Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.10.3 Deserts and Semi-arid Areas ====

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Drylands, which include hyper-arid, arid, semi-arid and dry sub-humid areas ( [[#IPCC--2019c|IPCC, 2019c]] ), lie on all continents and cover 46% of the global land area and host more than one-third of the current population ( [[#Olsson--2019|Olsson et al., 2019]] ). [[#Huang--2016b|Huang et al. (2016b)]] found that aridity changes have helped expand dryland area by about 4% from 1948 to 2004, with the largest expansion of drylands occurring in semi-arid regions since the early 1960s. [[IPCC:Wg1:Chapter:Chapter-4#4.5.1|Section 4.5.1]] assessed ''high confidence'' of a future poleward expansion of the Hadley cell, leading to a poleward shift of dryland areas in all scenarios considered. There is no evidence of a future global trend in aridification of drylands ( [[#IPCC--2019a|IPCC, 2019a]] ), but ''high confidence'' of aridification in some areas (e.g., Mediterranean, Central America, Southern Africa; [[#IPCC--2019a|IPCC, 2019a]] ; see also Figure 12.4j–l). However, drivers of desertification largely include land-cover changes and land-use management, along with climate change ( [[#IPCC--2019a|IPCC, 2019a]] ).

Warming temperatures and extreme heat are major climatic impact-drivers with multiple potential impacts on societies, health, and habitability in semi-arid and arid regions that are already near physiological limits for outdoor activities. Semi-arid regions will ''very likely'' undergo a warming in all future scenarios ( [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] and Atlas) and ''likely'' undergo an increase in duration, magnitude and frequency of heatwaves (Chapter 11) (Figure 12.4a–c). It is ''likely'' that heat stress will be much more intense by the end of the century in many areas under all scenarios, such as deserts and semi-arid zones in Asia ( [[#Murari--2015|Murari et al., 2015]] ; [[#Mishra--2017|Mishra et al., 2017]] ), Australia and Africa ( [[#Zhao--2015|Zhao et al., 2015]] ; [[#Xia--2016|Xia et al., 2016]] ; [[#Guo--2017|Guo et al., 2017]] ; [[#Dosio--2018|Dosio et al., 2018]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ), with consequences for labour productivity with respect to high heat-humidity conditions (Figure 12.4d–f).

Drought is another major climatic impact-driver for semi-arid areas, imposing major challenges on agriculture given existing water availability constraints ( [[#Kusunose--2014|Kusunose and Lybbert, 2014]] ; [[#Barlow--2016|Barlow et al., 2016]] ; [[#Otto--2018|Otto et al., 2018]] ). Over the period 1961–2013, the annual area of drylands in drought has increased, on average by slightly more than 1% per year, with large interannual variability ( [[#Olsson--2019|Olsson et al., 2019]] ). In general, droughts have increased in several arid and semi-arid areas over the last decades ( ''medium confidence'' ), and are ''likely'' to increase in the future as indicated by a number of indices calculated from climate ( [[#Liu--2018b|Liu et al., 2018b]] ; [[#Zkhiri--2019|Zkhiri et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ; [[#Driouech--2021|Driouech et al., 2021]] ; see also Figure 12.4j–l).

Deserts and semi-arid areas are prone to dust storms, which can drive impacts on health and several other sectors (X. [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ; [[#Tong--2017|Tong et al., 2017]] ). The SRCCL indicated that the evolution of dust under climate change is uncertain ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), and there is a lack of evidence and agreement of a change in their frequency or intensity so far in most regions (Sections 12.4.1–12.4.9). Model projections of future changes in dust are hindered by the uncertainties in future regional wind and precipitation as the climate warms ( [[#Evan--2016|Evan et al., 2016]] ); in the effect of CO <sub>2</sub> fertilization on source extent ( [[#Huang--2017|Huang et al., 2017]] ); and in the impact of human activities upon the land surface ( [[#Ginoux--2012|Ginoux et al., 2012]] ; see Chapter 10). Projected trends in dust storms and dust loads in deserts and semi-arid areas vary from region to region. Dust loadings are expected to decrease over most of the Sahara and Sahel ( ''low confidence'' ) ( [[#12.4.1|Section 12.4.1]] ), increase over Mexico and the south-west USA ( ''medium confidence'' ) ( [[#12.4.6|Section 12.4.6]] ), and there is ''low confidence'' of a future trend due to climate change in other continents (Sections 12.4.2–12.4.5).

'''In conclusion, desert and semi-arid areas are strongly affected by climatic impact-drivers such as extreme heat, drought and dust storms. Heat hazards are''' very likely '''increasing in all future climate scenarios, but uncertainty remains regarding any broadly consistent future changes in other climatic impact-drivers for deserts and semi-arid regions.'''

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