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

==== 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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