Editing IPCC:AR6/SRCCL/Chapter-7 (section)

==== 7.2.2.8 Risks of desertification, land degradation and food insecurity under different Future Development Pathways ====

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Socio-economic developments and policy choices that govern land–climate interactions are an important driver of risk, along with climate change ( ''very high confidence'' ). Risks under two different Shared Socio-economic Pathways (SSPs) were assessed using emerging literature. SSP1 is characterised by low population growth, reduced inequalities, land-use regulation, low meat consumption, and moderate trade (Riahi et al. 2017 <sup>[[#fn:r157|157]]</sup> ; Popp et al. 2017a <sup>[[#fn:r158|158]]</sup> ). SSP3 is characterised by high population growth, higher inequalities, limited land-use regulation, resource-intensive consumption including meat-intensive diets, and constrained trade (for further details see Chapter 1 and Cross-Chapter Box 9 in Chapters 6 and 7). These two SSPs, among the set of five SSPs, were selected because they illustrate contrasting futures, ranging from low (SSP1) to high (SSP3) challenges to mitigation and adaptation. Figure 7.2 shows that for a given global mean temperature (GMT) change, risks are different under SSP1 compared to SSP3. In SSP1, global temperature change does not increase above 3°C even in the baseline case (i.e., with no additional mitigation measures) because in this pathway the combination of low population and autonomous improvements, for example, in terms of carbon intensity and/or energy intensity, effectively act as mitigation measures (Riahi et al. 2017 <sup>[[#fn:r159|159]]</sup> ). Thus Figure 7.2 does not indicate risks beyond this point in either SSP1 and SSP3. Literature based on such socio-economic and climate models is still emerging and there is a need for greater research on impacts of different pathways. There are few SSP studies exploring aspects of desertification and land degradation, but a greater number of SSP studies on food security (Supplementary Material). SSP1 reduces the vulnerability and exposure of human and natural systems and thus limits risks resulting from desertification, land degradation and food insecurity compared to SSP3 ( ''high confidence'' ).

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'''Figure 7.2'''

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'''Risks associated with desertification, land degradation and food security due to climate change and patterns of socio-economic development.Increasing risks associated with desertification include population exposed and vulnerable to water scarcity in drylands. Risks related to land degradation include increased habitat degradation, population exposed to wildfire and floods and costs of floods. Risks to food security […]'''
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Risks associated with desertification, land degradation and food security due to climate change and patterns of socio-economic development.Increasing risks associated with desertification include population exposed and vulnerable to water scarcity in drylands. Risks related to land degradation include increased habitat degradation, population exposed to wildfire and floods and costs of floods. Risks to food security include availability and access to food, including population at risk of hunger, food price increases and increases in disability adjusted life years attributable due to childhood underweight. The risks are assessed for two contrasted socio-economic futures (SSP1 and SSP3) under unmitigated climate change {3.6, 4.3.1.2, 5.2.2, 5.2.3, 5.2.4, 5.2.5, 6.2.4, 7.3}. Risks are not indicated beyond 3°C because SSP1 does not exceed this level of temperature change.

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Changes to the water cycle due to global warming are an essential driver of desertification and of the risks to livelihood, food production and vegetation in dryland regions. Changes in water scarcity due to climate change have already been detected in some dryland regions (Section 7.2.2.4) and therefore the transition to moderate risk occurred in recent decades ( ''high confidence'' ). IPCC (2018d) noted that in the case of risks to water resources, socio-economic drivers are expected to have a greater influence than the changes in climate ( ''medium confidence'' ). Indeed, in SSP1 there is only moderate risk even at 3°C of warming, due to the lower exposure and vulnerability of human population (Hanasaki et al. 2013a <sup>[[#fn:r160|160]]</sup> ; Arnell and Lloyd-Hughes 2014 <sup>[[#fn:r161|161]]</sup> ; Byers et al. 2018b <sup>[[#fn:r162|162]]</sup> ). Considering drylands only, Byers et al. (2018b) <sup>[[#fn:r163|163]]</sup> estimate, using a time-sampling approach for climate change and the 2050 population, that at 1.5°C, 2°C and 3°C, the dryland population exposed and vulnerable to water stress in SSP1 will be 2%, 3% and 3% respectively, thus indicating relatively stable moderate risks. In SSP3, the transition from moderate to high risk occurs in the range 1.2°C to 1.5°C ( ''medium confidence'' ) and the transition from ''high'' to very ''high risk'' is in the range 1.5°C to 2.8°C ( ''medium confidence'' ). Hanasaki et al. (2013b) <sup>[[#fn:r164|164]]</sup> found a consistent increase in water stress at higher warming levels due in large part to growth in population and demand for energy and agricultural commodities, and to a lesser extent due to hydrological changes induced by global warming. In SSP3, Byers et al. (2018b) <sup>[[#fn:r165|165]]</sup> estimate that at 1.5°C, 2°C and 3°C, the population exposed and vulnerable to water stress in drylands will steadily increase from 20% to 22% and 24% respectively, thus indicating overall much higher risks compared to SSP1 for the same global warming levels.

SSP studies relevant to land degradation assess risks such as: number of people exposed to fire; the costs of floods and coastal flooding; and loss of ES including the ability of land to sequester carbon. The risks related to permafrost melting (Section 7.2.2.7) are not considered here due to the lack of SSP studies addressing this topic. Climate change impacts on various components of land degradation have already been detected (Sections 7.2.2.3, 7.2.2.5 and 7.2.2.6) and therefore the transition from ''undetectable'' to ''moderate risk'' is in the range 0.7°C to 1°C ( ''high confidence'' ). Less than 100 million people are exposed to habitat degradation at 1.5°C under SSP1 in non-dryland regions, increasing to 257 million at 2°C (Byers et al. 2018 <sup>[[#fn:r166|166]]</sup> ). This suggests a gradual transition to high risk in the range 1.8°C to 2.8°C, but a ''low confidence'' is attributed due to the very limited evidence to constrain this transition.

By contrast in SSP3, there are already 107 million people exposed to habitat degradation at 1.5°C, increasing to 1156 million people at 3°C (Byers et al. 2018b <sup>[[#fn:r167|167]]</sup> ). Furthermore, Knorr et al. (2016b) <sup>[[#fn:r168|168]]</sup> estimate that 646 million people will be exposed to fire at 2°C warming, the main risk driver being the high population growth in SSP3 rather than increased burned area due to climate change. Exposure to extreme rainfall, a causative factor for soil erosion and flooding, also differs under SSPs. Under SSP1 up to 14% of the land and population experience five-day extreme precipitation events. Similar levels of exposure occur at lower temperatures in SSP3 (Zhang et al. 2018b <sup>[[#fn:r169|169]]</sup> ). Population exposed to coastal flooding is lowest under SSP1 and higher under SSP3 with a limited effect of enhanced protection in SSP3 already after 2°C warming (Hinkel et al. 2014 <sup>[[#fn:r170|170]]</sup> ). The transition from ''high'' to very ''high risk'' will occur at 2.2°Cto 2.8°C in SSP3 ( ''medium confidence'' ), whereas this level of risk is not expected to be reached in SSP1.

The greatest number of SSP studies explore climate change impacts relevant to food security, including population at risk of hunger, food price increases, increases in disability adjusted life years (Hasegawa et al. 2018a <sup>[[#fn:r171|171]]</sup> ; Wiebe et al. 2015a <sup>[[#fn:r172|172]]</sup> ; van Meijl et al. 2018a <sup>[[#fn:r173|173]]</sup> ; Byers et al. 2018b <sup>[[#fn:r174|174]]</sup> ). Changes in crop yields and food supply stability have already been attributed to climate change (Sections 7.2.2.1 and 7.2.2.2) and the transition from ''undetectable'' to ''moderate risk'' is placed at 0.5°C to 1°C ( ''medium confidence'' ). At 1.5°C, about two million people are exposed and vulnerable to crop yield change in SSP1 (Hasegawa et al. 2018b <sup>[[#fn:r175|175]]</sup> ; Byers et al. 2018b <sup>[[#fn:r176|176]]</sup> ), implying moderate risk. A transition from moderate to high risk is expected above 2.5°C ( ''medium confidence'' ) with population at risk of hunger of the order of 100 million (Byers et al. 2018b <sup>[[#fn:r177|177]]</sup> ). Under SSP3, high risks already exist at 1.5°C ( ''medium confidence'' ), with 20 million people exposed and vulnerable to crop yield change. By 2°C, 178 million are vulnerable and 854 million people are vulnerable at 3°C (Byers et al. 2018b <sup>[[#fn:r178|178]]</sup> ). This is supported by the higher food prices increase of up to 20% in 2050 in an RCP6.0 scenario (i.e., slightly below 2°C) in SSP3 compared to up to 5% in SSP1 (van Meijl et al. 2018 <sup>[[#fn:r179|179]]</sup> ). Furthermore in SSP3, restricted trade increase this price effect (Wiebe et al. 2015 <sup>[[#fn:r180|180]]</sup> ). In SSP3, the transition from ''high'' to ''very high'' risk is in the range 2°C to 2.7°C ( ''medium confidence'' ) while this transition is never reached in SSP1. This overall confirms that socio-economic development, by affecting exposure and vulnerability, has an even larger effect than climate change for future trends in the population at risk of hunger (O’Neill et al. 2017 <sup>[[#fn:r181|181]]</sup> , p.32). Changes can also threaten development gains ( ''medium confidence'' ). Disability adjusted life years due to childhood underweight decline in both SSP1 and SSP3 by 2030 (by 36.4 million disability adjusted life years in SSP1 and 16.2 million in SSP3). However by 2050, disability adjusted life years increase by 43.7 million in SSP3 (Ishida et al. 2014 <sup>[[#fn:r182|182]]</sup> ).

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