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

=== 3.1.1 Introduction ===

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In this report, desertification is defined as land degradation in arid, semi-arid, and dry sub-humid areas resulting from many factors, including climatic variations and human activities (United Nations Convention to Combat Desertification (UNCCD) 1994). Land degradation is a negative trend in land condition, caused by direct or indirect human-induced processes including anthropogenic climate change, expressed as long-term reduction or loss of at least one of the following: biological productivity, ecological integrity or value to humans (Section 4.1.3). Arid, semi-arid, and dry sub-humid areas, together with hyper-arid areas, constitute drylands (UNEP 1992 <sup>[[#fn:r1|1]]</sup> ), home to about 3 billion people (van der Esch et al. 2017 <sup>[[#fn:r2|2]]</sup> ). The difference between desertification and land degradation is not process-based but geographic. Although land degradation can occur anywhere across the world, when it occurs in drylands, it is considered desertification (FAQ 1.3). Desertification is not limited to irreversible forms of land degradation, nor is it equated to desert expansion, but represents all forms and levels of land degradation occurring in drylands.

The geographic classification of drylands is often based on the aridity index (AI) – the ratio of average annual precipitation amount (P) to potential evapotranspiration amount (PET, see Glossary) (Figure 3.1). Recent estimates, based on AI, suggest that drylands cover about 46.2% (±0.8%) of the global land area (Koutroulis 2019 <sup>[[#fn:r3|3]]</sup> ; Prăvălie 2016 <sup>[[#fn:r4|4]]</sup> ) ( ''low confidence'' ). Hyper-arid areas, where the aridity index is below 0.05, are included in drylands, but are excluded from the definition of desertification (UNCCD 1994 <sup>[[#fn:r5|5]]</sup> ). Deserts are valuable ecosystems (UNEP 2006 <sup>[[#fn:r6|6]]</sup> ; Safriel 2009 <sup>[[#fn:r7|7]]</sup> ) geographically located in drylands and vulnerable to climate change. However, they are not considered prone to desertification. Aridity is a long-term climatic feature characterised by low average precipitation or available water (Gbeckor-Kove 1989 <sup>[[#fn:r8|8]]</sup> ; Türkeş 1999 <sup>[[#fn:r9|9]]</sup> ). Thus, aridity is different from drought, which is a temporary climatic event (Maliva and Missimer 2012 <sup>[[#fn:r10|10]]</sup> ). Moreover, droughts are not restricted to drylands, but occur both in drylands and humid areas (Wilhite et al. 2014 <sup>[[#fn:r11|11]]</sup> ). Following the Synthesis Report (SYR) of the IPCC Fifth Assessment Report (AR5), drought is defined here as “a period of abnormally dry weather long enough to cause a serious hydrological imbalance” (Mach et al. 2014 <sup>[[#fn:r12|12]]</sup> ) (Cross-Chapter Box 5 in this chapter).

AI is not an accurate proxy for delineating drylands in an increasing CO <sub>2</sub> environment (Section 3.2.1). The suggestion that most of the world has become more arid, since the AI has decreased, is not supported by changes observed in precipitation, evaporation or drought (Sheffield et al. 2012 <sup>[[#fn:r13|13]]</sup> ; Greve et al. 2014 <sup>[[#fn:r14|14]]</sup> ). While climate change is expected to decrease the AI due to increases in potential evaporation, the assumptions that underpin the potential evaporation calculation are not consistent with a changing CO <sub>2</sub> environment and the effect this has on transpiration rates (Roderick et al. 2015 <sup>[[#fn:r15|15]]</sup> ; Milly and Dunne 2016 <sup>[[#fn:r16|16]]</sup> ; Greve et al. 2017 <sup>[[#fn:r17|17]]</sup> ) (Section 3.2.1). Given that future climate is characterised by significant increases in CO <sub>2</sub> , the usefulness of currently applied AI thresholds to estimate dryland areas is limited under climate change. If instead of the AI, other variables such as precipitation, soil moisture, and primary productivity are used to identify dryland areas, there is no clear indication that the extent of drylands will change overall under climate change (Roderick et al. 2015 <sup>[[#fn:r18|18]]</sup> ; Greve et al. 2017 <sup>[[#fn:r19|19]]</sup> ; Lemordant et al. 2018 <sup>[[#fn:r20|20]]</sup> ). Thus, some dryland borders will expand, while some others will contract ( ''high confidence'' ).

Approximately 70% of dryland areas are located in Africa and Asia (Figure 3.2). The biggest land use/cover in terms of area in drylands, if deserts are excluded, are grasslands, followed by forests and croplands (Figure 3.3). The category of ‘other lands’ in Figure 3.3 includes bare soil, ice, rock, and all other land areas that are not included within the other five categories (FAO 2016 <sup>[[#fn:r21|21]]</sup> ). Thus, hyper-arid areas contain mostly deserts, with some small exceptions, for example, where grasslands and croplands are cultivated under oasis conditions with irrigation (Section 3.7.4). Moreover, FAO (2016) <sup>[[#fn:r1786|1786]]</sup> defines grasslands as permanent pastures and meadows used continuously for more than five years. In drylands, transhumance, i.e. seasonal migratory grazing, often leads to non-permanent pasture systems, thus some of the areas under the ‘other land’ category are also used as non-permanent pastures (Ramankutty et al. 2008 <sup>[[#fn:r22|22]]</sup> ; Fetzel et al. 2017 <sup>[[#fn:r23|23]]</sup> ; Erb et al. 2016 <sup>[[#fn:r24|24]]</sup> ).

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<span id="figure-3.1"></span>
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'''Figure 3.1'''

<span id="geographical-distribution-of-drylands-delimited-based-on-the-aridity-index-ai.-the-classification-of-ai-is-humid-ai-0.65-dry-sub-humid-0.50-ai-0.65-semi-arid-0.20-ai-0.50-arid-0.05-ai-0.20-hyper-arid-ai-0.05.-data-terraclimate-precipitation-and-potential-evapotranspiration-19802015-abatzoglou-et-al.-2018."></span>
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'''Geographical distribution of drylands, delimited based on the aridity index (AI). The classification of AI is: Humid AI &gt; 0.65, Dry sub-humid 0.50 &lt; AI ≤ 0.65, Semi-arid 0.20 &lt; AI ≤ 0.50, Arid 0.05 &lt; AI ≤ 0.20, Hyper-arid AI &lt; 0.05. Data: TerraClimate precipitation and potential evapotranspiration (1980–2015) (Abatzoglou et al. 2018).'''
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[[File:2d902f3f9347deee691548e052becdac C3_Figure-3.1-1024x455.jpg]]

Geographical distribution of drylands, delimited based on the aridity index (AI). The classification of AI is: Humid AI &gt; 0.65, Dry sub-humid 0.50 &lt; AI ≤ 0.65, Semi-arid 0.20 &lt; AI ≤ 0.50, Arid 0.05 &lt; AI ≤ 0.20, Hyper-arid AI &lt; 0.05. Data: TerraClimate precipitation and potential evapotranspiration (1980–2015) (Abatzoglou et al. 2018 <sup>[[#fn:r1787|1787]]</sup> ).

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'''Figure 3.2'''

<span id="dryland-categories-across-geographical-areas-continents-and-pacific-region.-data-terraclimate-precipitation-and-potential-evapotranspiration-19802015-abatzoglou-et-al.-2018."></span>
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'''Dryland categories across geographical areas (continents and Pacific region). Data: TerraClimate precipitation and potential evapotranspiration (1980–2015) (Abatzoglou et al. 2018).'''
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[[File:83a75298bfa247ed8c5a870d124591cb Figure-3.2-1024x577.jpg]]

Dryland categories across geographical areas (continents and Pacific region). Data: TerraClimate precipitation and potential evapotranspiration (1980–2015) (Abatzoglou et al. 2018 <sup>[[#fn:r1788|1788]]</sup> ).

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In the earlier global assessments of desertification (since the 1970s), which were based on qualitative expert evaluations, the extent of desertification was found to range between 4% and 70% of the area of drylands (Safriel 2007 <sup>[[#fn:r25|25]]</sup> ). More recent estimates, based on remotely sensed data, show that about 24–29% of the global land area experienced reductions in biomass productivity between the 1980s and 2000s (Bai et al. 2008 <sup>[[#fn:r26|26]]</sup> ; Le et al. 2016 <sup>[[#fn:r27|27]]</sup> ), corresponding to about 9.2% of drylands (±0.5%) experiencing declines in biomass productivity during this period ( ''low confidence'' ), mainly due to anthropogenic causes. Both of these studies consider rainfall dynamics, thus, accounting for the effect of droughts. While less than 10% of drylands is undergoing desertification, it is occurring in areas that contain around 20% of dryland population (Klein Goldewijk et al. 2017 <sup>[[#fn:r28|28]]</sup> ). In these areas the population has increased from approximately 172 million in 1950 to over 630 million today (Figure 1.1).

Available assessments of the global extent and severity of desertification are relatively crude approximations with considerable uncertainties, for example, due to confounding effects of invasive bush encroachment in some dryland regions. Different indicator sets and approaches have been developed for monitoring and assessment of desertification from national to global scales (Imeson 2012 <sup>[[#fn:r29|29]]</sup> ; Sommer et al. 2011 <sup>[[#fn:r30|30]]</sup> ; Zucca et al. 2012 <sup>[[#fn:r31|31]]</sup> ; Bestelmeyer et al. 2013 <sup>[[#fn:r32|32]]</sup> ). Many indicators of desertification only include a single factor or characteristic of desertification, such as the patch size distribution of vegetation (Maestre and Escudero 2009 <sup>[[#fn:r33|33]]</sup> ; Kéfi et al. 2010 <sup>[[#fn:r34|34]]</sup> ), Normalized Difference Vegetation Index (NDVI) (Piao et al. 2005 <sup>[[#fn:r35|35]]</sup> ), drought-tolerant plant species (An et al. 2007), grass cover (Bestelmeyer et al. 2013 <sup>[[#fn:r36|36]]</sup> ), land productivity dynamics (Baskan et al. 2017 <sup>[[#fn:r37|37]]</sup> ), ecosystem net primary productivity (Zhou et al. 2015 <sup>[[#fn:r38|38]]</sup> ) or Environmentally Sensitive Land Area Index (Symeonakis et al. 2016 <sup>[[#fn:r39|39]]</sup> ). In addition, some synthetic indicators of desertification have also been used to assess desertification extent and desertification processes, such as climate, land use, soil, and socio-economic parameters (Dharumarajan et al. 2018 <sup>[[#fn:r40|40]]</sup> ), or changes in climate, land use, vegetation cover, soil properties and population as the desertification vulnerability index (Salvati et al. 2009 <sup>[[#fn:r41|41]]</sup> ). Current data availability and methodological challenges do not allow for accurately and comprehensively mapping desertification at a global scale (Cherlet et al. 2018 <sup>[[#fn:r42|42]]</sup> ). However, the emerging partial evidence points to a lower global extent of desertification than previously estimated ( ''medium confidence'' ) (Section 3.2).

This assessment examines the socio-ecological links between drivers (Section 3.1) and feedbacks (Section 3.3) that influence desertification–climate change interactions, and then examines associated observed and projected impacts (Sections 3.4 and 3.5) and responses (Section 3.6). Moreover, this assessment highlights that dryland populations are highly vulnerable to desertification and climate change (Sections 3.2 and 3.4). At the same time, dryland populations also have significant past experience and sources of resilience embodied in indigenous and local knowledge and practices in order to successfully adapt to climatic changes and address desertification (Section 3.6). Numerous site-specific technological response options are also available for SLM in drylands that can help increase the resilience of agricultural livelihood systems to climate change (Section 3.6). However, continuing environmental degradation combined with climate change is straining the resilience of dryland populations. Enabling policy responses for SLM and livelihoods diversification can help maintain and strengthen the resilience and adaptive capacities in dryland areas (Section 3.6). The assessment finds that policies promoting SLM in drylands will contribute to climate change adaptation and mitigation, with co-benefits for broader sustainable development ( ''high confidence'' ) (Section 3.4).

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<span id="figure-3.3"></span>
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'''Figure 3.3'''

<span id="land-use-and-land-cover-in-drylands-and-share-of-each-dryland-category-in-global-land-area.-source-fao-2016."></span>
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'''Land use and land cover in drylands and share of each dryland category in global land area. Source: FAO (2016).'''
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[[File:69315d223573506a44817d84bd4d03f4 Figure-3.3-1024x599.jpg]]

Land use and land cover in drylands and share of each dryland category in global land area. Source: FAO (2016) <sup>[[#fn:r1789|1789]]</sup> .

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