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

===== 4.3.3.5.4 Coastal protection by coastal and marine ecosystems =====

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Major ‘protection’ benefits derived from the above-mentioned coastal ecosystems include wave attenuation and shoreline stabilisation, for example, by coral reefs (Elliff and Silva, 2017 <sup>[[#fn:r1325|1325]]</sup> ; Siegle and Costa, 2017 <sup>[[#fn:r1326|1326]]</sup> ), mangroves (Zhang et al., 2012 <sup>[[#fn:r1327|1327]]</sup> ; Barbier, 2016 <sup>[[#fn:r1328|1328]]</sup> ; Menéndez et al., 2018 <sup>[[#fn:r1329|1329]]</sup> ) or salt marshes (Möller et al., 2014 <sup>[[#fn:r1330|1330]]</sup> ; Hu et al., 2015 <sup>[[#fn:r1331|1331]]</sup> ). Recently, a global meta-analysis of 69 studies demonstrated that, on average, these ecosystems together reduced wave heights between 35–71% at the limited locations considered (Narayan et al., 2016 <sup>[[#fn:r1332|1332]]</sup> ), with coral reefs, salt marshes, mangroves and seagrass/kelp beds reducing wave heights by 54–81%, 62–79%, 25–37% and 25–45% respectively (see Narayan et al., 2016 for map of locations considered). Additional studies suggest greater wave attenuation in mangrove systems (Horstman et al., 2014 <sup>[[#fn:r1333|1333]]</sup> ), and highlight broader complexities in wave attenuation related to total tidal wetland extent, water depth, and species. Global analyses show that natural and artificial seagrasses can attenuate wave height and energy by as much as 40% and 50%, respectively (Fonseca and Cahalan, 1992 <sup>[[#fn:r1334|1334]]</sup> ; John et al., 2015 <sup>[[#fn:r1335|1335]]</sup> ), while coral reefs have been observed to reduce total wave energy by 94–98% (n = 13; Ferrario et al., 2014 <sup>[[#fn:r1336|1336]]</sup> ) and wave driven flooding volume by 72% (Beetham et al., 2017 <sup>[[#fn:r1337|1337]]</sup> ). In addition, storm surge attenuation based on a recent literature review by Stark et al. (2015) <sup>[[#fn:r1338|1338]]</sup> range from -2–25 cm km <sup>–1</sup> length of marsh, where the negative value denotes actual amplification. Other ecosystems provide coastal protection, including macroalgae, oyster and mussel beds, and also beaches, dunes and barrier islands, but there is less understanding of the level of protection conferred by these other organisms and habitats (Spalding et al., 2014 <sup>[[#fn:r1339|1339]]</sup> ).

While there is little literature on the extent to which SLR specifically will affect coastal protection by coastal and marine ecosystems, it is estimated that SLR may reduce this ecosystem service ( ''limited evidence, high agreement'' ) through the above-described impacts on the ecosystems themselves, and in combination with the impacts of other climate-related changes to the ocean (e.g., ocean warming and acidification; Sections 5.3.1 to 5.3.6, 5.4.1). Wave attenuation by coral reefs, for example, is estimated to be negatively affected in the near future by changes in coral reefs’ structural complexity more than by SLR (Harris et al., 2018 <sup>[[#fn:r1340|1340]]</sup> ); changes in mean and ESL events will rather add a layer of stress. Beck et al. (2018) estimate that under RCP8.5 by 2100, a 1 m loss in coral reefs’ height will increase the global area flooded under a 100-year storm event by 116% compared to today, against +66% with no reef loss.

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