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

==== 4.2.3.1 Direct linkages with climate change ====

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The most important direct impacts of climate change on land degradation are the results of increasing temperatures, changing rainfall patterns, and intensification of rainfall. These changes will, in various combinations, cause changes in erosion rates and the processes driving both increases and decreases of soil erosion. From an attribution point of view, it is important to note that projections of precipitation are, in general, more uncertain than projections of temperature changes (Murphy et al. 2004 <sup>[[#fn:r233|233]]</sup> ; Fischer and Knutti 2015 <sup>[[#fn:r234|234]]</sup> ; IPCC 2013a <sup>[[#fn:r235|235]]</sup> ). Precipitation involves local processes of larger complexity than temperature, and projections are usually less robust than those for temperature (Giorgi and Lionello 2008 <sup>[[#fn:r236|236]]</sup> ; Pendergrass 2018 <sup>[[#fn:r237|237]]</sup> ).

Theoretically the intensification of the hydrological cycle as a result of human-induced climate change is well established (Guerreiro et al. 2018 <sup>[[#fn:r238|238]]</sup> ; Trenberth 1999 <sup>[[#fn:r239|239]]</sup> ; Pendergrass et al. 2017 <sup>[[#fn:r240|240]]</sup> ; Pendergrass and Knutti 2018 <sup>[[#fn:r241|241]]</sup> ) and also empirically observed (Blenkinsop et al. 2018 <sup>[[#fn:r242|242]]</sup> ; Burt et al. 2016a <sup>[[#fn:r243|243]]</sup> ; Liu et al. 2009 <sup>[[#fn:r244|244]]</sup> ; Bindoff et al. 2013 <sup>[[#fn:r245|245]]</sup> ). AR5 WGI concluded that heavy precipitation events have increased in frequency, intensity, and/or amount since 1950 ( ''likely'' ) and that further changes in this direction are ''likely'' to very ''likely'' during the 21st century (IPCC 2013 <sup>[[#fn:r246|246]]</sup> ). The IPCC Special Report on 1.5°C concluded that human-induced global warming has already caused an increase in the frequency, intensity and/or amount of heavy precipitation events at the global scale (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r247|247]]</sup> ). As an example, in central India, there has been a threefold increase in widespread extreme rain events during 1950–2015 which has influenced several land degradation processes, not least soil erosion (Burt et al. 2016b <sup>[[#fn:r248|248]]</sup> ). In Europe and North America, where observation networks are dense and extend over a long time, it is ''likely'' that the frequency or intensity of heavy rainfall have increased (IPCC 2013b <sup>[[#fn:r1644|1644]]</sup> ). It is also expected that seasonal shifts and cycles such as monsoons and El Niño–Southern Oscillation (ENSO) will further increase the intensity of rainfall events (IPCC 2013 <sup>[[#fn:r249|249]]</sup> ).

When rainfall regimes change, it is expected to drive changes in vegetation cover and composition, which may be a cause of land degradation in and of itself, as well as impacting on other aspects of land degradation. Vegetation cover, for example, is a key factor in determining soil loss through water (Nearing et al. 2005 <sup>[[#fn:r250|250]]</sup> ) and wind erosion (Shao 2008 <sup>[[#fn:r251|251]]</sup> ). Changing rainfall regimes also affect below-ground biological processes, such as fungi and bacteria (Meisner et al. 2018 <sup>[[#fn:r252|252]]</sup> ; Shuab et al. 2017 <sup>[[#fn:r253|253]]</sup> ; Asmelash et al. 2016 <sup>[[#fn:r254|254]]</sup> ).

Changing snow accumulation and snow melt alter volume and timing of hydrological flows in and from mountain areas (Brahney et al. 2017 <sup>[[#fn:r255|255]]</sup> ; Lutz et al. 2014 <sup>[[#fn:r256|256]]</sup> ), with potentially large impacts on downstream areas. Soil processes are also affected by changing snow conditions with partitioning between evaporation and streamflow and between subsurface flow and surface runoff (Barnhart et al. 2016 <sup>[[#fn:r257|257]]</sup> ). Rainfall intensity is a key climatic driver of soil erosion. Early modelling studies and theory suggest that light rainfall events will decrease while heavy rainfall events increase at about 7% per degree of warming (Liu et al. 2009 <sup>[[#fn:r258|258]]</sup> ; Trenberth 2011 <sup>[[#fn:r259|259]]</sup> ). Such changes result in increased intensity of rainfall, which increases the erosive power of rainfall (erosivity) and hence enhances the likelihood of water erosion. Increases in rainfall intensity can even exceed the rate of increase of atmospheric moisture content (Liu et al. 2009 <sup>[[#fn:r260|260]]</sup> ; Trenberth 2011 <sup>[[#fn:r261|261]]</sup> ). Erosivity is highly correlated to the product of total rainstorm energy and the maximum 30-minute rainfall intensity of the storm (Nearing et al. 2004 <sup>[[#fn:r262|262]]</sup> ) and increased erosivity will exacerbate water erosion substantially (Nearing et al. 2004 <sup>[[#fn:r263|263]]</sup> ). However, the effects will not be uniform, but highly variable across regions (Almagro et al. 2017 <sup>[[#fn:r264|264]]</sup> ; Mondal et al. 2016 <sup>[[#fn:r265|265]]</sup> ). Several empirical studies around the world have shown the increasing intensity of rainfall (IPCC 2013b <sup>[[#fn:r266|266]]</sup> ; Ma et al. 2015 <sup>[[#fn:r267|267]]</sup> , 2017 <sup>[[#fn:r268|268]]</sup> ) and also suggest that this will be accentuated with future increased global warming (Cheng and AghaKouchak 2015 <sup>[[#fn:r269|269]]</sup> ; Burt et al. 2016b <sup>[[#fn:r270|270]]</sup> ; O’Gorman 2015 <sup>[[#fn:r271|271]]</sup> ).

The very comprehensive database of direct measurements of water erosion presented by García-Ruiz et al. (2015) <sup>[[#fn:r272|272]]</sup> contains 4377 entries (North America: 2776, Europe: 847, Asia: 259, Latin America: 237, Africa: 189, Australia and Pacific: 67), even though not all entries are complete (Figure 4.3).

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'''Map of observed soil erosion rates in database of 4,377 entries by García-Ruiz et al. (2015). The map was published by Li and Fang (2016).'''
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Map of observed soil erosion rates in database of 4,377 entries by García-Ruiz et al. (2015) <sup>[[#fn:r1645|1645]]</sup> . The map was published by Li and Fang (2016) <sup>[[#fn:r1646|1646]]</sup> .

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An important finding from that database is that almost any erosion rate is possible under almost any climatic condition (García-Ruiz et al. 2015 <sup>[[#fn:r273|273]]</sup> ). Even if the results show few clear relationships between erosion and land conditions, the authors highlighted four observations (i) the highest erosion rates were found in relation to agricultural activities – even though moderate erosion rates were also found in agricultural settings, (ii) high erosion rates after forest fires were not observed (although the cases were few), (iii) land covered by shrubs showed generally low erosion rates, (iv) pasture land showed generally medium rates of erosion. Some important findings for the link between soil erosion and climate change can be noted from erosion measurements: erosion rates tend to increase with increasing mean annual rainfall, with a peak in the interval of 1000 to 1400 mm annual rainfall (García-Ruiz et al. 2015 <sup>[[#fn:r274|274]]</sup> ) ( ''low confidence'' ). However, such relationships are overshadowed by the fact that most rainfall events do not cause any erosion, instead erosion is caused by a few high-intensity rainfall events (Fischer et al. 2016 <sup>[[#fn:r275|275]]</sup> ; Zhu et al. 2019 <sup>[[#fn:r276|276]]</sup> ). Hence, mean annual rainfall is not a good predictor of erosion (Gonzalez-Hidalgo et al. 2012, 2009 <sup>[[#fn:r277|277]]</sup> ). In the context of climate change, it means that the tendency for rainfall patterns to change towards more intensive precipitation events is serious. Such patterns have already been observed widely, even in cases where the total rainfall is decreasing (Trenberth 2011 <sup>[[#fn:r278|278]]</sup> ). The findings generally confirm the strong consensus about the importance of vegetation cover as a protection against soil erosion, emphasising how extremely important land management is for controlling erosion.

In the Mediterranean region, the observed and expected decrease in annual rainfall due to climate change is accompanied by an increase of rainfall intensity, and hence erosivity (Capolongo et al. 2008 <sup>[[#fn:r279|279]]</sup> ). In tropical and sub-tropical regions, the on-site impacts of soil erosion dominate, and are manifested in very high rates of soil loss, in some cases exceeding 100 t ha–1 yr–1 (Tadesse 2001 <sup>[[#fn:r280|280]]</sup> ; García-Ruiz et al. 2015 <sup>[[#fn:r281|281]]</sup> ). In temperate regions, the off-site costs of soil erosion are often a greater concern, for example, siltation of dams and ponds, downslope damage to property, roads and other infrastructure (Boardman 2010). In cases where water erosion occurs, the downstream effects, such as siltation of dams, are often significant and severe in terms of environmental and economic damages (Kidane and Alemu 2015 <sup>[[#fn:r282|282]]</sup> ; Reinwarth et al. 2019 <sup>[[#fn:r283|283]]</sup> ; Quiñonero-Rubio et al. 2016 <sup>[[#fn:r284|284]]</sup> ; Adeogun et al. 2018 <sup>[[#fn:r285|285]]</sup> ; Ben Slimane et al. 2016 <sup>[[#fn:r286|286]]</sup> ).

The distribution of wet and dry spells also affects land degradation, although uncertainties remain depending on resolution of climate models used for prediction (Kendon et al. 2014 <sup>[[#fn:r287|287]]</sup> ). Changes in timing of rainfall events may have significant impacts on processes of soil erosion through changes in wetting and drying of soils (Lado et al. 2004 <sup>[[#fn:r288|288]]</sup> ).

Soil moisture content is affected by changes in evapotranspiration and evaporation, which may influence the partitioning of water into surface and subsurface runoff (Li and Fang 2016 <sup>[[#fn:r289|289]]</sup> ; Nearing et al. 2004 <sup>[[#fn:r290|290]]</sup> ). This portioning of rainfall can have a decisive effect on erosion (Stocking et al. 2001 <sup>[[#fn:r291|291]]</sup> ).

Wind erosion is a serious problem in agricultural regions, not only in drylands (Wagner 2013 <sup>[[#fn:r292|292]]</sup> ). Near-surface wind speeds over land areas have decreased in recent decades (McVicar and Roderick 2010 <sup>[[#fn:r293|293]]</sup> ), partly as a result of changing surface roughness (Vautard et al. 2010 <sup>[[#fn:r294|294]]</sup> ). Theoretically (Bakun 1990 <sup>[[#fn:r295|295]]</sup> ; Bakun et al. 2015 <sup>[[#fn:r296|296]]</sup> ) and empirically (Sydeman et al. 2014 <sup>[[#fn:r297|297]]</sup> ; England et al. 2014 <sup>[[#fn:r298|298]]</sup> ) average winds along coastal regions worldwide have increased with climate change ( ''medium evidence, high agreement'' ). Other studies of wind and wind erosion have not detected any long-term trend, suggesting that climate change has altered wind patterns outside drylands in a way that can significantly affect the risk of wind erosion (Pryor and Barthelmie 2010 <sup>[[#fn:r299|299]]</sup> ; Bärring et al. 2003 <sup>[[#fn:r300|300]]</sup> ). Therefore, the findings regarding wind erosion and climate change are inconclusive, partly due to inadequate measurements.

Global mean temperatures are rising worldwide, but particularly in the Arctic region ( ''high confidence'' ) (IPCC 2018a <sup>[[#fn:r301|301]]</sup> ). Heat stress from extreme temperatures and heatwaves (multiple days of hot weather in a row) have increased markedly in some locations in the last three decades ( ''high confidence'' ), and are ''virtually certain'' to continue during the 21st century (Olsson et al. 2014a <sup>[[#fn:r302|302]]</sup> ). The IPCC Special Report on Global Warming of 1.5°C concluded that human-induced global warming has already caused more frequent heatwaves in most of land regions, and that climate models project robust differences between present-day and global warming up to 1.5°C and between 1.5°C and 2°C (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r303|303]]</sup> ). Direct temperature effects on soils are of two kinds. Firstly, permafrost thawing leads to soil degradation in boreal and high-altitude regions (Yang et al. 2010 <sup>[[#fn:r304|304]]</sup> ; Jorgenson and Osterkamp 2005 <sup>[[#fn:r305|305]]</sup> ). Secondly, warming alters the cycling of nitrogen and carbon in soils, partly due to impacts on soil microbiota (Solly et al. 2017 <sup>[[#fn:r306|306]]</sup> ). There are many studies with particularly strong experimental evidence, but a full understanding of cause and effect is contextual and elusive (Conant et al. 2011a <sup>[[#fn:r307|307]]</sup> ,b <sup>[[#fn:r308|308]]</sup> ; Wu et al. 2011 <sup>[[#fn:r309|309]]</sup> ). This is discussed comprehensively in Chapter 2.

Climate change, including increasing atmospheric CO <sub>2</sub> levels, affects vegetation structure and function and hence conditions for land degradation. Exactly how vegetation responds to changes remains a research task. In a comparison of seven global vegetation models under four representative concentration pathways, Friend et al. (2014) <sup>[[#fn:r310|310]]</sup> found that all models predicted increasing vegetation carbon storage, however, with substantial variation between models. An important insight compared with previous understanding is that structural dynamics of vegetation seems to play a more important role for carbon storage than vegetation production (Friend et al. 2014 <sup>[[#fn:r311|311]]</sup> ). The magnitude of CO <sub>2</sub> fertilisation of vegetation growth, and hence conditions for land degradation, is still uncertain (Holtum and Winter 2010 <sup>[[#fn:r312|312]]</sup> ), particularly in tropical rainforests (Yang et al. 2016 <sup>[[#fn:r313|313]]</sup> ). For more discussion on this topic, see Chapter 2 in this report.

In summary, rainfall changes attributed to human-induced climate change have already intensified drivers of land degradation ( ''robust evidence, high agreement'' ) but attributing land degradation to climate change is challenging because of the importance of land management ( ''medium evidence, high agreement'' ). Changes in climate variability modes, such as in monsoons and El Niño–Southern Oscillation (ENSO) events, can also affect land degradation ( ''low evidence, low agreement'' ).

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