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

=== 3.5.1 Future projections of desertification ===

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Assessing the impact of climate change on future desertification is difficult as several environmental and anthropogenic variables interact to determine its dynamics. The majority of modelling studies regarding the future evolution of desertification rely on the analysis of specific climate change scenarios and Global Climate Models (GCMs) and their effect on a few processes or drivers that trigger desertification (Cross-Chapter Box 1 in Chapter 1).

With regards to climate impacts, the analysis of global and regional climate models concludes that under all representative concentration pathways (RCPs) potential evapotranspiration (PET) would increase worldwide as a consequence of increasing surface temperatures and surface water vapour deficit (Sherwood and Fu 2014 <sup>[[#fn:r850|850]]</sup> ). Consequently, there would be associated changes in aridity indices that depend on this variable ( ''high agreement, robust evidence'' ) (Cook et al. 2014a <sup>[[#fn:r851|851]]</sup> ; Dai 2011 <sup>[[#fn:r852|852]]</sup> ; Dominguez et al. 2010 <sup>[[#fn:r853|853]]</sup> ; Feng and Fu 2013 <sup>[[#fn:r854|854]]</sup> ; Ficklin et al. 2016 <sup>[[#fn:r855|855]]</sup> ; Fu et al. 2016 <sup>[[#fn:r856|856]]</sup> ; Greve and Seneviratne 1999 <sup>[[#fn:r857|857]]</sup> ; Koutroulis 2019 <sup>[[#fn:r858|858]]</sup> ; Scheff and Frierson 2015 <sup>[[#fn:r859|859]]</sup> ). Due to the large increase in PET and decrease in precipitation over some subtropical land areas, aridity index will decrease in some drylands (Zhao and Dai 2015 <sup>[[#fn:r860|860]]</sup> ), with one model estimating approximately 10% increase in hyper-arid areas globally (Zeng and Yoon 2009 <sup>[[#fn:r861|861]]</sup> ). Increases in PET are projected to continue due to climate change (Cook et al. 2014a <sup>[[#fn:r862|862]]</sup> ; Fu et al. 2016 <sup>[[#fn:r863|863]]</sup> ; Lin et al. 2015 <sup>[[#fn:r864|864]]</sup> ; Scheff and Frierson 2015 <sup>[[#fn:r865|865]]</sup> ). However, as noted in Sections 3.1.1 and 3.2.1, these PET calculations use assumptions that are not valid in an environment with changing CO <sub>2</sub> . Evidence from precipitation, runoff or photosynthetic uptake of CO <sub>2</sub> suggest that a future warmer world will be less arid (Roderick et al. 2015 <sup>[[#fn:r866|866]]</sup> ). Observations in recent decades indicate that the Hadley cell has expanded poleward in both hemispheres (Fu et al. 2006 <sup>[[#fn:r867|867]]</sup> ; Hu and Fu 2007 <sup>[[#fn:r868|868]]</sup> ; Johanson et al. 2009 <sup>[[#fn:r869|869]]</sup> ; Seidel and Randel 2007 <sup>[[#fn:r870|870]]</sup> ), and under all RCPs would continue expanding (Johanson et al. 2009 <sup>[[#fn:r871|871]]</sup> ; Lu et al. 2007 <sup>[[#fn:r872|872]]</sup> ). This expansion leads to the poleward extension of subtropical dry zones and hence an expansion in drylands on the poleward edge (Scheff and Frierson 2012 <sup>[[#fn:r873|873]]</sup> ). Overall, this suggests that while aridity will increase in some places ( ''high confidence'' ), there is insufficient evidence to suggest a global change in dryland aridity ( ''medium confidence'' ).

Regional modelling studies confirm the outcomes of Global Climate Models (Africa: Terink et al. 2013 <sup>[[#fn:r874|874]]</sup> ; China: Yin et al. 2015 <sup>[[#fn:r875|875]]</sup> ; Brazil: Marengo and Bernasconi 2015 <sup>[[#fn:r876|876]]</sup> ; Cook et al. 2012 <sup>[[#fn:r877|877]]</sup> ; Greece: Nastos et al. 2013 <sup>[[#fn:r878|878]]</sup> ; Italy: Coppola and Giorgi 2009 <sup>[[#fn:r879|879]]</sup> ). According to the IPCC AR5 (IPCC 2013) <sup>[[#fn:r880|880]]</sup> , decreases in soil moisture are detected in the Mediterranean, southwest USA and southern African regions. This is in line with alterations in the Hadley circulation and higher surface temperatures. This surface drying will continue to the end of this century under the RCP8.5 scenario ( ''high confidence'' ). Ramarao et al. (2015) <sup>[[#fn:r881|881]]</sup> showed that a future climate projection based on RCP4.5 scenario indicated the possibility for detecting the summer-time soil drying signal over the Indian region during the 21st century in response to climate change. The IPCC Special Report on Global Warming of 1.5°C (SR15) (Chapter 3; Hoegh-Guldberg et al. 2018 <sup>[[#fn:r882|882]]</sup> ) concluded with ‘ ''medium confidence'' ’ that global warming by more than 1.5°C increases considerably the risk of aridity for the Mediterranean area and southern Africa. Miao et al. (2015b) <sup>[[#fn:r883|883]]</sup> showed an acceleration of desertification trends under the RCP8.5 scenario in the middle and northern part of Central Asia and some parts of north-western China. It is also useful to consider the effects of the dynamic–thermodynamical feedback of the climate. Schewe and Levermann (2017) <sup>[[#fn:r884|884]]</sup> show increases of up to 300% in the Central Sahel rainfall by the end of the century due to an expansion of the West African monsoon. Warming could trigger an intensification of monsoonal precipitation due to increases in ocean moisture availability.

The impacts of climate change on dust storm activity are not yet comprehensively studied and represent an important knowledge gap. Currently, GCMs are unable to capture recent observed dust emission and transport (Evan 2018 <sup>[[#fn:r885|885]]</sup> ; Evan et al. 2014 <sup>[[#fn:r886|886]]</sup> ), limiting confidence in future projections. Literature suggests that climate change decreases wind erosion/dust emission overall, with regional variation ( ''low confidence'' ). Mahowald et al. (2006) <sup>[[#fn:r887|887]]</sup> and Mahowald (2007) <sup>[[#fn:r888|888]]</sup> found that climate change led to a decrease in desert dust source areas globally using CMIP3 GCMs. Wang et al. (2009) <sup>[[#fn:r889|889]]</sup> found a decrease in sand dune movement by 2039 (increasing thereafter) when assessing future wind-erosion-driven desertification in arid and semi-arid China using a range of SRES scenarios and HadCM3 simulations. Dust activity in the Southern Great Plains in the USA was projected to increase, while in the Northern Great Plains it was projected to decrease under RCP8.5 climate change scenario (Pu and Ginoux 2017 <sup>[[#fn:r890|890]]</sup> ). Evan et al. (2016) <sup>[[#fn:r891|891]]</sup> project a decrease in African dust emission associated with a slowdown of the tropical circulation in the high CO <sub>2</sub> RCP8.5 scenario.

Global estimates of the impact of climate change on soil salinisation show that under the IS92a emissions scenario (a scenario prepared in 1992 that contains ‘business as usual’ assumptions) (Leggett et al. 1992 <sup>[[#fn:r892|892]]</sup> ) the area at risk of salinisation would increase in the future ( ''limited evidence, high agreement'' ) (Schofield and Kirkby 2003 <sup>[[#fn:r893|893]]</sup> ). Climate change has an influence on soil salinisation that induces further land degradation through several mechanisms that vary in their level of complexity. However, only a few examples can be found to illustrate this range of impacts, including the effect of groundwater table depletion (Rengasamy 2006 <sup>[[#fn:r894|894]]</sup> ) and irrigation management (Sivakumar 2007 <sup>[[#fn:r895|895]]</sup> ), salt migration in coastal aquifers with decreasing water tables (Sherif and Singh 1999 <sup>[[#fn:r896|896]]</sup> ) (Section 4.10.7), and surface hydrology and vegetation that affect wetlands and favour salinisation (Nielsen and Brock 2009 <sup>[[#fn:r897|897]]</sup> ).

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==== 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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