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

==== 3.4.1.1 Impacts on ecosystems and their services in drylands ====

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The Millenium Ecosystem Assessement (2005) <sup>[[#fn:r542|542]]</sup> proposed four classes of ecosystem services: provisioning, regulating, supporting and cultural services (Cross-Chapter Box 8 in Chapter 6). These ecosystem services in drylands are vulnerable to the impacts of climate change due to high variability in temperature, precipitation and soil fertility (Enfors and Gordon 2008 <sup>[[#fn:r543|543]]</sup> ; Mortimore 2005 <sup>[[#fn:r544|544]]</sup> ). There is ''high confidence'' that desertification processes such as soil erosion, secondary salinisation, and overgrazing have negatively impacted provisioning ecosystem services in drylands, particularly food and fodder production (Majeed and Muhammad 2019 <sup>[[#fn:r545|545]]</sup> ; Mirzabaev et al. 2016 <sup>[[#fn:r546|546]]</sup> ; Qadir et al. 2009 <sup>[[#fn:r547|547]]</sup> ; Van Loo et al. 2017 <sup>[[#fn:r548|548]]</sup> ; Tokbergenova et al. 2018 <sup>[[#fn:r549|549]]</sup> ) (Section 3.4.2.2). Zika and Erb (2009) <sup>[[#fn:r550|550]]</sup> reported an estimation of NPP losses between 0.8 and 2.0 GtC yr– <sup>1</sup> due to desertification, comparing the potential NPP and the NPP calculated for the year 2000. In terms of climatic factors, although climatic changes between 1976 and 2016 were found to be favourable for crop yields overall in Russia (Ivanov et al. 2018 <sup>[[#fn:r551|551]]</sup> ), yield decreases of up to 40–60% in dryland areas were caused by severe and extensive droughts (Ivanov et al. 2018 <sup>[[#fn:r552|552]]</sup> ). Increase in temperature can have a direct impact on animals in the form of increased physiological stress (Rojas-Downing et al. 2017 <sup>[[#fn:r553|553]]</sup> ), increased water requirements for drinking and cooling, a decrease in the production of milk, meat and eggs, increased stress during conception and reproduction (Nardone et al. 2010 <sup>[[#fn:r554|554]]</sup> ) or an increase in seasonal diseases and epidemics (Thornton et al. 2009 <sup>[[#fn:r555|555]]</sup> ; Nardone et al. 2010 <sup>[[#fn:r556|556]]</sup> ). Furthermore, changes in temperature can indirectly impact livestock through reducing the productivity and quality of feed crops and forages (Thornton et al. 2009 <sup>[[#fn:r557|557]]</sup> ; Polley et al. 2013 <sup>[[#fn:r558|558]]</sup> ). On the other hand, fewer days with extreme cold temperatures during winter in the temperate zones are associated with lower livestock mortality. The future projection of impacts on ecosystems is presented in Section 3.5.2.

Over-extraction is leading to groundwater depletion in many dryland areas ( ''high confidence'' ) (Mudd 2000 <sup>[[#fn:r559|559]]</sup> ; Mays 2013 <sup>[[#fn:r560|560]]</sup> ; Mahmod and Watanabe 2014 <sup>[[#fn:r561|561]]</sup> ; Jolly et al. 2008 <sup>[[#fn:r562|562]]</sup> ). Globally, groundwater reserves have been reduced since 1900, with the highest rate of estimated reductions of 145 km <sup>3</sup> yr– <sup>1</sup> between 2000 and 2008 (Konikow 2011 <sup>[[#fn:r563|563]]</sup> ). Some arid lands are very vulnerable to groundwater reductions, because the current natural recharge rates are lower than during the previous wetter periods (e.g., the Atacama Desert, and Nubian aquifer system in Africa) (Squeo et al. 2006 <sup>[[#fn:r564|564]]</sup> ; Mahmod and Watanabe 2014 <sup>[[#fn:r565|565]]</sup> ; Herrera et al. 2018 <sup>[[#fn:r566|566]]</sup> ).

Among regulating services, desertification can influence levels of atmospheric CO <sub>2</sub> . In drylands, the majority of carbon is stored below ground in the form of biomass and SOC (FAO 1995 <sup>[[#fn:r567|567]]</sup> ) (Section 3.3.3). Land-use changes often lead to reductions in SOC and organic matter inputs into soil (Albaladejo et al. 2013 <sup>[[#fn:r568|568]]</sup> ; Almagro et al. 2010 <sup>[[#fn:r569|569]]</sup> ; Hoffmann et al. 2012 <sup>[[#fn:r570|570]]</sup> ; Lavee et al. 1998 <sup>[[#fn:r571|571]]</sup> ; Rey et al. 2011 <sup>[[#fn:r572|572]]</sup> ), increasing soil salinity and soil erosion (Lavee et al. 1998 <sup>[[#fn:r573|573]]</sup> ; Martinez-Mena et al. 2008 <sup>[[#fn:r574|574]]</sup> ). In addition to the loss of soil, erosion reduces soil nutrients and organic matter, thereby impacting land’s productive capacity. To illustrate, soil erosion by water is estimated to result in the loss of 23–42 Mt of nitrogen and 14.6–26.4 Mt of phosphorus from soils globally each year (Pierzynski et al. 2017 <sup>[[#fn:r575|575]]</sup> ).

Precipitation, by affecting soil moisture content, is considered to be the principal determinant of the capacity of drylands to sequester carbon (Fay et al. 2008 <sup>[[#fn:r576|576]]</sup> ; Hao et al. 2008 <sup>[[#fn:r577|577]]</sup> ; Mi et al. 2015 <sup>[[#fn:r578|578]]</sup> ; Serrano-Ortiz et al. 2015 <sup>[[#fn:r579|579]]</sup> ; Vargas et al. 2012 <sup>[[#fn:r580|580]]</sup> ; Sharkhuu et al. 2016 <sup>[[#fn:r581|581]]</sup> ). Lower annual rainfall resulted in the release of carbon into the atmosphere for a number of sites located in Mongolia, China and North America (Biederman et al. 2017 <sup>[[#fn:r582|582]]</sup> ; Chen et al. 2009 <sup>[[#fn:r583|583]]</sup> ; Fay et al. 2008 <sup>[[#fn:r584|584]]</sup> ; Hao et al. 2008 <sup>[[#fn:r585|585]]</sup> ; Mi et al. 2015 <sup>[[#fn:r586|586]]</sup> ; Sharkhuu et al. 2016 <sup>[[#fn:r587|587]]</sup> ). Low soil water availability promotes soil microbial respiration, yet there is insufficient moisture to stimulate plant productivity (Austin et al. 2004 <sup>[[#fn:r588|588]]</sup> ), resulting in net carbon emissions at an ecosystem level. Under even drier conditions, photo degradation of vegetation biomass may often constitute an additional loss of carbon from an ecosystem (Rutledge et al. 2010 <sup>[[#fn:r589|589]]</sup> ). In contrast, years of good rainfall in drylands resulted in the sequestration of carbon (Biederman et al. 2017 <sup>[[#fn:r590|590]]</sup> ; Chen et al. 2009 <sup>[[#fn:r591|591]]</sup> ; Hao et al. 2008 <sup>[[#fn:r592|592]]</sup> ). In an exceptionally rainy year (2011) in the southern hemisphere, the semi-arid ecosystems of this region contributed 51% of the global net carbon sink (Poulter et al. 2014 <sup>[[#fn:r593|593]]</sup> ). These results suggest that arid ecosystems could be an important global carbon sink, depending on soil water availability (medium evidence, high agreement). However, drylands are generally predicted to become warmer with an increasing frequency of extreme drought and high rainfall events (Donat et al. 2016 <sup>[[#fn:r594|594]]</sup> ).

When desertification reduces vegetation cover, this alters the soil surface, affecting the albedo and the water balance (Gonzalez-Martin et al. 2014 <sup>[[#fn:r595|595]]</sup> ) (Section 3.3). In such situations, erosive winds have no more obstacles, which favours the occurrence of wind erosion and dust storms. Mineral aerosols have an important influence on the dispersal of soil nutrients and lead to changes in soil characteristics (Goudie and Middleton 2001 <sup>[[#fn:r596|596]]</sup> ; Middleton 2017 <sup>[[#fn:r597|597]]</sup> ). Thereby, the soil formation as a supporting ecosystem service is negatively affected (Section 3.3.1). Soil erosion by wind results in a loss of fine soil particles (silt and clay), reducing the ability of soil to sequester carbon (Wiesmeier et al. 2015 <sup>[[#fn:r598|598]]</sup> ). Moreover, dust storms reduce crop yields by loss of plant tissue caused by sandblasting (resulting in loss of plant leaves and hence reduced photosynthetic activity (Field et al. 2010 <sup>[[#fn:r599|599]]</sup> ), exposing crop roots, crop seed burial under sand deposits, and leading to losses of nutrients and fertiliser from topsoil (Stefanski and Sivakumar 2009 <sup>[[#fn:r600|600]]</sup> )). Dust storms also impact crop yields by reducing the quantity of water available for irrigation; they can decrease the storage capacity of reservoirs by siltation, and block conveyance canals (Middleton 2017 <sup>[[#fn:r601|601]]</sup> ; Middleton and Kang 2017 <sup>[[#fn:r602|602]]</sup> ; Stefanski and Sivakumar 2009 <sup>[[#fn:r603|603]]</sup> ). Livestock productivity is reduced by injuries caused by dust storms (Stefanski and Sivakumar 2009 <sup>[[#fn:r604|604]]</sup> ). Additionally, dust storms favour the dispersion of microbial and plant species, which can make local endemic species vulnerable to extinction and promote the invasion of plant and microbial species (Asem and Roy 2010 <sup>[[#fn:r605|605]]</sup> ; Womack et al. 2010 <sup>[[#fn:r606|606]]</sup> ). Dust storms increase microbial species in remote sites ( ''high confidence'' ) (Kellogg et al. 2004 <sup>[[#fn:r607|607]]</sup> ; Prospero et al. 2005 <sup>[[#fn:r608|608]]</sup> ; Griffin et al. 2006 <sup>[[#fn:r609|609]]</sup> ; Schlesinger et al. 2006 <sup>[[#fn:r610|610]]</sup> ; Griffin 2007 <sup>[[#fn:r611|611]]</sup> ; De Deckker et al. 2008 <sup>[[#fn:r612|612]]</sup> ; Jeon et al. 2011 <sup>[[#fn:r613|613]]</sup> ; Abed et al. 2012 <sup>[[#fn:r614|614]]</sup> ; Favet et al. 2013 <sup>[[#fn:r615|615]]</sup> ; Woo et al. 2013 <sup>[[#fn:r616|616]]</sup> ; Pointing and Belnap 2014 <sup>[[#fn:r617|617]]</sup> ).

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