Editing IPCC:AR6/SRCCL/Chapter-3 (section)

==== 3.5.1.1 Future vulnerability and risk of desertification ====

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Following the conceptual framework developed in the Special Report on extreme events (SREX) (IPCC 2012 <sup>[[#fn:r898|898]]</sup> ), future risks are assessed by examining changes in exposure (that is, presence of people; livelihoods; species or ecosystems; environmental functions, service, and resources; infrastructure; or economic, social or cultural assets; see Glossary), changes in vulnerability (that is, propensity or predisposition to be adversely affected; see Glossary) and changes in the nature and magnitude of hazards (that is, potential occurrence of a natural or human-induced physical event that causes damage; see Glossary). Climate change is expected to further exacerbate the vulnerability of dryland ecosystems to desertification by increasing PET globally (Sherwood and Fu 2014 <sup>[[#fn:r899|899]]</sup> ). Temperature increases between 2°C and 4°C are projected in drylands by the end of the 21st century under RCP4.5 and RCP8.5 scenarios, respectively (IPCC 2013 <sup>[[#fn:r900|900]]</sup> ). An assessment by Carrão et al. 2017 <sup>[[#fn:r901|901]]</sup> showed an increase in drought hazards by late-century (2071–2099) compared to a baseline (1971–2000) under high RCPs in drylands around the Mediterranean, south-eastern Africa, and southern Australia. In Latin America, Morales et al. (2011) <sup>[[#fn:r902|902]]</sup> indicated that areas affected by drought will increase significantly by 2100 under SRES scenarios A2 and B2. The countries expected to be affected include Guatemala, El Salvador, Honduras and Nicaragua. In CMIP5 scenarios, Mediterranean types of climate are projected to become drier (Alessandri et al. 2014 <sup>[[#fn:r903|903]]</sup> ; Polade et al. 2017 <sup>[[#fn:r904|904]]</sup> ), with the equatorward margins being potentially replaced by arid climate types (Alessandri et al. 2014 <sup>[[#fn:r905|905]]</sup> ). Globally, climate change is predicted to intensify the occurrence and severity of droughts ( ''medium confidence'' ) (Dai 2013 <sup>[[#fn:r906|906]]</sup> ; Sheffield and Wood 2008 <sup>[[#fn:r907|907]]</sup> ; Swann et al. 2016 <sup>[[#fn:r908|908]]</sup> ; Wang 2005 <sup>[[#fn:r909|909]]</sup> ; Zhao and Dai 2015 <sup>[[#fn:r910|910]]</sup> ; Carrão et al. 2017 <sup>[[#fn:r911|911]]</sup> ; Naumann et al. 2018 <sup>[[#fn:r912|912]]</sup> ) (Section 2.2). Ukkola et al. (2018) <sup>[[#fn:r913|913]]</sup> showed large discrepancies between CMIP5 models for all types of droughts, limiting the confidence that can be assigned to projections of drought.

Drylands are characterised by high climatic variability. Climate impacts on desertification are not only defined by projected trends in mean temperature and precipitation values but are also strongly dependent on changes in climate variability and extremes (Reyer et al. 2013 <sup>[[#fn:r914|914]]</sup> ). The responses of ecosystems depend on diverse vegetation types. Drier ecosystems are more sensitive to changes in precipitation and temperature (Li et al. 2018 <sup>[[#fn:r915|915]]</sup> ; Seddon et al. 2016 <sup>[[#fn:r916|916]]</sup> ; You et al. 2018 <sup>[[#fn:r917|917]]</sup> ), increasing vulnerability to desertification. It has also been reported that areas with high variability in precipitation tend to have lower livestock densities and that those societies that have a strong dependence on livestock that graze natural forage are especially affected (Sloat et al. 2018 <sup>[[#fn:r918|918]]</sup> ). Social vulnerability in drylands increases as a consequence of climate change that threatens the viability of pastoral food systems (Dougill et al. 2010 <sup>[[#fn:r919|919]]</sup> ; López-i-Gelats et al. 2016 <sup>[[#fn:r920|920]]</sup> ). Social drivers can also play an important role with regards to future vulnerability (Máñez Costa et al. 2011 <sup>[[#fn:r921|921]]</sup> ). In the arid region of north-western China, Liu et al. (2016b) <sup>[[#fn:r922|922]]</sup> estimated that under RCP4.5 areas of increased vulnerability to climate change and desertification will surpass those with decreased vulnerability.

Using an ensemble of global climate, integrated assessment and impact models, Byers et al. (2018) <sup>[[#fn:r923|923]]</sup> investigated 14 impact indicators at different levels of global mean temperature change and socio-economic development. The indicators cover water, energy and land sectors. Of particular relevance to desertification are the water (e.g., water stress, drought intensity) and the land (e.g., habitat degradation) indicators. Under shared socio-economic pathway SSP2 (‘Middle of the Road’) at 1.5°C, 2°C and 3°C of global warming, the numbers of dryland populations exposed (vulnerable) to various impacts related to water, energy and land sectors (e.g., water stress, drought intensity, habitat degradation) are projected to reach 951 (178) million, 1152 (220) million and 1285 (277) million, respectively. While at global warming of 2°C, under SSP1 (‘Sustainability’), the exposed (vulnerable) dryland population is 974 (35) million, and under SSP3 (‘Fragmented World’) it is 1267 (522) million. Steady increases in the exposed and vulnerable populations are seen for increasing global mean temperatures. However much larger differences are seen in the vulnerable population under different SSPs. Around half the vulnerable population is in South Asia, followed by Central Asia, West Africa and East Asia.

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