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

=== 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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