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

== 4.4 Projections of land degradation in a changing climate ==

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Land degradation will be affected by climate change in both direct and indirect ways, and land degradation will, to some extent, also feed back into the climate system. The direct impacts are those in which climate and land interact directly in time and space. Examples of direct impacts are when increasing rainfall intensity exacerbates soil erosion, or when prolonged droughts reduce the vegetation cover of the soil, making it more prone to erosion and nutrient depletion. The indirect impacts are those where climate change impacts and land degradation are separated in time and/or space. Examples of such impacts are when declining agricultural productivity due to climate change drives an intensification of agriculture elsewhere, which may cause land degradation. Land degradation, if sufficiently widespread, may also feed back into the climate system by reinforcing ongoing climate change.

Although climate change is exacerbating many land degradation processes ( ''high to very high confidence'' ), prediction of future land degradation is challenging because land management practices determine, to a very large extent, the state of the land. Scenarios of climate change in combination with land degradation models can provide useful knowledge on what kind and extent of land management will be necessary to avoid, reduce and reverse land degradation.

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=== 4.4.1 Direct impacts on land degradation ===

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There are two main levels of uncertainty in assessing the risks of future climate-change-induced land degradation. The first level, where uncertainties are comparatively low, involves changes of the degrading agent, such as erosive power of precipitation, heat stress from increasing temperature extremes (Hüve et al. 2011 <sup>[[#fn:r528|528]]</sup> ), water stress from droughts, and high surface wind speed. The second level of uncertainties, and where the uncertainties are much larger, relates to the above – and below-ground ecological changes as a result of changes in climate, such as rainfall, temperature, and increasing level of CO <sub>2</sub> . Vegetation cover is crucial to protect against erosion (Mullan et al. 2012 <sup>[[#fn:r529|529]]</sup> ; García-Ruiz et al. 2015 <sup>[[#fn:r530|530]]</sup> ).

Changes in rainfall patterns, such as distribution in time and space, and intensification of rainfall events will increase the risk of land degradation, both in terms of likelihood and consequences ( ''high agreement, medium evidence'' ). Climate-induced vegetation changes will increase the risk of land degradation in some areas (where vegetation cover will decline) ( ''medium confidence'' ). Landslides are a form of land degradation, induced by extreme rainfall events. There is a strong theoretical reason for increasing landslide activity due to intensification of rainfall, but so far, the empirical evidence that climate change has contributed to landslides is lacking (Crozier 2010 <sup>[[#fn:r1649|1649]]</sup> ; Huggel et al. 2012 <sup>[[#fn:r532|532]]</sup> ; Gariano and Guzzetti 2016 <sup>[[#fn:r533|533]]</sup> ). Human disturbance may be a more important future trigger than climate change (Froude and Petley 2018 <sup>[[#fn:r534|534]]</sup> ).

Erosion of coastal areas as a result of sea level rise will increase worldwide ( ''very high confidence'' ). In cyclone-prone areas (such as the Caribbean, Southeast Asia, and the Bay of Bengal) the combination of sea level rise and more intense cyclones (Walsh et al. 2016b <sup>[[#fn:r535|535]]</sup> ) and, in some areas, land subsidence (Yang et al. 2019 <sup>[[#fn:r536|536]]</sup> ; Shirzaei and Bürgmann 2018 <sup>[[#fn:r537|537]]</sup> ; Wang et al. 2018 <sup>[[#fn:r538|538]]</sup> ; Fuangswasdi et al. 2019 <sup>[[#fn:r539|539]]</sup> ; Keogh and Törnqvist 2019 <sup>[[#fn:r540|540]]</sup> ), will pose a serious risk to people and livelihoods ( ''very high confidence'' ), in some cases even exceeding limits to adaption (Sections 4.8.4.1, 4.9.6 and 4.9.8).

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<span id="changes-in-water-erosion-risk-due-to-precipitation-changes"></span>
==== 4.4.1.1 Changes in water erosion risk due to precipitation changes ====

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The hydrological cycle is intensifying with increasing warming of the atmosphere. The intensification means that the number of heavy rainfall events is increasing, while the total number of rainfall events tends to decrease (Trenberth 2011 <sup>[[#fn:r541|541]]</sup> ; Li and Fang 2016 <sup>[[#fn:r542|542]]</sup> ; Kendon et al. 2014 <sup>[[#fn:r543|543]]</sup> ; Guerreiro et al. 2018 <sup>[[#fn:r544|544]]</sup> ; Burt et al. 2016a <sup>[[#fn:r545|545]]</sup> ; Westra et al. 2014 <sup>[[#fn:r546|546]]</sup> ; Pendergrass and Knutti 2018 <sup>[[#fn:r547|547]]</sup> ) ( ''robust evidence, high agreement'' ). Modelling of the changes in land degradation that are a result of climate change alone is hard because of the importance of local contextual factors. As shown above, actual erosion rate is extremely dependent on local conditions, primarily vegetation cover and topography (García-Ruiz et al. 2015 <sup>[[#fn:r548|548]]</sup> ). Nevertheless, modelling of soil erosion risks has advanced substantially in recent decades, and such studies are indicative of future changes in the risk of soil erosion, while actual erosion rates will still primarily be determined by land management. In a review article, Li and Fang (2016) <sup>[[#fn:r549|549]]</sup> summarised 205 representative modelling studies around the world where erosion models were used in combination with downscaled climate models to assess future (between 2030 to 2100) erosion rates. The meta-study by Li and Fang, where possible, considered climate change in terms of temperature increase and changing rainfall regimes and their impacts on vegetation and soils. Almost all of the sites had current soil loss rates above 1 t ha–1 (assumed to be the upper limit for acceptable soil erosion in Europe) and 136 out of 205 studies predicted increased soil erosion rates. The percentage increase in erosion rates varied between 1.2% to as much as over 1600%, whereas 49 out of 205 studies projected more than 50% increase. Projected soil erosion rates varied substantially between studies because the important of local factors, hence climate change impacts on soil erosion, should preferably be assessed at the local to regional scale, rather than the global (Li and Fang 2016 <sup>[[#fn:r550|550]]</sup> ).

Mesoscale convective systems (MCS), typically thunder storms, have increased markedly in the last three to four decades in the USA and Australia and they are projected to increase substantially (Prein et al. 2017 <sup>[[#fn:r551|551]]</sup> ). Using a climate model with the ability to represent MCS, Prein and colleagues were able to predict future increases in frequency, intensity and size of such weather systems. Findings include the 30% decrease in number of MCS of &lt;40 mm h <sup>-1</sup> , but a sharp increase of 380% in the number of extreme precipitation events of &gt;90 mm h <sup>–1</sup> over the North American continent. The combined effect of increasing precipitation intensity and increasing size of the weather systems implies that the total amount of precipitation from these weather systems is expected to increase by up to 80% (Prein et al. 2017 <sup>[[#fn:r552|552]]</sup> ), which will substantially increase the risk of land degradation in terms of landslides, extreme erosion events, flashfloods, and so on.

The potential impacts of climate change on soil erosion can be assessed by modelling the projected changes in particular variables of climate change known to cause erosion, such as erosivity of rainfall. A study of the conterminous United States based on three climate models and three scenarios (A2, A1B, and B1) found that rainfall erosivity will increase in all scenarios, even if there are large spatial differences – a strong increase in the north-east and north-west, and either weak or inconsistent trends in the south-west and mid-west (Segura et al. 2014 <sup>[[#fn:r553|553]]</sup> ).

In a study of how climate change will impact on future soil erosion processes in the Himalayas, Gupta and Kumar (2017) <sup>[[#fn:r554|554]]</sup> estimated that soil erosion will increase by about 27% in the near term (2020s) and 22% in the medium term (2080s), with little difference between scenarios. A study from Northern Thailand estimated that erosivity will increase by 5% in the near term (2020s) and 14% in the medium term (2080s), which would result in a similar increase of soil erosion, all other factors being constant (Plangoen and Babel 2014 <sup>[[#fn:r555|555]]</sup> ). Observed rainfall erosivity has increased significantly in the lower Niger Basin (Nigeria) and is predicted to increase further based on statistical downscaling of four General Circulation Models (GCM) scenarios, with an estimated increase of 14%, 19% and 24% for the 2030s, 2050s, and 2070s respectively (Amanambu et al. 2019 <sup>[[#fn:r556|556]]</sup> ).

Many studies from around the world where statistical downscaling of GCM results have been used in combination with process-based erosion models show a consistent trend of increasing soil erosion.

Using a comparative approach, Serpa et al. (2015) <sup>[[#fn:r557|557]]</sup> studied two Mediterranean catchments (one dry and one humid) using a spatially explicit hydrological model – soil and water assessment tool (SWAT) – in combination with land-use and climate scenarios for 2071–2100. Climate change projections showed, on the one hand, decreased rainfall and streamflow for both catchments, whereas sediment export decreased only for the humid catchment; projected land-use change, from traditional to more profitable, on the other hand, resulted in increase in streamflow. The combined effect of climate and land-use change resulted in reduced sediment export for the humid catchment (–29% for A1B; –22% for B1) and increased sediment export for the dry catchment (+222% for A1B; +5% for B1). Similar methods have been used elsewhere, also showing the dominant effect of land-use/land cover for runoff and soil erosion (Neupane and Kumar 2015 <sup>[[#fn:r558|558]]</sup> ).

A study of future erosion rates in Northern Ireland, using a spatially explicit erosion model in combination with downscaled climate projections (with and without sub-daily rainfall intensity changes), showed that erosion rates without land management changes would decrease by the 2020s, 2050s and 2100s, irrespective of changes in intensity, mainly as a result of a general decline in rainfall (Mullan et al. 2012 <sup>[[#fn:r559|559]]</sup> ). When land management scenarios were added to the modelling, the erosion rates started to vary dramatically for all three time periods, ranging from a decrease of 100% for no-till land use, to an increase of 3621% for row crops under annual tillage and sub-days intensity changes (Mullan et al. 2012 <sup>[[#fn:r560|560]]</sup> ). Again, it shows how crucial land management is for addressing soil erosion, and the important role of rainfall intensity changes.

There is a large body of literature based on modelling future land degradation due to soil erosion concluding that, in spite of the increasing trend of erosive power of rainfall, ( ''medium evidence, high agreement'' ) land degradation is primarily determined by land management ( ''very high confidence'' ).

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==== 4.4.1.2 Climate-induced vegetation changes, implications for land degradation ====

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The spatial mosaic of vegetation is determined by three factors: the ability of species to reach a particular location, how species tolerate the environmental conditions at that location (e.g., temperature, precipitation, wind, the topographic and soil conditions), and the interaction between species (including above/below ground species (Settele et al. 2015 <sup>[[#fn:r562|562]]</sup> ). Climate change is projected to alter the conditions and hence impact on the spatial mosaic of vegetation, which can be considered a form of land degradation. Warren et al. (2018) <sup>[[#fn:r563|563]]</sup> estimated that only about 33% of globally important biodiversity conservation areas will remain intact if global mean temperature increases to 4.5°C, while twice that area (67%) will remain intact if warming is restricted to 2°C. According to AR5, the clearest link between climate change and ecosystem change is when temperature is the primary driver, with changes of Arctic tundra as a response to significant warming as the best example (Settele et al. 2015 <sup>[[#fn:r564|564]]</sup> ). Even though distinguishing climate-induced changes from land-use changes is challenging, Boit et al. (2016) <sup>[[#fn:r565|565]]</sup> suggest that 5–6% of biomes in South America will undergo biome shifts until 2100, regardless of scenario, attributed to climate change. The projected biome shifts are primarily forests shifting to shrubland and dry forests becoming fragmented and isolated from more humid forests (Boit et al. 2016 <sup>[[#fn:r566|566]]</sup> ). Boreal forests are subject to unprecedented warming in terms of speed and amplitude (IPCC 2013b <sup>[[#fn:r567|567]]</sup> ), with significant impacts on their regional distribution (Juday et al. 2015 <sup>[[#fn:r568|568]]</sup> ). Globally, tree lines are generally expanding northward and to higher elevations, or remaining stable, while a reduction in tree lines was rarely observed, and only where disturbances occurred (Harsch et al. 2009 <sup>[[#fn:r569|569]]</sup> ). There is limited evidence of a slow northward migration of the boreal forest in eastern North America (Gamache and Payette 2005 <sup>[[#fn:r570|570]]</sup> ). The thawing of permafrost may increase drought-induced tree mortality throughout the circumboreal zone (Gauthier et al. 2015 <sup>[[#fn:r571|571]]</sup> ).

Forests are a prime regulator of hydrological cycling, both fluxes of atmospheric moisture and precipitation, hence climate and forests are inextricably linked (Ellison et al. 2017 <sup>[[#fn:r572|572]]</sup> ; Keys et al. 2017 <sup>[[#fn:r573|573]]</sup> ). Forest management influences the storage and flow of water in forested

watersheds. In particular, harvesting, forest thinning and the construction of roads increase the likelihood of floods as an outcome of extreme climate events (Eisenbies et al. 2007 <sup>[[#fn:r574|574]]</sup> ). Water balance of at least partly forested landscapes is, to a large extent, controlled by forest ecosystems (Sheil and Murdiyarso 2009 <sup>[[#fn:r575|575]]</sup> ; Pokam et al. 2014 <sup>[[#fn:r576|576]]</sup> ). This includes surface runoff, as determined by evaporation and transpiration and soil conditions, and water flow routing (Eisenbies et al. 2007 <sup>[[#fn:r577|577]]</sup> ). Water-use efficiency (i.e., the ratio of water loss to biomass gain) is increasing with increased CO <sub>2</sub> levels (Keenan et al. 2013 <sup>[[#fn:r578|578]]</sup> ), hence transpiration is predicted to decrease which, in turn, will increase surface runoff (Schlesinger and Jasechko 2014 <sup>[[#fn:r579|579]]</sup> ). However, the interaction of several processes makes predictions challenging (Frank et al. 2015 <sup>[[#fn:r580|580]]</sup> ; Trahan and Schubert 2016 <sup>[[#fn:r581|581]]</sup> ). Surface runoff is an important agent in soil erosion.

Generally, removal of trees through harvesting or forest death (Anderegg et al. 2012 <sup>[[#fn:r582|582]]</sup> ) will reduce transpiration and hence increase the runoff during the growing season. Management-induced soil disturbance (such as skid trails and roads) will affect water flow routing to rivers and streams (Zhang et al. 2017 <sup>[[#fn:r583|583]]</sup> ; Luo et al. 2018 <sup>[[#fn:r584|584]]</sup> ; Eisenbies et al. 2007 <sup>[[#fn:r585|585]]</sup> ).

Climate change affects forests in both positive and negative ways (Trumbore et al. 2015 <sup>[[#fn:r586|586]]</sup> ; Price et al. 2013 <sup>[[#fn:r587|587]]</sup> ) and there will be regional and temporal differences in vegetation responses (Hember et al. 2017 <sup>[[#fn:r1650|1650]]</sup> ; Midgley and Bond 2015 <sup>[[#fn:r589|589]]</sup> ). Several climate-change-related drivers interact in complex ways, such as warming, changes in precipitation and water balance, CO <sub>2</sub> fertilisation, and nutrient cycling, which makes projections of future net impacts challenging (Kurz et al. 2013 <sup>[[#fn:r590|590]]</sup> ; Price et al. 2013 <sup>[[#fn:r591|591]]</sup> ) (Section 2.3.1.2). In high latitudes, a warmer climate will extend the growing seasons. However, this could be constrained by summer drought (Holmberg et al. 2019 <sup>[[#fn:r592|592]]</sup> ), while increasing levels of atmospheric CO <sub>2</sub> will increase water-use efficiency but not necessarily tree growth (Giguère-Croteau et al. 2019 <sup>[[#fn:r593|593]]</sup> ). Improving one growth-limiting factor will only enhance tree growth if other factors are not limiting (Norby et al. 2010 <sup>[[#fn:r594|594]]</sup> ; Trahan and Schubert 2016 <sup>[[#fn:r595|595]]</sup> ; Xie et al. 2016 <sup>[[#fn:r596|596]]</sup> ; Frank et al. 2015 <sup>[[#fn:r597|597]]</sup> ). Increasing forest productivity has been observed in most of Fennoscandia (Kauppi et al. 2014 <sup>[[#fn:r598|598]]</sup> ; Henttonen et al. 2017 <sup>[[#fn:r599|599]]</sup> ), Siberia and the northern reaches of North America as a response to a warming trend (Gauthier et al. 2015 <sup>[[#fn:r600|600]]</sup> ) but increased warming may also decrease forest productivity and increase risk of tree mortality and natural disturbances (Price et al. 2013 <sup>[[#fn:r601|601]]</sup> ; Girardin et al. 2016 <sup>[[#fn:r602|602]]</sup> ; Beck et al. 2011 <sup>[[#fn:r603|603]]</sup> ; Hember et al. 2016 <sup>[[#fn:r604|604]]</sup> ; Allen et al. 2011 <sup>[[#fn:r605|605]]</sup> ). The climatic conditions in high latitudes are changing at a magnitude faster than the ability of forests to adapt with detrimental, yet unpredictable, consequences (Gauthier et al. 2015 <sup>[[#fn:r606|606]]</sup> ).

Negative impacts dominate, however, and have already been documented (Lewis et al. 2004 <sup>[[#fn:r607|607]]</sup> ; Bonan et al. 2008 <sup>[[#fn:r608|608]]</sup> ; Beck et al. 2011 <sup>[[#fn:r609|609]]</sup> ) and are predicted to increase (Miles et al. 2004 <sup>[[#fn:r610|610]]</sup> ; Allen et al. 2010 <sup>[[#fn:r611|611]]</sup> ; Gauthier et al. 2015 <sup>[[#fn:r612|612]]</sup> ; Girardin et al. 2016 <sup>[[#fn:r613|613]]</sup> ; Trumbore et al. 2015 <sup>[[#fn:r614|614]]</sup> ). Several authors have emphasised a concern that tree mortality (forest dieback) will increase due to climate-induced physiological stress as well as interactions between physiological stress and other stressors, such as insect pests, diseases, and wildfires (Anderegg et al. 2012 <sup>[[#fn:r615|615]]</sup> ; Sturrock et al. 2011 <sup>[[#fn:r616|616]]</sup> ; Bentz et al. 2010 <sup>[[#fn:r617|617]]</sup> ; McDowell et al. 2011 <sup>[[#fn:r618|618]]</sup> ). Extreme events such as extreme heat and drought, storms, and floods also pose increased threats to forests in both high – and low-latitude forests (Lindner et al. 2010 <sup>[[#fn:r619|619]]</sup> ; Mokria et al. 2015 <sup>[[#fn:r620|620]]</sup> ). However, comparing observed forest dieback with modelled climate-induced damages did not show a general link between climate change and forest dieback (Steinkamp and Hickler 2015 <sup>[[#fn:r621|621]]</sup> ). Forests are subject to increasing frequency and intensity of wildfires which is projected to increase substantially with continued climate change (Price et al. 2013 <sup>[[#fn:r622|622]]</sup> ) (Cross-Chapter Box 3 in Chapter 2, and Chapter 2). In the tropics, interaction between climate change, CO <sub>2</sub> and fire could lead to abrupt shifts between woodland – and grassland-dominated states in the future (Shanahan et al. 2016 <sup>[[#fn:r623|623]]</sup> ).

Within the tropics, much research has been devoted to understanding how climate change may alter regional suitability of various crops. For example, coffee is expected to be highly sensitive to both temperature and precipitation changes, both in terms of growth and yield, and in terms of increasing problems of pests (Ovalle-Rivera et al. 2015 <sup>[[#fn:r624|624]]</sup> ). Some studies conclude that the global area of coffee production will decrease by 50% (Bunn et al. 2015 <sup>[[#fn:r625|625]]</sup> ). Due to increased heat stress, the suitability of Arabica coffee is expected to deteriorate in Mesoamerica, while it can improve in high-altitude areas in South America. The general pattern is that the climatic suitability for Arabica coffee will deteriorate at low altitudes of the tropics as well as at the higher latitudes (Ovalle-Rivera et al. 2015 <sup>[[#fn:r626|626]]</sup> ). This means that climate change in and of itself can render unsustainable previously sustainable land-use and land management practices, and vice versa (Laderach et al. 2011 <sup>[[#fn:r627|627]]</sup> ).

Rangelands are projected to change in complex ways due to climate change. Increasing levels of atmospheric CO <sub>2</sub> directly stimulate plant growth and can potentially compensate for negative effects from drying by increasing rain-use efficiency. But the positive effect of increasing CO <sub>2</sub> will be mediated by other environmental conditions, primarily water availability, but also nutrient cycling, fire regimes and invasive species. Studies over the North American rangelands suggest, for example, that warmer and dryer climatic conditions will reduce NPP in the southern Great Plains, the Southwest, and northern Mexico, but warmer and wetter conditions will increase NPP in the northern Plains and southern Canada (Polley et al. 2013 <sup>[[#fn:r628|628]]</sup> ).

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==== 4.4.1.3 Coastal erosion ====

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Coastal erosion is expected to increase dramatically by sea level rise and, in some areas, in combination with increasing intensity of cyclones (highlighted in Section 4.9.6) and cyclone-induced coastal erosion. Coastal regions are also characterised by high population density, particularly in Asia (Bangladesh, China, India, Indonesia, Vietnam), whereas the highest population increase in coastal regions is projected in Africa (East Africa, Egypt, and West Africa) (Neumann et al. 2015 <sup>[[#fn:r629|629]]</sup> ). For coastal regions worldwide, and particularly in developing countries with high population density in low-lying coastal areas, limiting the warming to 1.5°C to 2.0°C will have major socio-economic benefits compared with higher temperature scenarios (IPCC 2018a <sup>[[#fn:r630|630]]</sup> ; Nicholls et al. 2018 <sup>[[#fn:r631|631]]</sup> ). For more in-depth discussions on coastal process, please refer to Chapter 4 of the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC SROCC).

Despite the uncertainty related to the responses of the large ice sheets of Greenland and west Antarctica, climate-change-induced sea level rise is largely accepted and represents one of the biggest threats faced by coastal communities and ecosystems (Nicholls et al. 2011 <sup>[[#fn:r632|632]]</sup> ; Cazenave and Cozannet 2014 <sup>[[#fn:r633|633]]</sup> ; DeConto and Pollard 2016 <sup>[[#fn:r634|634]]</sup> ; Mengel et al. 2016 <sup>[[#fn:r635|635]]</sup> ). With significant socio-economic effects, the physical impacts of projected sea level rise, notably coastal erosion, have received considerable scientific attention (Nicholls et al. 2011 <sup>[[#fn:r636|636]]</sup> ; Rahmstorf 2010 <sup>[[#fn:r637|637]]</sup> ; Hauer et al. 2016 <sup>[[#fn:r638|638]]</sup> ).

Rates of coastal erosion or recession will increase due to rising sea levels and, in some regions, also in combination with increasing oceans waves (Day and Hodges 2018 <sup>[[#fn:r639|639]]</sup> ; Thomson and Rogers 2014 <sup>[[#fn:r640|640]]</sup> ; McInnes et al. 2011 <sup>[[#fn:r641|641]]</sup> ; Mori et al. 2010 <sup>[[#fn:r642|642]]</sup> ), lack or absence of sea-ice (Savard et al. 2009 <sup>[[#fn:r643|643]]</sup> ; Thomson and Rogers 2014 <sup>[[#fn:r644|644]]</sup> ) thawing of permafrost (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r645|645]]</sup> ), and changing cyclone paths (Tamarin-Brodsky and Kaspi 2017 <sup>[[#fn:r646|646]]</sup> ; Lin and Emanuel 2016a <sup>[[#fn:r647|647]]</sup> ). The respective role of the different climate factors in the coastal erosion process will vary spatially. Some studies have shown that the role of sea level rise on the coastal erosion process can be less important than other climate factors, like wave heights, changes in the frequency of the storms, and the cryogenic processes (Ruggiero 2013 <sup>[[#fn:r648|648]]</sup> ; Savard et al. 2009 <sup>[[#fn:r649|649]]</sup> ). Therefore, in order to have a complete picture of the potential effects of sea level rise on rates of coastal erosion, it is crucial to consider the combined effects of the aforementioned climate controls and the geomorphology of the coast under study.

Coastal wetlands around the world are sensitive to sea level rise. Projections of the impacts on global coastlines are inconclusive, with some projections suggesting that 20% to 90% (depending on sea level rise scenario) of present day wetlands will disappear during the 21st century (Spencer et al. 2016 <sup>[[#fn:r650|650]]</sup> ). Another study, which included natural feedback processes and management responses, suggested that coastal wetlands may actually increase (Schuerch et al. 2018 <sup>[[#fn:r651|651]]</sup> ).

Low-lying coastal areas in the tropics are particularly subject to the combined effect of sea level rise and increasing intensity of tropical cyclones, conditions that, in many cases, pose limits to adaptation (Section 4.8.5.1).

Many large coastal deltas are subject to the additional stress of shrinking deltas as a consequence of the combined effect of reduced sediment loads from rivers due to damming and water use, and land subsidence resulting from extraction of ground water or natural gas, and aquaculture (Higgins et al. 2013 <sup>[[#fn:r652|652]]</sup> ; Tessler et al. 2016 <sup>[[#fn:r653|653]]</sup> ; Minderhoud et al. 2017 <sup>[[#fn:r654|654]]</sup> ; Tessler et al. 2015 <sup>[[#fn:r655|655]]</sup> ; Brown and Nicholls 2015 <sup>[[#fn:r656|656]]</sup> ; Szabo et al. 2016 <sup>[[#fn:r657|657]]</sup> ; Yang et al. 2019 <sup>[[#fn:r658|658]]</sup> ; Shirzaei and Bürgmann 2018 <sup>[[#fn:r659|659]]</sup> ; Wang et al. 2018 <sup>[[#fn:r660|660]]</sup> ; Fuangswasdi et al. 2019 <sup>[[#fn:r661|661]]</sup> ). In some cases the rate of subsidence can outpace the rate of sea level rise by one order of magnitude (Minderhoud et al. 2017 <sup>[[#fn:r662|662]]</sup> ) or even two (Higgins et al. 2013 <sup>[[#fn:r663|663]]</sup> ). Recent findings from the Mississippi Delta raise the risk of a systematic underestimation of the rate of land subsidence in coastal deltas (Keogh and Törnqvist 2019 <sup>[[#fn:r664|664]]</sup> ).

In sum, from a land degradation point of view, low-lying coastal areas are particularly exposed to the nexus of climate change and increasing concentration of people (Elliott et al. 2014 <sup>[[#fn:r665|665]]</sup> ) ( ''robust evidence, high agreement'' ) and the situation will become particularly acute in delta areas shrinking from both reduced sediment loads and land subsidence ( ''robust evidence, high agreement'' ).

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=== 4.4.2 Indirect impacts on land degradation ===

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Indirect impacts of climate change on land degradation are difficult to quantify because of the many conflating factors. The causes of land-use change are complex, combining physical, biological and socio-economic drivers (Lambin et al. 2001 <sup>[[#fn:r666|666]]</sup> ; Lambin and Meyfroidt 2011 <sup>[[#fn:r667|667]]</sup> ). One such driver of land-use change is the degradation of agricultural land, which can result in a negative cycle of natural land being converted to agricultural land to sustain production levels. The intensive management of agricultural land can lead to a loss of soil function, negatively impacting on the many ecosystem services provided by soils, including maintenance of water quality and soil carbon sequestration (Smith et al. 2016a <sup>[[#fn:r668|668]]</sup> ). The degradation of soil quality due to cropping is of particular concern in tropical regions, where it results in a loss of productive potential of the land, affecting regional food security and driving conversion of non-agricultural land, such as forestry, to agriculture (Lambin et al. 2003 <sup>[[#fn:r669|669]]</sup> ; Drescher et al. 2016 <sup>[[#fn:r670|670]]</sup> ; Van der Laan et al. 2017 <sup>[[#fn:r671|671]]</sup> ). Climate change will exacerbate these negative cycles unless sustainable land management practices are implemented.

Climate change impacts on agricultural productivity (see Chapter 5) will have implications for the intensity of land use and hence exacerbate the risk of increasing land degradation. There will be both localised effects (i.e., climate change impacts on productivity affecting land use in the same region) and teleconnections (i.e., climate change impacts and land-use changes that are spatially and temporally separate) (Wicke et al. 2012 <sup>[[#fn:r672|672]]</sup> ; Pielke et al. 2007 <sup>[[#fn:r673|673]]</sup> ). If global temperature increases beyond 3°C it will have negative yield impacts on all crops (Porter et al. 2014 <sup>[[#fn:r674|674]]</sup> ) which, in combination with a doubling of demands by 2050 (Tilman et al. 2011 <sup>[[#fn:r675|675]]</sup> ), and increasing competition for land from the expansion of negative emissions technologies (IPCC 2018a <sup>[[#fn:r676|676]]</sup> ; Schleussner et al. 2016 <sup>[[#fn:r677|677]]</sup> ), will exert strong pressure on agricultural lands and food security.

In sum, reduced productivity of most agricultural crops will drive land-use changes worldwide ( ''robust evidence, medium agreement'' ), but predicting how this will impact on land degradation is challenging because of several conflating factors. Social change, such as widespread changes in dietary preferences, will have a huge impact on agriculture and hence land degradation ( ''medium evidence, high agreement'' ).

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