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

=== 3.4.1 Impacts on natural and managed ecosystems ===

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==== 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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==== 3.4.1.2 Impacts on biodiversity: Plant and wildlife ====

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''Plant biodiversity''

Over 20% of global plant biodiversity centres are located within drylands (White and Nackoney 2003 <sup>[[#fn:r618|618]]</sup> ). Plant species located within these areas are characterised by high genetic diversity within populations (Martínez-Palacios et al. 1999 <sup>[[#fn:r619|619]]</sup> ). The plant species within these ecosystems are often highly threatened by climate change and desertification (Millennium Ecosystem Assessment 2005b <sup>[[#fn:r620|620]]</sup> ; Maestre et al. 2012 <sup>[[#fn:r621|621]]</sup> ). Increasing aridity exacerbates the risk of extinction of some plant species, especially those that are already threatened due to small populations or restricted habitats (Gitay et al. 2002 <sup>[[#fn:r622|622]]</sup> ). Desertification, including through land-use change, already contributed to the loss of biodiversity across drylands ( ''medium confidence'' ) (Newbold et al. 2015 <sup>[[#fn:r623|623]]</sup> ; Wilting et al. 2017 <sup>[[#fn:r624|624]]</sup> ). For example, species richness decreased from 234 species in 1978 to 95 in 2011 following long periods of drought and human driven degradation on the steppe land of south-western Algeria (Observatoire du Sahara et du Sahel 2013 <sup>[[#fn:r625|625]]</sup> ). Similarly, drought and overgrazing led to loss of biodiversity in Pakistan to the point that only drought-adapted species can now survive on the arid rangelands (Akhter and Arshad 2006 <sup>[[#fn:r626|626]]</sup> ). Similar trends were observed in desert steppes of Mongolia (Khishigbayar et al. 2015 <sup>[[#fn:r627|627]]</sup> ). In contrast, the increase in annual moistening of southern European Russia from the late 1980s to the beginning of the 21st century caused the restoration of steppe vegetation, even under conditions of strong anthropogenic pressure (Ivanov et al. 2018 <sup>[[#fn:r628|628]]</sup> ). The seed banks of annual species can often survive over the long term, germinating in wet years, suggesting that these species could be resilient to some aspects of climate change (Vetter et al. 2005 <sup>[[#fn:r629|629]]</sup> ). Yet, Hiernaux and Houérou (2006) <sup>[[#fn:r630|630]]</sup> showed that overgrazing in the Sahel tended to decrease the seed bank of annuals, which could make them vulnerable to climate change over time. Perennial species, considered as the structuring element of the ecosystem, are usually less affected as they have deeper roots, xeromorphic properties and physiological mechanisms that increase drought tolerance (Le Houérou 1996 <sup>[[#fn:r631|631]]</sup> ). However, in North Africa, long-term monitoring (1978–2014) has shown that important plant perennial species have also disappeared due to drought ( ''Stipa tenacissima and Artemisia herba alba'' ) (Hirche et al. 2018 <sup>[[#fn:r633|633]]</sup> ; Observatoire du Sahara et du Sahel 2013 <sup>[[#fn:r634|634]]</sup> ). The aridisation of the climate in the south of Eastern Siberia led to the advance of the steppes to the north and to the corresponding migration of steppe mammal species between 1976 and 2016 (Ivanov et al. 2018 <sup>[[#fn:r635|635]]</sup> ). The future projection of impacts on plant biodiversity is presented in Section 3.5.2.

''Wildlife biodiversity''

Dryland ecosystems have high levels of faunal diversity and endemism (MEA 2005 <sup>[[#fn:r636|636]]</sup> ; Whitford 2002 <sup>[[#fn:r637|637]]</sup> ). Over 30% of the endemic bird areas are located within these regions, which is also home to 25% of vertebrate species (Maestre et al. 2012 <sup>[[#fn:r638|638]]</sup> ; MEA 2005 <sup>[[#fn:r639|639]]</sup> ). Yet, many species within drylands are threatened with extinction (Durant et al. 2014 <sup>[[#fn:r640|640]]</sup> ; Walther 2016 <sup>[[#fn:r641|641]]</sup> ). Habitat degradation and desertification are generally associated with biodiversity loss (Ceballos et al. 2010 <sup>[[#fn:r642|642]]</sup> ; Tang et al. 2018 <sup>[[#fn:r643|643]]</sup> ; Newbold et al. 2015 <sup>[[#fn:r644|644]]</sup> ). The ‘grazing value’ of land declines with both a reduction in vegetation cover and shrub encroachment, with the former being more detrimental to native vertebrates (Parsons et al. 2017 <sup>[[#fn:r645|645]]</sup> ). Conversely, shrub encroachment may buffer desertification by increasing resource and microclimate availability, resulting in an increase in vertebrate species abundance and richness observed in the shrub-encroached arid grasslands of North America (Whitford 1997 <sup>[[#fn:r646|646]]</sup> ) and Australia (Parsons et al. 2017 <sup>[[#fn:r647|647]]</sup> ). However, compared to historically resilient drylands, these encroached habitats and their new species assemblages may be more sensitive to droughts, which may become more prevalent with climate change (Schooley et al. 2018 <sup>[[#fn:r648|648]]</sup> ). Mammals and birds may be particularly sensitive to droughts because they rely on evaporative cooling to maintain their body temperatures within an optimal range (Hetem et al. 2016 <sup>[[#fn:r649|649]]</sup> ) and risk lethal dehydration in water limited environments (Albright et al. 2017 <sup>[[#fn:r650|650]]</sup> ). The direct effects of reduced rainfall and water availability are ''likely'' to be exacerbated by the indirect effects of desertification through a reduction in primary productivity. A reduction in the quality and quantity of resources available to herbivores due to desertification under changing climate can have knock-on consequences for predators and may ultimately disrupt trophic cascades ( ''limited evidence, low agreement'' ) (Rey et al. 2017 <sup>[[#fn:r651|651]]</sup> ; Walther 2010 <sup>[[#fn:r652|652]]</sup> ). Reduced resource availability may also compromise immune response to novel pathogens, with increased pathogen dispersal associated with dust storms (Zinabu et al. 2018 <sup>[[#fn:r653|653]]</sup> ). Responses to desertification are species-specific and mechanistic models are not yet able to accurately predict individual species’ responses to the many factors associated with desertification (Fuller et al. 2016 <sup>[[#fn:r654|654]]</sup> ).

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