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

==== 8.4.1.6 Aridity and Drought ====

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The AR5 concluded that regional to global-scale projections of aridity and drought remained relatively uncertain compared to other aspects of the water cycle. It reported that there is a ''likely'' increase in drought occurrence ( ''medium confidence'' ) by 2100 in regions that are currently drought-prone under the RCP8.5 scenario due to projected decreases in soil moisture. It stated that it is ''likely'' that the most prominent projected decreases in soil moisture would occur in the Mediterranean, south-western USA, and southern Africa, consistent with projected changes in the Hadley circulation and increased surface temperatures. These AR5 conclusions are generally supported by more recent analyses of CMIP5 models ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Berg--2017|Berg et al., 2017]] ; [[#Cook--2018|Cook et al., 2018]] ).

Results from the latest generation of models in CMIP6 are largely congruent with CMIP5. Consistent with the coherent nature of warming in future projections, increases in vapour pressure deficit and evaporative demand are widespread and consistent across regions, seasons, and models, increasing in magnitude in accordance with the emissions scenario ( ''high confidence'' ) (Figure 8.19; [[#Scheff--2014|Scheff and Frierson, 2014]] , 2015; [[#Vicente-Serrano--2020|Vicente-Serrano et al., 2020]] ). Even under a low-emissions scenario (SSP1-2.6), projections of soil moisture show significant decreases in the Mediterranean, southern Africa, and the Amazonian basin ( ''high confidence'' ) (Figure 8.19). Under mid- and high-emissions scenarios (SSP2-4.5 and SSP5-8.5), coherent declines emerge across Europe, westernmost North Africa, south-western Australia, Central America, south-western North America, and south-western South America ( ''high confidence'' ) (Figure 8.19; [[#Cook--2020|Cook et al., 2020]] ). Compared to CMIP5 results, CMIP6 models exhibit more consistent drying in the Amazonian basin ( [[#Parsons--2020|Parsons, 2020]] ), more extensive declines in total soil moisture in Siberia ( [[#Cook--2020|Cook et al., 2020]] ), and stronger declines in westernmost North Africa and south-western Australia (Figure 8.19).

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[[File:4a50edd8b2c74bd6af406606bf94d850 IPCC_AR6_WGI_Figure_8_19.png]]

'''Figure 8.19 |''' '''Projected long-term relative changes in annual mean soil moisture and vapour pressure deficit.''' Global maps of projected relative changes (%) in annual mean vapor pressure deficit (left), surface soil moisture (top 10cm, middle) and total column soil moisture (right) from available CMIP6 models (number provided at the top right of each panel) for the SSP1.2-6 '''(a, b, c)''' , SSP2-4.5 '''(d, e, f)''' and SSP5-8.5 '''(g, h, i)''' scenarios respectively. All changes are estimated for 2081–2100 relative to a 1995–2014 base period. Uncertainty is represented using the simple approach. No overlay indicates regions with high model agreement (‘Robust change’), where ≥80% of models agree on sign of change, diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Soil moisture in the top soil layer (10 cm) shows more widespread drying than total soil moisture, reflecting a greater sensitivity of the upper soil layer to increasing evaporative demand (Figure 8.19; [[#Berg--2017|Berg et al., 2017]] ). Conversely, total column soil moisture represents the carry-over of moisture from previous seasons deeper in the soil column, and potentially higher sensitivity to vegetation processes ( [[#Berg--2017|Berg et al., 2017]] ; [[#Kumar--2019|Kumar et al., 2019]] ). Central America, the Amazonian basin, the Mediterranean region, southern Africa, and south-western Australia are projected to experience significant declines in total soil moisture, whereas declines in Europe (north of the Mediterranean), western Siberia, and north-eastern North America are limited to the surface (Figure 8.19). It should be noted that because models differ in their number of hydrologically active layers, there is less confidence in total soil moisture projections than surface soil moisture projections. Based on surface soil moisture projections, more than 40% of global land areas (excluding Antarctica and Greenland) are expected to experience robust year-round drying, even under lower emissions scenarios ( [[#Cook--2020|Cook et al., 2020]] ). The percentage of land area experiencing drying is slightly lower when runoff is used as an aridity metric instead (20–30%); taking this into consideration, it is estimated that about a third of global land areas will experience at least moderate drying in response to anthropogenic emissions, even under SSP1-2.6 ( ''medium confidence'' ) ( [[#Cook--2020|Cook et al., 2020]] ).

Although there are regions where multiple models predict consistent and significant changes in soil moisture, as with evapotranspiration ( [[#8.4.1.4|Section 8.4.1.4]] ), there is still uncertainty in these projections related to the response of plants to elevated CO <sub>2</sub> . Most models project increases in two variables that have opposite effects on surface water availability: plant water use efficiency (WUE) and leaf area index (LAI; see [[#8.4.1.4|Section 8.4.1.4]] ). As discussed in Sections 8.2.3.3, 8.3.1.4 and 8.4.1.4, there is ''low confidence'' in how these changes in plant physiology will affect future projections of evapotranspiration, and likewise, drought and aridity.

Changes in meteorological (precipitation-based) drought duration and intensity in CMIP6 models are more robust than projected changes in mean precipitation, more than found in CMIP5 projections ( [[#Ukkola--2020|Ukkola et al., 2020]] ). Significant increases in drought duration are expected in Central America, the Amazonian basin, south-western South America, the Mediterranean, westernmost North Africa, southern Africa, and south-western Australia, on the order of 0.5 to 1 month for a moderate emissions scenario (SSP2-4.5) and two months for a high-emissions scenario (SSP5-8.5; [[#Ukkola--2020|Ukkola et al., 2020]] ). Drought intensity is projected to increase in the tropics, mainly in the Amazonian basin, Central Africa, and southern Asia, as well as in Central America and south-western South America ( [[#Ukkola--2020|Ukkola et al., 2020]] ). The CORDEX South Asia multi-model ensemble projections indicate an increase in the frequency and severity of droughts over central and northern India during the 21st century, under the RCP4.5 and RCP8.5 scenarios ( ''medium confidence'' ) ( [[#Mujumdar--2020|Mujumdar et al., 2020]] ). Under intermediate or high-emissions scenarios, the likelihood of extreme droughts (events that have magnitudes equal to or less than the 10th percentile of the 1851–1880 baseline period) increases by 200–300% in the Amazonian basin, south-western North America, Central America, the Mediterranean, southern Africa, and south-western South America ( [[#Cook--2020|Cook et al., 2020]] ). Even under a low-emissions scenario (SSP1-2.6), the likelihood of extreme droughts increases by 100% in south-western North America, south-western South America, the Amazon, the Mediterranean, and southern Africa ( [[#Cook--2020|Cook et al., 2020]] ). Thus, there is ''high confidence'' that drought severity and intensity will increase in the Mediterranean, southern Africa, south-western South America, south-western North America, south-western Australia, Central America and the Amazonian basin.

Paleoclimate records provide context for these future expected changes in drought and aridity. In the Mediterranean, western North America, and Central Chile, there is ''high confidence'' that climate change will shift soil moisture (as represented by the Palmer Drought Severity Index) outside the range of observed and reconstructed values spanning the last millennium (Figure 8.20; [[#Cook--2014|Cook et al., 2014]] ; [[#Otto-Bliesner--2016|Otto-Bliesner et al., 2016]] ). Warmer temperatures, leading to increased evaporative losses, are clearly implicated in the projected future drying in these semi-arid regions ( [[#Dai--2018|Dai et al., 2018]] ), emphasizing the central role that warming plays in driving increased evaporative demand ( [[#Vicente-Serrano--2020|Vicente-Serrano et al., 2020]] ). In contrast, future trajectories are more uncertain in regions like Central Asia and eastern Australia–New Zealand where projected changes in precipitation and soil moisture are less coherent (Figure 8.19 and 8.20; [[#Hessl--2018|Hessl et al., 2018]] ). More information on projected changes in drought, including specific categories or drought, can be found in [[IPCC:Wg1:Chapter:Chapter-11#11.6.5|Section 11.6.5]] and [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] .

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[[File:9a77c8b4542932f9c263c3a08a9cc58c IPCC_AR6_WGI_Figure_8_20.png]]

'''Figure 8.20 |''' '''Past-to-future drought variability in paleoclimate reconstructions and models for select regions.''' On the left '''(a, c, e, g, i)''' , tree-ring reconstructed Palmer Drought Severity Index (PDSI) series (black line) for the Mediterranean (10°W–45°E, 30°–47°N; E.R. [[#Cook--2015|]] [[#Cook--2015|]] [[#Cook--2015|Cook et al., 2015]] ; [[#Cook--2016a|Cook et al., 2016a]] ), central Chile (70°W–74°W, 32°S–37°S; [[#Morales--2020|Morales et al., 2020]] ), western North America (117°W–124°W, 32°N–38°N; [[#Cook--2010|Cook et al., 2010]] ; [[#Griffin--2014|Griffin and Anchukaitis, 2014]] ), Eastern Australia and New Zealand (136°E–178°E, 46°S–11°S; [[#Palmer--2015|Palmer et al., 2015]] ), and Central Asia (99°E–107°E, 47°N–49°N; [[#Pederson--2014|Pederson et al., 2014]] ; [[#Hessl--2018|Hessl et al., 2018]] ) plotted in comparison to the past-to-future, fully-forced simulations from four ensemble members (thin blue lines) from the NCAR CESM Last Millennium Ensemble (thick blue line = ensemble mean) ( [[#Otto-Bliesner--2016|Otto-Bliesner et al., 2016]] ) for the same regions. The shaded area represents the range (10th to 90th percentile) of historical and future (RCP8.5) PDSI (Penman–Monteith) simulations from 15 CMIP5 models and 34 ensemble members for the same regions (1900–2100; [[#Cook--2014|Cook et al., 2014]] ). On the right '''(b, d, f, h, j)''' , the distribution of annual PDSI values from the past and present (850 to 2005 CE) (black) is compared to the future distribution (2006 to 2100 CE) (blue). The distributions show each of the four ensemble members from the CESM LME simulations. The future component of the CESM LME follows the RCP8.5 scenario. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

In summary, there is ''high confidence'' that soil moisture will decline in semi-arid, winter-rainfall dominated areas including the Mediterranean, southern Africa, south-western North America, south-western South America, and south-western Australia, as well as in Central America and the Amazonian basin. In general, these regions are expected to become drier both due to reduced precipitation ( ''medium confidence'' ) and increases in evaporative demand ( ''high confidence'' ). These same regions are ''likely'' to experience increases in drought duration and/or severity ( ''high confidence'' ). The magnitude of expected change scales with emissions scenarios ( ''high confidence'' ) but even under low-emissions trajectories, large changes in drought and aridity are expected to occur ( ''high confidence'' ) with consequences for regional water availability. In the Mediterranean, Central Chile, and western North America, future aridification will far exceed the magnitude of change seen over the last millennium ( ''high co'' ''nfidence'' ).

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