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

==== 8.3.1.6 Aridity and Drought ====

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The AR5 reported ''low confidence'' that changes in drought since the mid-20th century could be attributed to human influence, owing to observational uncertainties and difficulties in distinguishing decadal-scale variability from long-term trends. Changes in soil moisture, a metric of aridity, were not assessed thoroughly in AR5. Since AR5, new satellite products, land surface reanalyses, and land surface models have been used to document recent changes in soil moisture at the global scale. The science of detection and attribution has also progressed considerably ( [[#Trenberth--2015|Trenberth et al., 2015]] ; [[#Easterling--2016|Easterling et al., 2016]] ; [[#Stott--2016|Stott et al., 2016]] ). Attribution efforts have further benefited from the increased use of paleoclimate information, which provides an important constraint on natural variability that is insufficiently sampled by short observational record ( [[#Cook--2018|Cook et al., 2018]] ; [[#Kageyama--2018|Kageyama et al., 2018]] ).

Several studies have identified a persistent ‘fingerprint’ of anthropogenic forcing in global trends in aridity spanning the last 120 years. Using a combination of tree ring data, CMIP5 model simulations, and reanalysis products, [[#Marvel--2019|Marvel et al. (2019)]] determined that the dominant trend in aridity since 1900, characterized by drying in North and Central America and the Mediterranean, is detectable and attributable to external forcing from 1900 to 1949. This trend weakens from 1950 to 1975, possibly due to aerosol forcing ( [[#Marvel--2019|Marvel et al., 2019]] ), but then emerges again from 1981 to present, although it is not detectable in the GLEAM nor MERRA-2 soil moisture reanalysis products. Likewise, [[#Bonfils--2020|Bonfils et al. (2020)]] investigated changes in precipitation, temperature and continental aridity in CMIP5 historical simulations and found that the dominant multivariate fingerprint, an amplification of wet–dry latitudinal patterns and progressive continental aridification, was associated with greenhouse gas emissions (Figure 8.9a , d), and the second leading fingerprint was associated with anthropogenic aerosols (Figure 8.9e , h). This study found that the anthropogenic greenhouse gas signal is statistically detectable in reanalyses over the 1950–2014 period (signal-to-noise ratio above 1.96). [[#Gu--2019|Gu et al. (2019)]] found that a global trend in declining soil moisture is detectable in the GLDAS-2 reanalysis product and is attributable to greenhouse gas forcing. [[#Padrón--2020|Padrón et al. (2020)]] reconstructed the global patterns of dry season water availability from 1902–2014, and found it ''extremely likely'' (99% range) that trends in the last three decades of the analysis period could be attributed to anthropogenic forcing, mainly due to increases in evapotranspiration. It is ''very likely'' (&gt;90% range) that anthropogenic forcing has affected global patterns of soil moisture over the 20th century.

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[[File:44c55b7cf362f36b7ea9f26f789671a6 IPCC_AR6_WGI_Figure_8_9.png]]

'''Figure 8.9 |''' '''Spatial expressions (a–c, e–g) of the leading multivariate fingerprints of temperature (°C), precipitation (mm day''' <sup>–1</sup> '''), and aridity (CMI; the Climate Moisture Index) in CMIP5 historical simulations and the corresponding temporal evolution in both CMIP5 and reanalysis products (d, h).'''
The first leading fingerprint is associated with greenhouse gas forcing (a–d) and the second leading fingerprint is associated with aerosol forcing (e–h). CMI is a dimensionless aridity indicator that combines precipitation and atmospheric evaporative demand. Figure after [[#Bonfils--2020|Bonfils et al. (2020)]] . Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

On a regional scale, the robustness of trend attribution for drought and aridity varies widely. Key trends and their attributions are summarized here, while a complete regional assessment of observed trends in drought and aridity is in [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] (Sections 11.6.2, 12.3.2 and 12.4).

Several studies have analyzed CMIP5 and land surface models and detected a significant summer drying trend in the NH across the late 20th century that is attributable to anthropogenic forcings ( [[#Mueller--2016|Mueller and Zhang, 2016]] ; [[#Douville--2017|Douville and Plazzotta, 2017]] ). This trend is mainly driven by dryland areas such as the western USA and west-central Asia, where both reanalysis products and satellite data confirm there has been a persistent decline in soil moisture since 1990 (Y. [[#Liu--2019|]] [[#Liu--2019|Liu et al., 2019]] a). In the western USA, snow deficits have ''very likely'' contributed to recent drying ( [[#Mote--2018|Mote et al., 2018]] ). Spring snow water equivalent across the Sierra Nevada Mountains reached a record low in 2015 ( [[#Margulis--2016|Margulis et al., 2016]] ; [[#Mote--2016|Mote et al., 2016]] ), possibly the lowest of the last five hundred years ( [[#Belmecheri--2016|Belmecheri et al., 2016]] ). Over the longer California drought (2011–2015) anthropogenic warming alone reduced snowpack levels in the Sierras by 25% ( [[#Berg--2017|Berg and Hall, 2017]] ). The north-western USA also experienced snow drought in 2015, despite near-normal levels of total cold season precipitation ( [[#Mote--2016|Mote et al., 2016]] ; [[#Marlier--2017|Marlier et al., 2017]] ). There is ''high confidence'' that anthropogenic warming contributed to these recent snow droughts ( [[#Belmecheri--2016|Belmecheri et al., 2016]] ; [[#Mote--2016|Mote et al., 2016]] ).

In the western USA, anthropogenic warming is amplifying drought and aridity by increasing evaporative demand and water loss to the atmosphere ( [[#Weiss--2009|Weiss et al., 2009]] ; [[#Overpeck--2013|Overpeck, 2013]] ; [[#Cook--2014|Cook et al., 2014]] ; [[#Griffin--2014|Griffin and Anchukaitis, 2014]] ; [[#Williams--2020|Williams et al., 2020]] ). For the California drought between 2012–2014, [[#Griffin--2014|Griffin and Anchukaitis (2014)]] used paleoclimate reconstructions to determine that while rainfall deficits were not unprecedented, record-high temperatures drove an exceptional decline in soil moisture relative to the last millennium. [[#Williams--2015|Williams et al. (2015)]] concluded that anthropogenic warming accounted for 8–27% of these soil moisture deficits. [[#Robeson--2015|Robeson (2015)]] estimated that the California drought was a 1-in-10,000 year event. Tree ring reconstructions indicate that prolonged megadroughts have occurred in the western USA throughout the last 1200 years ( Cook et al. , 2004, 2010; B.I. Cook et al. , 2015 ), forced by internal variability ( [[#Coats--2016|Coats et al., 2016]] ; [[#Cook--2016b|Cook et al., 2016b]] ). However, [[#Williams--2020|Williams et al. (2020)]] determined that 2000–2018 drought across the south-western USA was the second driest 19-year period since 800 CE, and attributed nearly half the magnitude of this event to anthropogenic forcing (see also [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.3|Section 10.4.2.3]] ). Evidence for human signals in drought can also be found in western North American streamflow records, as noted above in [[#8.3.1.5|Section 8.3.1.5]] . There is ''high confidence'' that anthropogenic forcing has contributed to recent droughts and drying trends in western North America.

Large areas of east-central Asia experienced drying in the early 2000s as a result of warmer temperatures, lower humidity, and declining soil moisture ( [[#Wei--2013|Wei and Wang, 2013]] ; Z. Li et al. , 2017; Hessl et al. , 2018). Paleoclimate data from the Mongolian plateau suggest that this recent central Asian drought exceeds the 900-year return interval, but is not unprecedented in the last 2060 years ( [[#Hessl--2018|Hessl et al., 2018]] ). There is ''low confidence'' due to ''limited evidence'' that recent droughts in central Asia can be attributed to anthropogenic forcing.

The Mediterranean region has experienced notable changes in drought and aridity. A number of studies have identified a decline in precipitation since 1960 and attributed this to anthropogenic forcing ( [[#Hoerling--2012|Hoerling et al., 2012]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ; [[#Seager--2019b|Seager et al., 2019b]] ). [[#Kelley--2015|Kelley et al. (2015)]] showed that climate change caused a three-fold increase in the likelihood of the 2007–2010 meteorological drought in the eastern Mediterranean. However, historical trends in precipitation across the Mediterranean are spatially variable and contain substantial decadal variability, such that an anthropogenic influence may not be detectable in all areas ( [[#Zittis--2018|Zittis, 2018]] ; [[#Vicente-Serrano--2021|Vicente-Serrano et al., 2021]] ). Records of soil moisture provide a clearer signal, indicating that higher temperatures and increased atmospheric demand have played a strong role in driving Mediterranean aridity ( [[#Vicente-Serrano--2014|Vicente-Serrano et al., 2014]] ). Hydrological modeling suggests that the recent decline in soil moisture in the Mediterranean is unprecedented in the last 250 years ( [[#Hanel--2018|Hanel et al., 2018]] ). Paleoclimate evidence extends this view, additionally indicating that dryness in the Mediterranean is approaching an extreme condition compared to the last millennium ( [[#Markonis--2018|Markonis et al., 2018]] ) and that the 15-year drought in the Levant (1998–2012) has an 89% likelihood of being the driest of the last 900 years ( [[#Cook--2016a|Cook et al., 2016a]] ). [[#Marvel--2019|Marvel et al. (2019)]] found that the Mediterranean region contributes strongly to the anthropogenic warming component of the global trend in aridity. There is ''high confidence'' that anthropogenic forcings are causing increased aridity and drought severity in the Mediterranean region.

Both central and north-eastern Africa have experienced a decline in rainfall since about 1980 ( ''high confidence'' ) ( [[#Lyon--2012|Lyon and Dewitt, 2012]] ; [[#Lyon--2014|Lyon, 2014]] ; [[#Hua--2016|Hua et al., 2016]] ; [[#Nicholson--2017|Nicholson, 2017]] ). In Central Africa, the decline has been attributed to atmospheric responses to Indo-Pacific sea surface temperature variability ( [[#Hua--2018|Hua et al., 2018]] ). In north-eastern Africa, droughts have become longer and more intense in recent decades, continuing across rainy seasons ( [[#Hoell--2017b|Hoell et al., 2017b]] ; [[#Nicholson--2017|Nicholson, 2017]] ), and this trend appears to be unusual in the context of the last 1500 years ( [[#Tierney--2015|Tierney et al., 2015]] ). [[#Knutson--2018|Knutson and Zeng (2018)]] attribute decreased annual precipitation over the Sudan to anthropogenic forcing, but other studies argue that the recent trend cannot yet be distinguished from natural variability, at least over parts of this region ( [[#Hoell--2017b|Hoell et al., 2017b]] ; [[#Philip--2018|Philip et al., 2018]] ). There remains ''low confidence'' due to ''limited evidence'' that drying the north-eastern Africa is attributable to human influence. In the Western Cape region of South Africa, human influence increased the likelihood of the severe 2015–2017 drought by a factor of 3–6, depending on the analysis ( [[#Otto--2018|Otto et al., 2018]] ; [[#Pascale--2020|Pascale et al., 2020]] ). Anthropogenic forcing also contributed to the 2018 drought, mainly by increasing evapotranspiration ( [[#Nangombe--2020|Nangombe et al., 2020]] ). While some analysis of instrumental precipitation data in this region detect a slight long-term drying trend consistent with the simulated anthropogenic response ( [[#Seager--2019b|Seager et al., 2019b]] ), there is strong multi-decadal variability in the data ( [[#Wolski--2021|Wolski et al., 2021]] ). However, a study of streamflow in southern Africa detected a significant decline ( [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ; see also [[IPCC:Wg1:Chapter:Chapter-10#10.6.2|Section 10.6.2]] ). There is ''medium confidence'' in the long-term drying trend in this region and its attribution to anthropogenic forcing, and ''medium confidence'' that anthropogenic warming has contributed to recent severe drought events.

Several subtropical, semi-arid regions in the Southern Hemisphere have experienced long-term drying trends in the late 20th century. South-western South America (central Chile) experienced a multi-decadal decline in precipitation and streamflow culminating in a post-2010 megadrought that has been partly attributed to anthropogenic GHG emissions and ozone depletion (Boisier et al. , 2016, 2018; Saurral et al. , 2017; [[#Knutson--2018|Knutson and Zeng, 2018]] ; Seager et al. , 2019b; Garreaud et al. , 2020). There is ''medium confidence'' that drying in central Chile can be attributed to human influence. The tree-ring paleoclimate record demonstrates that the mid-century increase in exteme drought events in southern South America is unusual in the context of the last 600 years, suggesting an emerging influence of anthropogenic forcing ( [[#Morales--2020|Morales et al., 2020]] ).

There has been a 20% decrease in winter (May to July) rainfall in south-western Australia since 1970, with the decline increasing to around 28% since 2000 ( [[#Delworth--2014|Delworth and Zeng, 2014]] ; BoM and CSIRO, 2020). There has also been a significant increase in the average intensity of seasonal droughts in the region since 1911in response to both lower precipitation and increased atmospheric evaporative demand ( [[#Gallant--2013|Gallant et al., 2013]] ). Several studies attribute the precipitation declines in south-western Australia to anthropogenic changes in GHG and ozone ( [[#Delworth--2014|Delworth and Zeng, 2014]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ; [[#Seager--2019b|Seager et al., 2019b]] ). There is ''high confidence'' that the observed drying in south-western Australia can be attributed to anthropogenic forcing.

In south-eastern Australia, the average length of droughts have increased significantly, lasting between 10 and 69% longer than droughts during the first half of the 20th century ( [[#Gallant--2013|Gallant et al., 2013]] ). Paleoclimate reconstructions indicate a 97.1% probability that the decadal rainfall anomaly recorded during the 1997–2009 Millennium drought in south-eastern Australia was the worst experienced since 1783 ( [[#Gergis--2012|Gergis et al., 2012]] ), and that the spatial extent and duration of cool season (April to September) rainfall anomalies were either very much below average or unprecedented over at least the last 400 years ( [[#Freund--2017|Freund et al., 2017]] ). Other paleoclimate studies suggest that the Millennium drought in eastern Australia was not unusual in the context of natural variability reconstructed over the past millennium (Palmer et al. , 2015; Cook et al. , 2016c; Kiem et al. , 2020). While there is currently ''low confidence'' that recent droughts in eastern Australia can be clearly attributed to human influence ( [[#Cai--2014|Cai et al., 2014]] ; [[#Delworth--2014|Delworth and Zeng, 2014]] ; [[#Rauniyar--2020|Rauniyar and Power, 2020]] ), there is emerging evidence that declines in April to October rainfall in south-eastern Australia since the 1990s would not have been as large without the influence of increasing levels of atmospheric GHGs ( [[#Rauniyar--2020|Rauniyar and Power, 2020]] ).

In summary, it is ''very likely'' that anthropogenic factors have influenced global trends in aridity, mainly through competing changes in evapotranspiration and/or atmospheric evaporative demand due to anthropogenic emissions of GHG and aerosols. There is ''high confidence'' that the frequency and the severity of droughts has increased over the last decades in the Mediterranean, western North America, and south-western Australia and that this can be attributed to anthropogenic warming. There is ''medium confidence'' that recent drying and severe droughts in southern Africa and south-western South America can be attributed to human influence. In some regions of western North America and the Mediterranean, paleoclimate evidence suggests that recent warming has resulted in droughts that are of similar or greater intensity than those reconstructed over the last millennium ( ''medium co'' ''nfidence'' ).

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