructions of historical soil moisture trends with data-driven models and process-based land surface models indicate drier dry seasons predominantly in extratropical latitudes, including Europe, western North America, northern Asia, southern South America, Australia and eastern Africa, consistent with climate model simulations of changes due to human-induced climate change ( [[#Padrón--2020|Padrón et al., 2020]] ). Furthermore, reduced water availability in the dry season is generally a consequence of increasing ET rather than decreasing precipitation ( [[#Padrón--2020|Padrón et al., 2020]] ).

While observationally based data for soil moisture are now more widely available, regional trends remain uncertain due to disagreements between data sets, so confident assessments of soil moisture changes remain a knowledge gap.

In summary, global mean soil moisture has slightly decreased, but regional changes vary, with both increases and decreases of 20% or more in some regions ( ''medium confidence'' ). Drying soil moisture trends are more widespread than wetting trends, not only in arid areas but also in humid and transitional areas ( ''medium confidence'' ). Reduced dry-season water availability is driven mainly by increasing transpiration ( ''medium confidence'' )

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