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

== Executive Summary ==

<div id="h1-1-siblings" class="h1-siblings"></div>

<div id="Introduction" class="h4-container"></div>

<span id="introduction"></span>
===== Introduction =====

<div id="h4-1-siblings" class="h4-siblings"></div>

This cross-chapter paper on ‘Deserts, semiarid areas and desertification’ updates and extends [[IPCC:Wg2:Chapter:Chapter-3|Chapter 3]] on ‘Desertification’ in the IPCC Special Report on Climate Change and Land (SRCCL) ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ). It assesses new information and links it to the findings across the chapters of Working Group II’s contribution as well as relevant chapters of Working Group I’s contribution to the IPCC Sixth Assessment Report (AR6), with an added focus on deserts which were outside the scope of the SRCCL.

<div id="Where" class="h4-container"></div>

<span id="where-are-we-now-observed-impacts-and-adaptation-responses"></span>
===== Where are we now: Observed impacts and adaptation responses =====

<div id="h4-2-siblings" class="h4-siblings"></div>

'''Deserts and semiarid areas have already been affected by climate change, with some areas experiencing increases in aridity. Mixed trends of decreases and increases in vegetation productivity have been observed, depending on the time period, geographic region, detection methods used and vegetation type under consideration (''' '''''high confidence''''' [[#footnote-001|1]] '''). These changes have had varying and location-specific impacts on biodiversity, and have altered ecosystem carbon balance, water availability and the provision of ecosystem services (''' '''''high confidence''''' '''). There is no evidence, however, of a global trend in dryland expansion based on analyses of vegetation patterns, precipitation and soil moisture, with overall, more greening than drying in drylands since the 1980s (''' '''''medium confidence''''' ''').''' Deserts and semiarid areas host unique biodiversity, rich cultural heritage and provide globally valuable ecosystem services. They are also highly vulnerable to climate change. The vitality of natural ecosystems in arid and semiarid regions greatly depends on water availability, as they are highly sensitive to changes in precipitation and potential evapotranspiration, as well as to land management practices. Multiple lines of evidence from 1920–2015 indicate that surface warming of 1.2 ° C–1.3 ° C over global drylands ( [[IPCC:Wg2:Chapter:Chapter-1#1.1.1|Section 1.1.1]] ) exceeded the 0.8 ° C–1.0 ° C warming over humid lands. From 1982 to 2015, unsustainable land use and climate change combined caused desertification of 6% of the global dryland area, while 41% showed significant increases in vegetation productivity (greening) and 53% of the area had no notable change, although greening rates are slowing or declining in some locations. Greening may cause biodiversity loss and ecosystem service degradation in relation to livelihood systems. Observed trends in deserts and semiarid areas have led to varying impacts on flora, fauna, soil, nutrient cycling, the carbon cycle and water resources. Ecological changes in dryland ecosystems detected and attributed primarily to climate change include tree mortality and losses of mesic tree species at specific sites in the African Sahel, particularly during the droughts of the 1970s and 1980s, and in North Africa from 1970 to 2007; and losses of bird species in the Mojave Desert of North America from 1908 to 2016. In contrast, growth in herbaceous vegetation production has increased in some drylands since the 1980s. Widespread woody encroachment has occurred in many shrublands and savannas in Africa, Australia, North America and South America, due to a combination of land use change, changes in rainfall, fire suppression and CO 2 fertilization which, together with unsustainable management, alters biodiversity and reduces ecosystem services, such as water availability and grazing potential {3.2.1, 3.2.2} .

'''The impacts of climate change have affected the ecosystem services that humans can harness from drylands, with largely negative implications for livelihoods, human health and well-being, particularly in deserts and semiarid areas with lower adaptive capacities (''' '''''high confidence''''' ''')''' . Ecosystem degradation ( [[IPCC:Wg2:Chapter:Chapter-16#16.5.2.3|Section 16.5.2.3.2]] ) and desertification threaten the abilities of both natural and human systems to adapt to climate change ( ''high confidence'' ). Changes in desert and semiarid ecosystem services most acutely affect people who are directly dependent on natural resources for their livelihoods and survival. These groups also often have lower capacities to adapt, particularly given structural limitations of some drylands where healthcare, sanitation, infrastructure and efficient markets are lacking, reinforcing existing inequalities ( ''high confidence'' ). In rural drylands in tropical and Mediterranean areas, human populations are steadily expanding with mixed implications for ecosystem services under climate change, while rapid urbanisation in new and existing dryland megacities puts additional pressure on water ecosystem services ( ''high confidence'' ). Impacts resulting from consumption of dryland ecosystem services elsewhere, alongside other teleconnections associated with health, trade, conflict and migration, mean that dryland adaptive capacities have far reaching implications for other locations, while other locations affect dryland adaptation options. {3.1.1, 3.2.1, 3.2.2, 3.4} ''.''

<span id="where-are-we-going-risks-and-adaptation-under-warming-pathways"></span>
===== Where are we going? Risks and adaptation under warming pathways =====

<div id="h4-3-siblings" class="h4-siblings"></div>

'''Some drylands will expand by 2100, while others will shrink (''' '''''high confidence''''' '''). Climate change affects drylands through increased temperatures and more irregular rainfall, with important differences between areas with different rainfall distributions linked to the dominant climate systems in each location. Projections are nevertheless uncertain and not well supported by observed trends, while different methodological approaches and indices exhibit different strengths and weaknesses (''' '''''medium confidence''''' ''').''' A fundamental methodological challenge is how to attribute projected impacts to climate change when background climate variability in drylands is so high. Some projections show aridity (as measured by the Aridity Index, AI) to expand substantially on all continents, except Antarctica. Expansion of arid regions is probable in southwest North America, the northern fringe of Africa, southern Africa and Australia. The main areas of semiarid expansion are ''likely'' [[#footnote-000|2]] to occur on the north side of the Mediterranean, southern Africa and North and South America. India, parts of northern China, eastern equatorial Africa and the southern Saharan regions are projected to have shrinking drylands. Under Representative Concentration Pathway (RCP) 8.5, aridity zones could expand by one-quarter of the 1990 area by 2100, increasing to over half the global terrestrial area. Lower greenhouse gas emissions, under RCP4.5, could limit expansion to one-tenth of the 1990 area by 2100. Nevertheless, the utility of the AI in delineating dryland biomes is limited under an increasing CO 2 environment ( ''medium confidence'' ) and how well the index fits observed trends has been questioned in recent research. The impacts of climate change on sand and dust storm activity are projected to be substantial, however, there is large regional variability in terms of rainfall seasonality, land management practices and differences in rates of change and the scales at which the projections are undertaken. The characteristics and speed of human responses and adaptations also affect future risks and impacts ( ''high confidence'' ). Increased temperature and rainfall variability will significantly change the interannual variability in the global carbon cycle, which is strongly influenced by the world’s drylands and the ways they are managed ( ''medium confidence'' ). Increased variability of precipitation would generally contribute to increased vulnerability for people in drylands, intensifying the challenges that people living in deserts and semiarid areas will face for their sustainable development ( ''medium confidence'' ). {3.3.1, 3.3.2}

<div id="Contributions" class="h4-container"></div>

<span id="contributions-of-adaptation-measures-to-climate-resilient-development"></span>
===== Contributions of adaptation measures to climate resilient development =====

<div id="h4-4-siblings" class="h4-siblings"></div>

'''Drivers of desert expansion and greening are numerous, are attributed to environmental and human processes and differ across dryland types, yet a suite of adaptations can help to address human drivers of change, support resilience and build the adaptive capacity of dryland people (''' '''''medium confidence''''' ''').''' Deserts and semiarid areas have a rich cultural heritage, and Indigenous knowledge and local knowledge (IKLK) which enrich and influence sustainability and land use globally. Growing research evidence and experience highlight the necessary features of an enabling environment for dryland adaptation ( [[IPCC:Wg2:Chapter:Chapter-8#8.5.2|Section 8.5.2]] ). Key enablers include supportive policies, institutions and governance approaches that strengthen the adaptive capacities of dryland farmers, pastoralists and other dryland resource users ( ''high confidence'' ), addressing drivers (proximate and underlying) as well as symptoms of desertification. For instance, the skills and capacities held by the mobile and adaptive approach of pastoralists may provide lessons for society at large in adapting to climate change and dealing with increased uncertainty. Such a policy would stand in contrast to previous attempts at settling pastoralists. There is a persistent gap in terms of scaling-up already known good practices, combining nature-based, land-based and ecosystem-based approaches that facilitate sustainable land management, with contextually appropriate and responsible governance systems (e.g., including those supporting communal land tenure arrangements and IKLK; ''medium confidence'' ). Land-based adaptations can help manage dryland changes, including sand and dust storms and desertification ( ''high confidence'' ), while technological options linked to water management draw from both traditional practices and new innovations. Adequate financing and investment is required to harness multiple benefits for managing the impacts of climate change and desertification while accelerating progress towards sustainable development in deserts and semiarid areas. {3.4}

<div id="CCP3.1" class="h1-container"></div>

<span id="ccp3.1-introduction"></span>