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

=== 3.1.4 Processes and drivers of desertification under climate change ===

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==== 3.1.4.1 Processes of desertification and their climatic drivers ====

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'''Processes of desertification''' are mechanisms by which drylands are degraded. Desertification consists of both biological and non-biological processes. These processes are classified under broad categories of degradation of physical, chemical and biological properties of terrestrial ecosystems. The number of desertification processes is large and they are extensively covered elsewhere (IPBES 2018a <sup>[[#fn:r111|111]]</sup> ; Lal 2016 <sup>[[#fn:r112|112]]</sup> ; Racine 2008 <sup>[[#fn:r113|113]]</sup> ; UNCCD 2017 <sup>[[#fn:r114|114]]</sup> ). Section 4.2.1 and Tables 4.1 and 4.2 in Chapter 4 highlight those which are particularly relevant for this assessment in terms of their links to climate change and land degradation, including desertification.

'''Drivers of desertification''' are factors which trigger desertification processes. Initial studies of desertification during the early-to-mid 20th century attributed it entirely to human activities. In one of the influential publications of that time, Lavauden (1927) <sup>[[#fn:r115|115]]</sup> stated that: “Desertification is purely artificial. It is only the act of the man…” However, such a uni-causal view of desertification was shown to be invalid (Geist et al. 2004 <sup>[[#fn:r116|116]]</sup> ; Reynolds et al. 2007 <sup>[[#fn:r117|117]]</sup> ) (Sections 3.1.4.2 and 3.1.4.3). Tables 4.1 and 4.2 in Chapter 4 summarise the drivers, linking them to the specific processes of desertification and land degradation under changing climate.

Erosion refers to removal of soil by the physical forces of water, wind, or often caused by farming activities such as tillage (Ginoux et al. 2012 <sup>[[#fn:r118|118]]</sup> ). The global estimates of soil erosion differ significantly, depending on scale, study period and method used (García-Ruiz et al. 2015 <sup>[[#fn:r119|119]]</sup> ), ranging from approximately 20 Gt yr– <sup>1</sup> to more than 200 Gt yr– <sup>1</sup> (Boix-Fayos et al. 2006 <sup>[[#fn:r120|120]]</sup> ; FAO 2015 <sup>[[#fn:r121|121]]</sup> ). There is a significant potential for climate change to increase soil erosion by water, particularly in those regions where precipitation volumes and intensity are projected to increase (Panthou et al. 2014 <sup>[[#fn:r122|122]]</sup> ; Nearing et al. 2015 <sup>[[#fn:r123|123]]</sup> ). On the other hand, while it is a dominant form of erosion in areas such as West Asia and the Arabian Peninsula (Prakash et al. 2015 <sup>[[#fn:r124|124]]</sup> ; Klingmüller et al. 2016 <sup>[[#fn:r125|125]]</sup> ), there is ''limited evidence'' concerning climate change impacts on wind erosion (Tables 4.1 and 4.2 in Chapter 4, and Section 3.5).

Saline and sodic soils (see Glossary) occur naturally in arid, semi-arid and dry sub-humid regions of the world. Climate change or hydrological change can cause soil salinisation by increasing the mineralised groundwater level. However, secondary salinisation occurs when the concentration of dissolved salts in water and soil is increased by anthropogenic processes, mainly through poorly managed irrigation schemes. The threat of soil and groundwater salinisation induced by sea level rise and seawater intrusion are amplified by climate change (Section 4.9.7).

Global warming is expected to accelerate soil organic carbon (SOC) turnover, since the decomposition of the soil organic matter by microbial activity begins with low soil water availability, but this moisture is insufficient for plant productivity (Austin et al. 2004 <sup>[[#fn:r126|126]]</sup> ) (Section 3.4.1.1). SOC is also lost due to soil erosion (Lal 2009 <sup>[[#fn:r127|127]]</sup> ); therefore, in some dryland areas leading to SOC decline (Sections 3.3.3 and 3.5.2) and the transfer of carbon (C) from soil to the atmosphere (Lal 2009 <sup>[[#fn:r128|128]]</sup> ).

Sea surface temperature (SST) anomalies can drive rainfall changes, with implications for desertification processes. North Atlantic SST anomalies are positively correlated with Sahel rainfall anomalies (Knight et al. 2006 <sup>[[#fn:r129|129]]</sup> ; Gonzalez-Martin et al. 2014 <sup>[[#fn:r130|130]]</sup> ; Sheen et al. 2017 <sup>[[#fn:r131|131]]</sup> ). While the eastern tropical Pacific SST anomalies have a negative correlation with Sahel rainfall (Pomposi et al. 2016 <sup>[[#fn:r132|132]]</sup> ), a cooler North Atlantic is related to a drier Sahel, with this relationship enhanced if there is a simultaneous relative warming of the South Atlantic (Hoerling et al. 2006 <sup>[[#fn:r133|133]]</sup> ). Huber and Fensholt (2011) <sup>[[#fn:r134|134]]</sup> explored the relationship between SST anomalies and satellite observed Sahel vegetation dynamics, finding similar relationships but with substantial west–east variations in both the significant SST regions and the vegetation response. Concerning the paleoclimatic evidence on aridification after the early Holocene ‘Green Sahara’ period (11,000 to 5000 years ago), Tierney et al. (2017) <sup>[[#fn:r135|135]]</sup> indicate that a cooling of the North Atlantic played a role (Collins et al. 2017 <sup>[[#fn:r136|136]]</sup> ; Otto-Bliesner et al. 2014 <sup>[[#fn:r137|137]]</sup> ; Niedermeyer et al. 2009 <sup>[[#fn:r138|138]]</sup> ) similar to that found in modern observations. Besides these SST relationships, aerosols have also been suggested as a potential driver of the Sahel droughts (Rotstayn and Lohmann 2002 <sup>[[#fn:r139|139]]</sup> ; Booth et al. 2012 <sup>[[#fn:r140|140]]</sup> ; Ackerley et al. 2011 <sup>[[#fn:r141|141]]</sup> ). For eastern Africa, both recent droughts and decadal declines have been linked to human-induced warming in the western Pacific (Funk et al. 2018 <sup>[[#fn:r142|142]]</sup> ).

Invasive plants contributed to desertification and loss of ecosystem services in many dryland areas in the last century ( ''high confidence'' ) (Section 3.7.3). Extensive woody plant encroachment altered runoff and soil erosion across much of the drylands, because the bare soil between shrubs is very susceptible to water erosion, mainly in high-intensity rainfall events (Manjoro et al. 2012 <sup>[[#fn:r143|143]]</sup> ; Pierson et al. 2013 <sup>[[#fn:r144|144]]</sup> ; Eldridge et al. 2015 <sup>[[#fn:r145|145]]</sup> ). Rising CO <sub>2</sub> levels due to global warming favour more rapid expansion of some invasive plant species in some regions. An example is the Great Basin region in western North America where over 20% of ecosystems have been significantly altered by invasive plants, especially exotic annual grasses and invasive conifers, resulting in loss of biodiversity. This land-cover conversion has resulted in reductions in forage availability, wildlife habitat, and biodiversity (Pierson et al. 2011, 2013 <sup>[[#fn:r146|146]]</sup> ; Miller et al. 2013 <sup>[[#fn:r147|147]]</sup> ).

The wildfire is a driver of desertification, because it reduces vegetation cover, increases runoff and soil erosion, reduces soil fertility and affects the soil microbial community (Vega et al. 2005 <sup>[[#fn:r148|148]]</sup> ; Nyman et al. 2010 <sup>[[#fn:r149|149]]</sup> ; Holden et al. 2013 <sup>[[#fn:r150|150]]</sup> ; Pourreza et al. 2014 <sup>[[#fn:r151|151]]</sup> ; Weber et al. 2014 <sup>[[#fn:r152|152]]</sup> ; Liu and Wimberly 2016 <sup>[[#fn:r153|153]]</sup> ). Predicted increases in temperature and the severity of drought events across some dryland areas (Section 2.2) can increase chances of wildfire occurrence ( ''medium confidence'' ) (Jolly et al. 2015 <sup>[[#fn:r154|154]]</sup> ; Williams et al. 2010 <sup>[[#fn:r155|155]]</sup> ; Clarke and Evans 2018 <sup>[[#fn:r156|156]]</sup> ) (Cross-Chapter Box 3 in Chapter 2). In semi-arid and dry sub-humid areas, fire can have a profound influence on observed vegetation and particularly the relative abundance of grasses to woody plants (Bond et al. 2003 <sup>[[#fn:r157|157]]</sup> ; Bond and Keeley 2005 <sup>[[#fn:r158|158]]</sup> ; Balch et al. 2013 <sup>[[#fn:r159|159]]</sup> ).

While large uncertainty exists concerning trends in droughts globally (AR5) (Section 2.2), examining the drought data by Ziese et al. (2014) <sup>[[#fn:r160|160]]</sup> for drylands only reveals a large inter-annual variability combined with a trend toward increasing dryland area affected by droughts since the 1950s (Figure 1.1). Thus, over the period 1961–2013, the annual area of drylands in drought has increased, on average, by slightly more than 1% per year, with large inter-annual variability.

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==== 3.1.4.2 Anthropogenic drivers of desertification under climate change ====

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The literature on the human drivers of desertification is substantial (e.g., D’Odorico et al. 2013 <sup>[[#fn:r161|161]]</sup> ; Sietz et al. 2011 <sup>[[#fn:r162|162]]</sup> ; Yan and Cai 2015 <sup>[[#fn:r163|163]]</sup> ; Sterk et al. 2016 <sup>[[#fn:r164|164]]</sup> ; Varghese and Singh 2016 <sup>[[#fn:r165|165]]</sup> ) and there have been several comprehensive reviews and assessments of these drivers very recently (Cherlet et al. 2018 <sup>[[#fn:r166|166]]</sup> ; IPBES 2018a <sup>[[#fn:r167|167]]</sup> ; UNCCD 2017 <sup>[[#fn:r168|168]]</sup> ). IPBES (2018a) identified cropland expansion, unsustainable land management practices including overgrazing by livestock, urban expansion, infrastructure development, and extractive industries as the main drivers of land degradation. IPBES (2018a) also found that the ultimate driver of land degradation is high and growing consumption of land-based resources, e.g., through deforestation and cropland expansion, escalated by population growth. What is particularly relevant in the context of the present assessment is to evaluate if, how and which human drivers of desertification will be modified by climate change effects.

Growing food demand is driving conversion of forests, rangelands, and woodlands into cropland (Bestelmeyer et al. 2015 <sup>[[#fn:r169|169]]</sup> ; D’Odorico et al. 2013 <sup>[[#fn:r170|170]]</sup> ). Climate change is projected to reduce crop yields across dryland areas (Sections 3.4.1 and 5.2.2), potentially reducing local production of food and feed. Without research breakthroughs mitigating these productivity losses through higher agricultural productivity, and reducing food waste and loss, meeting the increasing food demands of growing populations will require expansion of cropped areas to more marginal areas (with most prime areas in drylands already being under cultivation) (Lambin 2012 <sup>[[#fn:r171|171]]</sup> ; Lambin et al. 2013 <sup>[[#fn:r172|172]]</sup> ; Eitelberg et al. 2015 <sup>[[#fn:r173|173]]</sup> ; Gutiérrez-Elorza 2006 <sup>[[#fn:r174|174]]</sup> ; Kapović Solomun et al. 2018 <sup>[[#fn:r175|175]]</sup> ). Borrelli et al. (2017) <sup>[[#fn:r176|176]]</sup> showed that the primary driver of soil erosion in 2012 was cropland expansion. Although local food demands could also be met by importing from other areas, this would mean increasing the pressure on land in those areas (Lambin and Meyfroidt 2011 <sup>[[#fn:r177|177]]</sup> ). The net effects of such global agricultural production shifts on land condition in drylands are not known.

Climate change will exacerbate poverty among some categories of dryland populations (Sections 3.4.2 and 3.5.2). Depending on the context, this impact comes through declines in agricultural productivity, changes in agricultural prices and extreme weather events (Hertel and Lobell 2014 <sup>[[#fn:r178|178]]</sup> ; Hallegatte and Rozenberg 2017 <sup>[[#fn:r179|179]]</sup> ). There is ''high confidence'' that poverty limits both capacities to adapt to climate change and availability of financial resources to invest into SLM (Gerber et al. 2014 <sup>[[#fn:r180|180]]</sup> ; Way 2016 <sup>[[#fn:r181|181]]</sup> ; Vu et al. 2014 <sup>[[#fn:r182|182]]</sup> ) (Sections 3.5.2, 3.6.2 and 3.6.3).

Labour mobility is another key human driver that will interact with climate change. Although strong impacts of climate change on migration in dryland areas are disputed, in some places, it is ''likely'' to provide an added incentive to migrate (Section 3.4.2.7). Out-migration will have several contradictory effects on desertification. On one hand, it reduces an immediate pressure on land if it leads to less dependence on land for livelihoods (Chen et al. 2014 <sup>[[#fn:r183|183]]</sup> ; Liu et al. 2016a). Moreover, migrant remittances could be used to fund the adoption of SLM practices. Labour mobility from agriculture to non-agricultural sectors could allow land consolidation, gradually leading to mechanisation and agricultural intensification (Wang et al. 2014 <sup>[[#fn:r184|184]]</sup> , 2018 <sup>[[#fn:r185|185]]</sup> ). On the other hand, this can increase the costs of labour-intensive SLM practices due to lower availability of rural agricultural labour and/or higher rural wages. Out-migration increases the pressure on land if higher wages that rural migrants earn in urban centres will lead to their higher food consumption. Moreover, migrant remittances could also be used to fund land-use expansion to marginal areas (Taylor et al. 2016 <sup>[[#fn:r186|186]]</sup> ; Gray and Bilsborrow 2014 <sup>[[#fn:r187|187]]</sup> ). The net effect of these opposite mechanisms varies from place to place (Qin and Liao 2016 <sup>[[#fn:r188|188]]</sup> ). There is very little literature evaluating these joint effects of climate change, desertification and labour mobility (Section 7.3.2).

There are also many other institutional, policy and socio-economic drivers of desertification, such as land tenure insecurity, lack of property rights, lack of access to markets, and to rural advisory services, lack of technical knowledge and skills, agricultural price distortions, agricultural support and subsidies contributing to desertification, and lack of economic incentives for SLM (D’Odorico et al. 2013 <sup>[[#fn:r189|189]]</sup> ; Geist et al. 2004 <sup>[[#fn:r190|190]]</sup> ; Moussa et al. 2016 <sup>[[#fn:r191|191]]</sup> ; Mythili and Goedecke 2016 <sup>[[#fn:r192|192]]</sup> ; Sow et al. 2016 <sup>[[#fn:r193|193]]</sup> ; Tun et al. 2015 <sup>[[#fn:r194|194]]</sup> ; García-Ruiz 2010 <sup>[[#fn:r195|195]]</sup> ). There is no evidence that these factors will be materially affected by climate change, however, serving as drivers of unsustainable land management practices, they do play a very important role in modulating responses for climate change adaptation and mitigation (Section 3.6.3).

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==== 3.1.4.3 Interaction of drivers: Desertification syndrome versus drylands development paradigm ====

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Two broad narratives have historically emerged to describe responses of dryland populations to environmental degradation. The first is ‘desertification syndrome’ which describes the vicious cycle of resource degradation and poverty, whereby dryland populations apply unsustainable agricultural practices leading to desertification, and exacerbating their poverty, which then subsequently further limits their capacities to invest in SLM (MEA 2005 <sup>[[#fn:r196|196]]</sup> ; Safriel and Adeel 2008 <sup>[[#fn:r197|197]]</sup> ). The alternative paradigm is one of ‘drylands development’, which refers to social and technical ingenuity of dryland populations as a driver of dryland sustainability (MEA 2005; Reynolds et al. 2007 <sup>[[#fn:r198|198]]</sup> ; Safriel and Adeel 2008 <sup>[[#fn:r199|199]]</sup> ). The major difference between these two frameworks is that the ‘drylands development’ paradigm recognises that human activities are not the sole and/or most important drivers of desertification, but there are interactions of human and climatic drivers within coupled social-ecological systems (Reynolds et al. 2007 <sup>[[#fn:r200|200]]</sup> ). This led Behnke and Mortimore (2016) <sup>[[#fn:r201|201]]</sup> , and earlier Swift (1996) <sup>[[#fn:r202|202]]</sup> , to conclude that the concept of desertification as irreversible degradation distorts policy and governance in dryland areas. Mortimore (2016) <sup>[[#fn:r203|203]]</sup> suggested that instead of externally imposed technical solutions, what is needed is for populations in dryland areas to adapt to this variable environment which they cannot control. All in all, there is ''high confidence'' that anthropogenic and climatic drivers interact in complex ways in causing desertification. As discussed in Section 3.2.2, the relative influence of human or climatic drivers on desertification varies from place to place ( ''high confidence'' ) (Bestelmeyer et al. 2018 <sup>[[#fn:r204|204]]</sup> ; D’Odorico et al. 2013 <sup>[[#fn:r205|205]]</sup> ; Geist and Lambin 2004 <sup>[[#fn:r206|206]]</sup> ; Kok et al. 2016 <sup>[[#fn:r207|207]]</sup> ; Polley et al. 2013 <sup>[[#fn:r208|208]]</sup> ; Ravi et al. 2010 <sup>[[#fn:r209|209]]</sup> ; Scholes 2009 <sup>[[#fn:r210|210]]</sup> ; Sietz et al. 2017 <sup>[[#fn:r211|211]]</sup> ; Sietz et al. 2011 <sup>[[#fn:r212|212]]</sup> ).

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