Editing IPCC:AR6/WGII/Cross-Chapter-Paper-3 (section)

== CCP3.1 Introduction ==

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=== CCP3.1.1 Concepts, Definitions and Scope ===

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Deserts and semiarid areas are in ‘drylands’, which comprise hyper-arid, arid, semiarid and dry sub-humid areas (Figure CCP3.1). Drylands cover about 45–47% of the global land area ( [[#Prăvălie--2016|Prăvălie, 2016]] ; [[#Koutroulis--2019|Koutroulis, 2019]] ) and are home to about 3 billion people residing primarily in semiarid and dry sub-humid areas (van der Esch et al., 2017). Drylands host unique, rich biodiversity ( [[#Maestre--2015|Maestre et al., 2015]] ) and provide important ecosystem services ( [[#Bidak--2015|Bidak et al., 2015]] ; [[#Lu--2018|Lu et al., 2018]] ), while dryland people have a rich cultural and historical heritage. Rural human populations are growing in some Mediterranean and tropical drylands, while many are rapidly urbanising (Guengant Jean-Pierre, 2003; [[#Tabutin--2004|Tabutin and Schoumaker, 2004]] ; [[#Denis--2009|Denis and Moriconi-Ebrard, 2009]] ), with varying impacts on ecosystem services and adaptive capacities. In recent decades, 6% of global megacities have been established in arid areas and 2% in hyper-arid desert areas ( [[#Cherlet--2018|Cherlet et al., 2018]] ), with many of these areas suffering from severe water security challenges ( [[#Stringer--2021|Stringer et al., 2021]] ). Dryland inhabitants in many developing countries are also experiencing poverty ( [[IPCC:Wg2:Chapter:Chapter-16#16.1.4.3|Section 16.1.4.3]] ), hunger, poor health, land degradation, and economic and political marginalisation ( [[#Mbow--2019|Mbow et al., 2019]] ; [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), which sometimes limits their access to common pool resources. These challenges, together with a weak enabling environment, threaten opportunities to adapt to climate change.

The terms ‘desert’ and ‘desertification’ are subject to various interpretations due to the diverse components, processes and states they denote. Recognising ‘land degradation’ as a contested and perceptual term ( [[#Blaikie--1987|Blaikie and Brookfield, 1987]] ; [[#Behnke--2016|Behnke and Mortimore, 2016]] ; [[#Robbins--2020|Robbins, 2020]] ), this cross-chapter paper (CCP), defines land degradation as ‘a negative trend in land condition, caused by direct or indirect human-induced processes including climate change, expressed as long-term reduction or loss of at least one of the following: biological productivity, ecological integrity or value to humans’ ( [[#Olsson--2019|Olsson et al., 2019]] ). Desertification is land degradation in arid, semiarid and dry sub-humid areas ( [[#UNCCD--1994|UNCCD, 1994]] ). Following the above definitions, desertification is more common in arid and semiarid climates than in hyper-arid climates. When desertification does occur in arid and hyper-arid ecosystems it is often in oases and irrigated cultivated lands ( [[#Ezcurra--2006|Ezcurra, 2006]] ; [[#Dilshat--2015|Dilshat et al., 2015]] ). Hyper-arid areas, except wetlands such as oases, wadis and riverbanks, are excluded in the United Nations Convention to Combat Desertification (UNCCD) definition of desertification used here, yet many of the world’s deserts are in hyper-arid areas. Hyper-arid areas are therefore included when discussing deserts but not when discussing desertification. Deserts are not the end point in a desertification process ( [[#Ezcurra--2006|Ezcurra, 2006]] ). There is ''robust evidence'' of desertification in deserts, mostly driven by human activities and climate variability, expressed as loss of biological productivity, ecological integrity or value to humans to below their natural levels ( [[#Moridnejad--2015|Moridnejad et al., 2015]] ).

Interactions between climate change and desertification in drylands create challenges for both ecosystem and human resilience, affecting ecosystem services, biodiversity, food security, human health and well-being ( [[#Reed--2016|Reed and Stringer, 2016]] ). Dryland livelihoods that heavily rely on natural ecosystems face pressures, including high population growth rates, weak or poor governance, low investment, unemployment and poverty, market distortions and underestimates of the value of drylands ( [[#Stringer--2017|Stringer et al., 2017]] ; [[#Bawden--2018|Bawden, 2018]] ). These pressures intersect with broader societal challenges such as conflict and civil unrest ( [[#Okpara--2015|Okpara et al., 2015]] ; [[#Almer--2017|Almer et al., 2017]] ), which together, can contribute to human displacement ( [[IPCC:Wg2:Chapter:Chapter-16#16.2.3.10|Section 16.2.3.10]] ) in some drylands ( [[#Warner--2010|Warner, 2010]] ; [[#Abel--2019|Abel et al., 2019]] ). Nevertheless, evidence linking conflict with climate change and desertification is weak ( [[#Benjaminsen--2012|Benjaminsen et al., 2012]] ) and data are insufficient to draw robust conclusions.

Drylands yield important opportunities for adapting to and mitigating climate change. They offer abundant solar energy, which could support mitigation efforts, opportunities for cultural and nature-based tourism, rich plant biodiversity in some areas (e.g. Namibia), and extensive Indigenous knowledge and experience of adapting to dynamic climates ( [[#Christie--2014|Christie et al., 2014]] ; [[#Stringer--2017|Stringer et al., 2017]] ); for example, across West Asia and North Africa ( [[#Louhaichi--2010|Louhaichi and Tastad, 2010]] ; Hussein, 2011). Improved understanding of challenges and opportunities in drylands can be achieved by transdisciplinary, multi-scale and inter-sectoral approaches encompassing links between physical, biological, socioeconomic and institutional systems ( [[#Reynolds--2007|Reynolds et al., 2007]] ; [[#Stringer--2017|Stringer et al., 2017]] ).

[[IPCC:Wg2:Chapter:Chapter-3|Chapter 3]] of the IPCC Special Report on Climate Change and Land (SRCCL) focused on desertification ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), but links between climate change and deserts, desertification and semiarid areas have not been extensively considered in recent IPCC assessment cycles. Working Group II Assessment Report 5 noted that desertification contributes to atmospheric dust production, identifying desertification as needing consideration within climate change mitigation and adaptation governance and decision making ( [[#Boucher--2013|Boucher et al., 2013]] ; [[#Myhre--2013|Myhre et al., 2013]] ). This cross-chapter paper focuses on environmental and human aspects, finding that climate change impacts will intensify the challenges faced by dryland populations in advancing sustainable development. However, viable options exist for adapting to climate change, reducing desertification and supporting progress towards the Sustainable Development Goals (SDGs), particularly by combining modern science, IKLK, and livelihood and land management strategies that enable land-based adaptation, mitigation and nature-based solutions ( [[IPCC:Wg2:Chapter:Chapter-16#16.3.2.3|Section 16.3.2.3]] ).

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=== CCP3.1.2 Key Measurement Challenges and Observed Dryland Dynamics ===

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Maps of dryland extent commonly employ a climate-based approach measured using the Aridity Index (AI), or consider the extent of dryland vegetation. The two approaches sometimes do not demarcate the same geographical areas as being drylands, particularly when projecting future changes ( [[#Stringer--2021|Stringer et al., 2021]] ). Dryland dynamics therefore need to be viewed specifically in relation to the definitions and measurements being used. From 1982 to 2015, unsustainable land use and climate change combined caused desertification of 6% of the global dryland area, while 41% showed significant greening (i.e., increased vegetation productivity), and 53% of the area had no notable change (Figure CCP3.1; [[#Burrell--2020|Burrell et al., 2020]] ). In contrast [[#Yuan--2019|Yuan et al. (2019)]] conclude that during 1999–2015, trends of vegetation production reversed globally, and in drylands, showing extensive declines. Thus, while overall greening has occurred, this trend now appears to be declining. Analyses of vegetation, soil, and physical characteristics of over 50,000 sample points in drylands around the world indicate that aridification causes ecological degradation at three successive thresholds: vegetation decline at AI = 0.56, soil disruption at AI = 0.3 and loss of plant cover at AI = 0.2 ( [[#Berdugo--2020|Berdugo et al., 2020]] ). Drylands nevertheless show different dynamics depending on the index used and the variables assessed.

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'''Figure CCP3.1 |''' '''Aridity zone extent and observed changes in dryland areas as defined by the Aridity Index (AI).''' Aridity zones, according to UNESCO (1979) and [[#UNEP--1992|UNEP (1992)]] classifications, defined by the AI, consider the ratio of average annual precipitation to potential evapotranspiration: (i) dry sub-humid (0.5 ≤ AI &lt; 0.65), (ii) semiarid (0.2 ≤ AI &lt; 0.5), (iii) arid (0.05 ≤ AI &lt; 0.2) and (iv) hyper-arid (AI &lt;0.05). Drylands include land with AI &lt;0.65, humid lands are those with AI &gt;0.65 ( [[#UNEP--1992|UNEP, 1992]] ). Deserts represent a major part of the hyper-arid and arid zones. The aridity zones are shown for climate in the period 1988–2017 and changes in dryland area (combined area of four aridity zones) are shown between the periods 1901–1930 and 1988–2017, based on climate time series at 50 km spatial resolution ( [[#Harris--2020|Harris et al., 2020]] ). The AI has various limitations in assessing dryland expansion and different indices highlight different areas and different changes. This is known as the aridity paradox ( [[#Greve--2019|Greve et al., 2019]] ). See SRCCL [[IPCC:Wg2:Chapter:Chapter-3#3.2.1|Section 3.2.1]] ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ) for an in-depth analysis of limitations, and [[#Stringer--2021|Stringer et al. (2021)]] for a summary of different measures and indices used in the literature.

Based on the AI, some drylands are projected to expand and others to contract due to climate change. However, there is no evidence of a global trend in dryland expansion based on vegetation patterns, precipitation and soil moisture, based on the satellite record from the 1980s to the present ( ''medium confidence'' ). The AI will also be of limited use under a changing CO 2 environment due to higher water use efficiency by some plants ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), and it overvalues the role of potential evapotranspiration (PET) relative to rainfall. It also does not account for CO 2 impacts on evapotranspiration, and seasonality in rainfall and evapotranspiration. Higher annual PET because of increased temperatures will have little impact if temperature and actual evapotranspiration are not rising during the period of vegetation growth ( [[#Stringer--2021|Stringer et al., 2021]] ).

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