Editing IPCC:AR6/SRCCL/Chapter-4 (section)

=== 4.3.1 Land degradation ===

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There are no reliable global maps of the extent and severity of land degradation (Gibbs and Salmon 2015 <sup>[[#fn:r421|421]]</sup> ; Prince et al. 2018 <sup>[[#fn:r422|422]]</sup> ; van der Esch et al. 2017 <sup>[[#fn:r423|423]]</sup> ), despite the fact that land degradation is a severe problem (Turner et al. 2016 <sup>[[#fn:r424|424]]</sup> ). The reasons are both conceptual – that is, how land degradation is defined, using what baseline (Herrick et al. 2019 <sup>[[#fn:r425|425]]</sup> ) or over what time period – and methodological – that is, how it can be measured (Prince et al. 2018 <sup>[[#fn:r426|426]]</sup> ). Although there is a strong consensus that land degradation is a reduction in productivity of the land or soil, there are diverging views regarding the spatial and temporal scales at which land degradation occurs (Warren 2002 <sup>[[#fn:r427|427]]</sup> ), and how this can be quantified and mapped. Proceeding from the definition in this report, there are also diverging views concerning ecological integrity and the value to humans. A comprehensive treatment of the conceptual discussion about land degradation is provided by the recent report on land degradation from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) (Montanarella et al. 2018 <sup>[[#fn:r428|428]]</sup> ).

A review of different attempts to map global land degradation, based on expert opinion, satellite observations, biophysical models and a database of abandoned agricultural lands, suggested that between &lt;10 Mkm2 to 60 Mkm2 (corresponding to 8–45% of the ice-free land area) have been degraded globally (Gibbs and Salmon, 2015 <sup>[[#fn:r429|429]]</sup> ) ( ''very low confidence'' ).

One often-used global assessment of land degradation uses trends in NDVI as a proxy for land degradation and improvement during the period 1983 to 2006 (Bai et al. 2008b <sup>[[#fn:r430|430]]</sup> ,c <sup>[[#fn:r431|431]]</sup> ) with an update to 2011 (Bai et al. 2015 <sup>[[#fn:r432|432]]</sup> ). These studies, based on very coarse resolution satellite data (NOAA AVHRR data with a resolution of 8 km), indicated that, between 22% and 24% of the global ice-free land area was subject to a downward trend, while about 16% showed an increasing trend. The study also suggested, contrary to earlier assessments (Middleton and Thomas 1997 <sup>[[#fn:r433|433]]</sup> ), that drylands were not among the most affected regions. Another study using a similar approach for the period 1981–2006 suggested that about 29% of the global land area is subject to ‘land degradation hotspots’, that is, areas with acute land degradation in need of particular attention. These hotspot areas were distributed over all agro-ecological regions and land cover types. Two different studies have tried to link land degradation, identified by NDVI as a proxy, and number of people affected: Le et al. (2016) <sup>[[#fn:r434|434]]</sup> estimated that at least 3.2 billion people were affected, while Barbier and Hochard (2016 <sup>[[#fn:r435|435]]</sup> , 2018 <sup>[[#fn:r436|436]]</sup> ) estimated that 1.33 billion people were affected, of which 95% were living in developing countries.

Yet another study, using a similar approach and type of remote-sensing data, compared NDVI trends with biomass trends calculated by a global vegetation model over the period 1982–2010 and found that 17–36% of the land areas showed a negative NDVI trend, while a positive or neutral trend was predicted in modelled vegetation (Schut et al. 2015 <sup>[[#fn:r437|437]]</sup> ). The World Atlas of Desertification (3rd edition) includes a global map of land productivity change over the period 1999 to 2013, which is one useful proxy for land degradation (Cherlet et al. 2018 <sup>[[#fn:r438|438]]</sup> ). Over that period, about 20% of the global ice-free land area shows signs of declining or unstable productivity, whereas about 20% shows increasing productivity. The same report also summarised the productivity trends by land categories and found that most forest land showed increasing trends in productivity, while rangelands had more declining trends than increasing trends (Figure 4.4). These productivity assessments, however, do not distinguish between trends due to climate change and trends due to other factors. A recent analysis of ‘greening’ of the world using MODIS time series of NDVI 2000–2017, shows a striking increase in the greening over China and India. In China the greening is seen over forested areas, 42%, and cropland areas, in which 32% is increasing (Section 4.9.3). In India, the greening is almost entirely associated with cropland (82%) (Chen et al. 2019 <sup>[[#fn:r439|439]]</sup> ).

All these studies of vegetation trends show that there are regionally differentiated trends of either decreasing or increasing vegetation. When comparing vegetation trends with trends in climatic variables, Schut et al. (2015 <sup>[[#fn:r440|440]]</sup> ) found very few areas (1–2%) where an increase in vegetation trend was independent of the climate drivers, and that study suggested that positive vegetation trends are primarily caused by climatic factors.

In an attempt to go beyond the mapping of global vegetation trends for assessing land degradation, Borelli et al. (2017) <sup>[[#fn:r441|441]]</sup> used a soil erosion model (RUSLE) and suggested that soil erosion is mainly caused in areas of cropland expansion, particularly in Sub-Saharan Africa, South America and Southeast Asia. The method is controversial for conceptual reasons (i.e., the ability of the model to capture the most important erosion processes) and data limitations (i.e., the availability of relevant data at regional to global scales), and its validity for assessing erosion over large areas has been questioned by several studies (Baveye 2017 <sup>[[#fn:r442|442]]</sup> ; Evans and Boardman 2016a <sup>[[#fn:r443|443]]</sup> ,b <sup>[[#fn:r444|444]]</sup> ; Labrière et al. 2015 <sup>[[#fn:r445|445]]</sup> ).

An alternative to using remote sensing for assessing the state of land degradation is to compile field-based data from around the globe (Turner et al. 2016 <sup>[[#fn:r446|446]]</sup> ). In addition to the problems of definitions and baselines, this approach is also hampered by the lack of standardised methods used in the field. An assessment of the global severity of soil erosion in agriculture, based on 1673 measurements around the world (compiled from 201 peer-reviewed articles), indicated that the global net median rate of soil formation (i.e., formation minus erosion) is about 0.004 mm yr <sup>–1</sup> (about 0.05 t ha <sup>–1</sup> yr <sup>–1</sup> ) compared with the median net rate of soil loss in agricultural fields, 1.52 mm yr <sup>–1</sup> (about 18 t ha <sup>–1</sup> yr <sup>–1</sup> ) in tilled fields and 0.065 mm yr <sup>–1</sup> (about 0.8 t ha–1 yr <sup>–1</sup> ) in no-till fields (Montgomery 2007a <sup>[[#fn:r447|447]]</sup> ). This means that the rate of soil erosion from agricultural fields is between 380 (conventional tilling) and 16 times (no-till) the natural rate of soil formation ( ''medium agreement, limited evidence'' ). These approximate figures are supported by another large meta-study including over 4000 sites around the world (see Figure 4.4) where the average soil loss from agricultural plots was about 21 t ha <sup>–1</sup> yr <sup>–1</sup> (García-Ruiz et al. 2015 <sup>[[#fn:r448|448]]</sup> ). Climate change, mainly through the intensification of rainfall, will further increase these rates unless land management is improved ( ''high agreement, medium evidence'' ).

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<span id="proportional-global-land-productivity-trends-by-land-coverland-use-class.-cropland-includes-arable-land-permanent-crops-and-mixed-classes-with-over-50-crops-grassland-includes-natural-grassland-and-managed-pasture-land-rangelands-include-shrubland-herbaceous-and-sparsely-vegetated-areas-forest-land-includes-all-forest-categories-and-mixed-classes-with-tree-cover-greater-than-40.-data-source-copernicus"></span>
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'''Proportional global land productivity trends by land-cover/land-use class. (Cropland includes arable land, permanent crops and mixed classes with over 50% crops; grassland includes natural grassland and managed pasture land; rangelands include shrubland, herbaceous and sparsely vegetated areas; forest land includes all forest categories and mixed classes with tree cover greater than 40%.) Data source: Copernicus […]'''
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Proportional global land productivity trends by land-cover/land-use class. (Cropland includes arable land, permanent crops and mixed classes with over 50% crops; grassland includes natural grassland and managed pasture land; rangelands include shrubland, herbaceous and sparsely vegetated areas; forest land includes all forest categories and mixed classes with tree cover greater than 40%.) Data source: Copernicus Global Land SPOT VGT, 1999–2013, adapted from (Cherlet et al. 2018 <sup>[[#fn:r1647|1647]]</sup> ).

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Soils contain about 1500 Gt of organic carbon (median across 28 different estimates presented by Scharlemann et al. (2014)), which is about 1.8 times more carbon than in the atmosphere (Ciais et al. 2013 <sup>[[#fn:r449|449]]</sup> ) and 2.3–3.3 times more than what is held in the terrestrial vegetation of the world (Ciais et al. 2013 <sup>[[#fn:r450|450]]</sup> ). Hence, land degradation, including land conversion leading to soil carbon losses, has the potential to impact on the atmospheric concentration of CO <sub>2</sub> substantially. When natural ecosystems are cultivated they lose soil carbon that accumulated over long time periods.The loss rate depends on the type of natural vegetation and how the soil is managed. Estimates of the magnitude of loss vary but figures between 20% and 59% have been reported in several meta studies (Poeplau and Don 2015 <sup>[[#fn:r451|451]]</sup> ; Wei et al. 2015 <sup>[[#fn:r452|452]]</sup> ; Li et al. 2012 <sup>[[#fn:r453|453]]</sup> ; Murty et al. 2002 <sup>[[#fn:r454|454]]</sup> ; Guo and Gifford 2002 <sup>[[#fn:r455|455]]</sup> ). The amount of soil carbon lost explicitly due to land degradation after conversion is hard to assess due to large variation in local conditions and management, see also Chapter 2.

From a climate change perspective, land degradation plays an important role in the dynamics of nitrous oxide (N <sub>2</sub> O) and methane (CH <sub>4</sub> ). N <sub>2</sub> O is produced by microbial activity in the soil and the dynamics are related to both management practices and weather conditions, while CH <sub>4</sub> dynamics are primarily determined by the amount of soil carbon and to what extent the soil is subject to waterlogging (Palm et al. 2014 <sup>[[#fn:r456|456]]</sup> ), see also Chapter 2.

Several attempts have been made to map the human footprint on the planet (Čuček et al. 2012 <sup>[[#fn:r457|457]]</sup> ; Venter et al. 2016 <sup>[[#fn:r458|458]]</sup> ) but, in some cases, they confuse human impact on the planet with degradation. From our definition it is clear that human impact (or pressure) is not synonymous with degradation, but information on the human footprint provides a useful mapping of potential non-climatic drivers of degradation.

In summary, there are no uncontested maps of the location, extent and severity of land degradation. Proxy estimates based on remote sensing of vegetation dynamics provide one important information source, but attribution of the observed changes in productivity to climate change, human activities, or other drivers is hard. Nevertheless, the different attempts to map the extent of global land degradation using remotely sensed proxies show some convergence and suggest that about a quarter of the ice-free land area is subject to some form of land degradation ( ''limited evidence, medium agreement'' ) affecting about 3.2 billion people ( ''low confidence'' ). Attempts to estimate the severity of land degradation through soil erosion estimates suggest that soil erosion is a serious form of land degradation in croplands closely associated with unsustainable land management in combination with climatic parameters, some of which are subject to climate change ( ''limited evidence, high agreement'' ). Climate change is one among several causal factors in the status and current trends of land degradation ( ''limited evidence, high agreement'' ).

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