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

==== 4.3.3.3 Coastal Erosion and Projected Global Impacts of Enhanced Erosion on Human Systems ====

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Recent global assessments of coastal erosion indicate that land losses currently dominate over land gains and that human interventions are a major driver of shoreline changes (Cazenave and Cozannet, 2014 <sup>[[#fn:r1212|1212]]</sup> ; Luijendijk et al., 2018 <sup>[[#fn:r1213|1213]]</sup> ; Mentaschi et al., 2018 <sup>[[#fn:r1214|1214]]</sup> ). Luijendijk et al. (2018) <sup>[[#fn:r1215|1215]]</sup> estimate that over the 1984–2016 period, about a quarter of the world’s sandy beaches eroded at rates exceeding 0.5m yr–1 while about 28% accreted. While such global results can be challenged due to the relatively large detection threshold used (±0.5 m yr–1), there is growing literature indicating that coastal erosion is occurring or increasing, e.g. in the Arctic (Barnhart et al., 2014a <sup>[[#fn:r1216|1216]]</sup> ; Farquharson et al., 2018 <sup>[[#fn:r1217|1217]]</sup> ; Irrgang et al., 2019 <sup>[[#fn:r1218|1218]]</sup> ), Brazil (Amaro et al., 2015 <sup>[[#fn:r1219|1219]]</sup> ), China (Yang et al., 2017 <sup>[[#fn:r1220|1220]]</sup> ), Colombia (Rangel-Buitrago et al., 2015 <sup>[[#fn:r1221|1221]]</sup> ), India (Kankara et al., 2018 <sup>[[#fn:r1222|1222]]</sup> ), and along a large number of deltaic systems worldwide (e.g., Section 4.2.2.4).

Since AR5, however, there is growing appreciation and understanding of the ability of coastal systems to respond dynamically to SLR (Passeri et al., 2015 <sup>[[#fn:r1223|1223]]</sup> ; Lentz et al., 2016 <sup>[[#fn:r1224|1224]]</sup> ; Deng et al., 2017 <sup>[[#fn:r1225|1225]]</sup> ). Most low-lying coastal systems exhibit important feedbacks between biological and physical processes (e.g., Wright and Nichols, 2018), that have allowed them to maintain a relatively stable morphology under moderate rates of SLR (&lt;0.3 cm yr–1) over the past few millennia (Woodruff et al., 2013 <sup>[[#fn:r1226|1226]]</sup> ; Cross-Chapter Box 5 in Chapter 1). In a global review on multi-decadal changes in the land area of 709 atoll islands, Duvat (2019) <sup>[[#fn:r1227|1227]]</sup> shows that in a context of more rapid SLR than the global mean (Becker et al., 2012 <sup>[[#fn:r1228|1228]]</sup> ; Palanisamy et al., 2014 <sup>[[#fn:r1229|1229]]</sup> ), 73.1% of islands were stable in area, while respectively 15.5% and 11.4% increased and decreased in size. While anthropogenic drivers played a major role, especially in urban islands (e.g., shoreline stabilisation by coastal defences, increase in island size as a result of reclamation works), this study and others (e.g., McLean and Kench, 2015) suggest that these islands have had the capacity to maintain their land area by naturally adjusting to SLR over the past decades (high confidence). However, it has been argued that this capacity could be reduced in the coming decades, due to the combination of higher rates of SLR, increased wave energy (Albert et al., 2016 <sup>[[#fn:r1230|1230]]</sup> ), changes in run-up (Shope et al., 2017 <sup>[[#fn:r1231|1231]]</sup> ) and storm wave direction (Harley et al., 2017 <sup>[[#fn:r1232|1232]]</sup> ), effects of ocean warming and acidification on critical ecosystems such as coral reefs (Section 4.3.3.5.2), and a continued increase in anthropogenic pressure.

From a global scale perspective, based on AR4 SLR scenarios and without considering the potential benefits of adaptation, Hinkel et al. (2013b) estimate that about 6000 to 17,000 km2 of land is expected to be lost during the 21st century due to enhanced coastal erosion associated with SLR, in combination with other drivers. This could lead to a displacement of 1.6–5.3 million people and associated cumulative costs of 300 to 1000 billion USD (Section 4.4.3.5). Importantly, these global figures mask the wide diversity of local situations; and some literature is emerging on the non-physical and non-quantifiable impacts of coastal erosion, for example, on the loss of recreational grounds and the induced risks to the associated social dimensions (i.e., how local communities experience coastal erosion impacts; Karlsson et al., 2015 <sup>[[#fn:r1233|1233]]</sup> ).

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