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

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

<div id="section-3-1-4-1-processes-of-desertification-and-their-climatic-drivers-block-1"></div>

'''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.

<div id="section-3-1-4-2-anthropogenic-drivers-of-desertification-under-climate-change"></div>

<span id="anthropogenic-drivers-of-desertification-under-climate-change"></span>