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

=== 4.2.1 Observed Changes in Precipitation, Evapotranspiration and Soil Moisture ===

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==== 4.2.1.1 Observed Changes in Precipitation ====

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AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) concluded that GHG forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas ( ''medium confidence'' ), with a detectable precipitation increase in the northern high latitudes ( ''high confidence'' ). GHG forcing has also contributed to drying in dry summer climates, including the Mediterranean, southwestern Australia, southwestern South America, South Africa and western North America ( ''medium to high confidence'' ) (Figure 4.3). AR6 WGI ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) also concluded that the frequency and intensity of heavy precipitation events have ''likely'' increased at the global scale over most land regions with good observational coverage. Heavy precipitation has ''likely'' increased on the continental scale over North America, Europe and Asia. Regional increases in heavy precipitation frequency and (or) intensity have been observed with at least ''medium confidence'' for nearly half of the AR6 WGI climatic regions (Figure 4.3). Human influence, in particular GHG emissions, is ''likely'' the main driver of the observed global-scale intensification of heavy precipitation in land regions

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'''[[File:0c19bfccc496c614c6eb3417eaa9c58e IPCC_AR6_WGII_Figure_4_003.png]] Figure 4.3 |''' '''Observed mean and extreme precipitation changes and people experiencing the emergence of historically unfamiliar precipitation and changes in extreme precipitation.'''

'''(a)''' Percentage changes in annual mean precipitation over land (1891–2019) per °C global warming in the Global Precipitation Climatology Centre (GPCC) v2020 data set ( [[#Schneider--2017|Schneider et al., 2017]] ; [[#Schneider--2020|Schneider et al., 2020]] ). Green shows increasing precipitation; orange shows decreasing precipitation.

'''(b)''' Levels of unfamiliarity of wetter and drier climates, classified in terms of the ratio of the signal S of change to the noise N of variability, where the latter is defined as one standard deviation in annual data with the trend removed, that is, occurs approximately one in 6 years. Grey regions are either unobserved (oceans) or deserts (&lt;250 mm year –1 ). Stippling indicates where the signal of change is not significant. See [[#Hawkins--2020|Hawkins et al. (2020)]] for further details.

'''(c)''' Population densities in regions with annual precipitation classified as “emerging”.

'''(d)''' Precipitation trends from the GPCC data set in December, January and February (mm day –1 decade –1 ).

'''(e)''' As (d) for June-July-August.

'''(f)''' Changes in annual maximum 1-day precipitation (Rx1day) in the HadEX3 data set ( [[#Dunn--2020|Dunn et al., 2020]] ).

'''(g)''' Trend in annual mean consecutive dry days (CDD), 1950–2018, in HadEX3.

'''(h)''' Population densities per grid box where the trend in Rx1day is significantly different from zero.

'''(i)''' Population densities per grid box where the trend in CDD is significantly different from zero. Stipples in (h) and (i) show where HadEX3 data is available. Population data in (c), (h) and (i) are for 2020 from CIESIN (2018a; 2018b).

Large numbers of people live in regions where the annual mean precipitation is now ‘unfamiliar’ compared to the mean and variability between 1891 and 2016 (Figure 4.3c). “Unfamiliar” is defined as the long-term change being greater than one standard deviation in the annual data (Figure 4.3b). In 2020, approximately 498 million people lived in unfamiliarly wet areas, where the long-term average precipitation is as high as previously seen in only about one in 6 years ( ''medium confidence'' ) (Figure 4.3c). These areas are primarily in mid and high latitudes ( [[#Hawkins--2020|Hawkins et al., 2020]] ). On the other hand, approximately 163 million people lived in unfamiliarly dry areas, mostly in low latitudes ( ''medium confidence'' ). Due to high variability over time, the signal of long-term change in annual mean precipitation is not distinguishable from the noise of variability in many areas ( [[#Hawkins--2020|Hawkins et al., 2020]] ), implying that the local annual precipitation cannot yet be defined ‘unfamiliar’ by the above definition.

Notably, many regions have seen increased precipitation for part of the year and decreased precipitation at other times ( ''high confidence'' ) (Figure 4.3d,e), leading to small changes in the annual mean precipitation. Therefore, the numbers of people seeing unfamiliar seasonal precipitation levels are expected to be higher than those quoted above for unfamiliar annual precipitation changes ( ''medium confidence'' ). Still, quantified analysis of this is not yet available.

The intensity of heavy precipitation has increased in many regions ( ''high confidence'' ), including much of North America, most of Europe, most of the Indian sub-continent, parts of northern and southeastern Asia, much of southern South America, parts of southern Africa and parts of central, northern and western Australia (Figure 4.3 f) ( [[#Dunn--2020|Dunn et al., 2020]] ; [[#Sun--2020|Sun et al., 2020]] ). Conversely, heavy precipitation has decreased in some regions, including eastern Australia, northeastern South America and western Africa. The length of dry spells has also changed, with increases in annual mean consecutive dry days (CDD) in large areas of western, eastern and southern Africa, eastern and southwestern South America, and Southeast Asia, and decreases across much of North America (Figure 4.3g). Precipitation extremes have changed in some places where annual precipitation shows no trend. Some regions such as southern Africa and parts of southern South America are seeing increased heavy precipitation and longer dry spells. Many regions with changing extremes are highly populated, such as the Indian sub-continent, Southeast Asia, Europe and parts of North America, South America and southern Africa (Figure 4.3h,i). Substantially more people (~709 million) live in regions where annual maximum one-day precipitation has increased than in regions where it has decreased (~86 million) ( ''medium confidence'' ). However, more people are experiencing longer dry spells than shorter dry spells: approximately 711 million people live in places where annual mean CDD is longer than in the 1950s, and ~404 million in places with shorter CDD ( ''medium confidence'' ) (Figure 4.3i).

In summary, annual mean precipitation is increasing in many regions worldwide and decreasing over a smaller area, particularly in the tropics. Nearly half a billion people live in areas with historically unfamiliar wet conditions, and over 160 million in areas with historically unfamiliar dry conditions ( ''medium confidence'' ). Over 700 million people experience heavy precipitation significantly more intense than in the 1950s, but less than 90 million experience decreased heavy precipitation. Compared to the 1950s, 711 million people now experience longer dry spells and 404 million experience shorter dry spells.

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==== 4.2.1.2 Observed and Reconstructed Changes in Evapotranspiration ====

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WGI ( [[#Douville--2021|Douville et al., 2021]] ) conclude with ''high confidence'' that global terrestrial annual ET has increased since the early 1980s, driven by both increasing atmospheric water demand and vegetation greening ( ''medium confidence'' ), and can be partly attributed to anthropogenic forcing ( ''high confidence)'' .

Regional changes in ET depend on changes in both the climate and the properties of the land surface and ecosystems. The latter also responds to changes in climate and atmospheric composition. For example, a warming climate increases evaporative demand (Huang M et al., 2015; [[#Berg--2016|Berg et al., 2016]] ), although seasonal rainfall totals ( [[#Hovenden--2014|Hovenden et al., 2014]] ) affect the amount of soil moisture available for evaporation. Since transpiration accounts for much of the land-atmosphere water flux ( [[#Good--2015|Good et al., 2015]] ), vegetation changes also play a significant role in overall changes in ET.

With higher CO 2 , the increase in evaporative demand can, to some extent, be counteracted by reduced stomatal conductance (‘physiological effect’), which reduces transpiration and increases leaf-level water use efficiency (WUE), but is highly species-specific. There is evidence for recent increases in leaf-scale WUE from tree rings (14 ± 10%, broadleaf to 22 ± 6%, evergreen over the 20th century: ( [[#Frank--2015|Frank et al., 2015]] )), carbon isotopes (30 to 35% increase in 150 years: ( [[#van%20der%20Sleen--2014|van der Sleen et al., 2014]] )), and satellite-based measurements (1982–2008) combined with data-driven models (Huang M et al., 2015). WUE is also affected by aerodynamic conductance ( [[#Knauer--2017|Knauer et al., 2017]] ), nutrient limitation ( [[#Medlyn--2015|Medlyn et al., 2015]] ; [[#Donohue--2017|Donohue et al., 2017]] ), soil moisture availability ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ; [[#Medlyn--2015|Medlyn et al., 2015]] ), and ozone pollution ( [[#King--2013|King et al., 2013]] ; [[#Frank--2015|Frank et al., 2015]] ).

Higher CO 2 also increases photosynthesis rates, though this may not be maintained in the longer term ( [[#Warren--2015|Warren et al., 2015]] ; [[#Adams--2020|Adams et al., 2020]] ), particularly where temperatures exceed the thermal maxima for photosynthesis (Duffy et al., 2021). Higher photosynthesis increases leaf area index (LAI) (‘structural effect’) and therefore transpiration; 55 ± 25% of observed increases in ET (1980–2011) have been attributed to LAI change (Zeng Z. et al., 2018). Increases in ET driven by increased LAI (from satellite observations 1982–2012) are estimated at 0.32 ± 0.07 mm month –1 per decade, generating a climate forcing of −0.31 Wm– 2 per decade ( [[#Zeng--2017|Zeng et al., 2017]] ).

Overall regional transpiration change depends on the balance between the physiological and structural effects (e.g., [[#Tor-ngern--2015|Tor-ngern et al., 2015]] ; [[#Ukkola--2015|Ukkola et al., 2015]] ). In dry regions, ET may increase due to increasing LAI (Huang M et al., 2015), but in some densely vegetated regions, the stomatal effect dominates ( [[#Mao--2015|Mao et al., 2015]] ). Reductions in transpiration due to rising CO 2 concentrations may also be offset by a longer growing season ( [[#Frank--2015|Frank et al., 2015]] ; [[#Mankin--2019|Mankin et al., 2019]] ). Other factors modulate the transpiration effect both temporally and spatially, for example, additional vegetation structural changes ( [[#Kim--2015|Kim et al., 2015]] ; [[#Domec--2017|Domec et al., 2017]] ), vegetation disturbance and age ( [[#Donohue--2017|Donohue et al., 2017]] ) and species ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ).

Recent studies report global ET increases from the early 1980s to 2009 and 2013 (Table 4.1). Calculations informed by observations suggest that ET has increased in most regions, with statistically significant (p&lt;0.05) trends of up to 10 mm yr -2 observed in large parts of North America and northern Eurasia. Larger increases in ET are also observed in several regions, including northeast Brazil, western central Africa, southern Africa, southern India, southern China, and northern Australia. Decreases of around 10 mm yr -2 are reported for western Amazonia and central Africa ( [[#Miralles--2014|Miralles et al., 2014]] ), although not across all data sets ( [[#Zeng--2018|Zeng et al., 2018]] ). In estimates of past changes in long-term drying or wetting of the land surface driven by climate, uncertainties in ET observations or reconstructions make a more substantial contribution to the overall uncertainty than observed changes in precipitation ( [[#Greve--2014|Greve et al., 2014]] ). Other changes in ET are also driven strongly by land cover changes and irrigation ( [[#Bosmans--2017|Bosmans et al., 2017]] ).

'''Table 4.1 |''' Trends in global evapotranspiration for different periods between 1981–1982 and 2009–2013.

{| class="wikitable"
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! Trend (mm yr -2 )

! Period

! Data source

! Author(s)

|-
| +0.54

| 1981 to 2012

| Observations

| (Zhang Y. et al., 2016)

|-
| +1.18

| 1982 to 2010

| Observations

| ( [[#Mao--2015|Mao et al., 2015]] )

|-
| +0.93 ± 0.31

| 1982 to 2010

| LSMs

| ( [[#Mao--2015|Mao et al., 2015]] )

|-
| +0.88

| 1982 to 2013

| Remote-sensing data

| (Zhang K. et al., 2015)

|-
| +1.5

| 1982 to 2009

| Remote-sensing and surface observations

| ( [[#Zeng--2014|Zeng et al., 2014]] )

|}

The contribution of changes in WUE to observed changes in ET is a key knowledge gap. WGI assigned ''low confidence'' to this contribution. Estimating large-scale transpiration response to increased CO 2 based on leaf-level responses of WUE is not straightforward ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ; [[#Medlyn--2015|Medlyn et al., 2015]] ; [[#Tor-ngern--2015|Tor-ngern et al., 2015]] ; [[#Walker--2015|Walker et al., 2015]] ; [[#Kala--2016|Kala et al., 2016]] ) and new methodological approaches are needed.

In summary, there is ''high confidence'' that ET increased by between approximately 0.5 and 1.5 mm yr -2 between the 1980s and early 2010s due to warming-induced increased atmospheric demand worldwide and greening of vegetation in many regions. Increases in many areas are 10 mm yr –2 or more, but in some tropical land areas, ET has decreased by 10 mm yr –2 . Plant stomatal responses to rising CO 2 concentrations may play a role, but there is ''low confidence'' in quantifying this. Changes in land cover and irrigation have also changed regional ET ( ''medium confidence'' ).

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==== 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"
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! Areas

! % of the area with a drying trend

! % of the area with a wetting trend

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| Arid

| 38.4

| 2.9

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| 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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