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

===== 4.3.3.5.2 Coral reefs =====

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Coral reefs are considered to be the marine ecosystem most threatened by climate-related ocean change, especially ocean warming and acidification, even under an RCP2.6 scenario (Gattuso et al., 2015 <sup>[[#fn:r1293|1293]]</sup> ; Albright et al., 2018 <sup>[[#fn:r1294|1294]]</sup> ; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1295|1295]]</sup> ; Díaz et al., 2019 <sup>[[#fn:r1296|1296]]</sup> ; Section 5.3.4). AR5 concluded that ‘a number of coral reefs could […] keep up with the maximum rate of SLR of 15.1 mm yr–1 projected for the end of the century […] (medium confidence) [but a future net accretion rate lower] than during the Holocene (Perry et al., 2013 <sup>[[#fn:r1297|1297]]</sup> ) and increased turbidity (Storlazzi et al., 2011 <sup>[[#fn:r1298|1298]]</sup> ) will weaken this capability (very high confidence)’ (Wong et al., 2014: 379 <sup>[[#fn:r1299|1299]]</sup> ). Subsequently, some studies suggested that SLR may have negligible impacts on coral reefs’ vertical growth because the projected rate and magnitude of SLR by 2100 are within the potential accretion rates of most coral reefs (van Woesik et al., 2015 <sup>[[#fn:r1300|1300]]</sup> ). Other studies, however, stressed that the overall net vertical accretion of reefs may decrease after the first 30 years of rise in a 1.2 m SLR scenario (Hamylton et al., 2014 <sup>[[#fn:r1301|1301]]</sup> ), and that most reefs will not be able to keep up with SLR under RCP4.5 and beyond (Perry et al., 2018 <sup>[[#fn:r1302|1302]]</sup> ). The SR1.5 also concludes that coral reefs ‘are projected to decline by a further 70–90% at 1.5°C (high confidence) with larger losses (&gt;99%) at 2°C (very high confidence)’ (Hoegh-Guldberg et al., 2018: 10 <sup>[[#fn:r1303|1303]]</sup> ). A key point is that SLR will not act in isolation of other drivers. Cumulative impacts, including anthropogenic drivers, are estimated to reduce the ability of coral reefs to keep pace with future SLR (Hughes et al., 2017 <sup>[[#fn:r1304|1304]]</sup> ; Yates et al., 2017 <sup>[[#fn:r1305|1305]]</sup> ) and thereby reduce the capacity of reefs to provide sediments and protection to coastal areas. For example, the combination of reef erosion due to acidification and human-induced mechanical destruction is altering seafloor topography, increasing risks from SLR in carbonate sediment dominated regions (Yates et al., 2017 <sup>[[#fn:r1306|1306]]</sup> ). Both ocean acidification (Albright et al., 2018 <sup>[[#fn:r1307|1307]]</sup> ; Eyre et al., 2018 <sup>[[#fn:r1308|1308]]</sup> ) and ocean warming (Perry and Morgan, 2017 <sup>[[#fn:r1309|1309]]</sup> ) have been considered to slow future growth rates and reef accretion (Section 5.3.4). Recent literature also shows that alterations of coral reef 3D structure from changes in growth, breakage, disease or acidification can profoundly affect their ability to buffer waves impacts (through wave breaking and wave energy damping), and therefore keep-up with SLR (Yates et al., 2017 <sup>[[#fn:r1310|1310]]</sup> ; Harris et al., 2018 <sup>[[#fn:r1311|1311]]</sup> ). Such prospects contribute to raise concerns about the future ability of atoll islands to adjust naturally to SLR and persist (Section 4.3.3.3, Cross-Chapter Box 9). Another concern is that locally, even minimal SLR can increase turbidity on fringing reefs, reducing light and, therefore, photosynthesis and calcification. SLR-induced turbidity can be caused by increased coastal erosion and the transfer of sediment to nearby reefs and enhanced sediment resuspension (Field et al., 2011 <sup>[[#fn:r1312|1312]]</sup> ).

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