Editing IPCC:AR6/SROCC/Chapter-5 (section)

=== 5.3.4 Coral Reefs ===

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Human activities and warming have already led to major impacts on shallow water tropical coral reefs caused by species replacement, bleaching and decreased coral cover while warming, ocean acidification and climate hazards will put warm water corals at very high risk even if global warming can be limited to 1.5°C above pre-industrial level (Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1026|1026]]</sup> ; Kubicek et al., 2019 <sup>[[#fn:r1027|1027]]</sup> ; Sully et al., 2019 <sup>[[#fn:r1028|1028]]</sup> ). While providing new evidence to support these previous assessments (Kleypas, 2019 <sup>[[#fn:r1029|1029]]</sup> ), this assessment focuses on evaluating the variations in sensitivities and responses of coral reefs and their associated biota to highlight comparative risks and resiliences.

New evidence since AR5 and SR15 confirms the impacts of ocean warming (Kao et al., 2018 <sup>[[#fn:r1030|1030]]</sup> ; Jury and Toonen, 2019 <sup>[[#fn:r1031|1031]]</sup> ) and acidification (Jiang et al., 2018 <sup>[[#fn:r1032|1032]]</sup> ; Mollica et al., 2018 <sup>[[#fn:r1033|1033]]</sup> ; Bove et al., 2019 <sup>[[#fn:r1034|1034]]</sup> ) on coral reefs ( ''high confidence'' ), enhancing reef dissolution and bioerosion ( ''high confidence'' ), affecting coral species distribution, and leading to community changes (Agostini et al., 2018 <sup>[[#fn:r1035|1035]]</sup> ) ( ''high confidence'' ). The rate of SLR (primarily noticed in small reef islands) may outpace the growth of reefs to keep up although there is ''low agreement'' in the literature (Brown et al., 2011 <sup>[[#fn:r1036|1036]]</sup> ; Perry et al., 2018 <sup>[[#fn:r1037|1037]]</sup> ) ( ''low confidence'' ). Reefs are further exposed to other increased impacts, such as enhanced storm intensity (Lavender et al., 2018 <sup>[[#fn:r1038|1038]]</sup> ), turbidity and increased runoff from the land (Kleypas, 2019 <sup>[[#fn:r1039|1039]]</sup> ) ( ''high confidence'' ). Recovery of coral reefs resulting from repeated disturbance events is slow (Hughes et al., 2019a <sup>[[#fn:r1040|1040]]</sup> ; Ingeman et al., 2019 <sup>[[#fn:r1041|1041]]</sup> ) ( ''high confidence'' ). Only few coral reef areas show some resilience to global change drivers (Fine et al., 2019 <sup>[[#fn:r1042|1042]]</sup> ) ( ''low confidence'' ).

Globally, coral reefs and their associated communities are projected to change their species composition and biodiversity as a result of future interactions of multiple climatic and non-climatic hazards (Kleypas, 2019 <sup>[[#fn:r1043|1043]]</sup> ; Kubicek et al., 2019 <sup>[[#fn:r1044|1044]]</sup> ; Rinkevich, 2019 <sup>[[#fn:r1045|1045]]</sup> ) ( ''high evidence, very high agreement, very high confidence'' ). Multiple stressors act together to increase the risk of population declines or local extinction of reef-associated species through impacts of warming and ocean acidification on physiology and behaviours (Gunderson et al., 2017 <sup>[[#fn:r1046|1046]]</sup> ) ( ''high confidence'' ). Alteration of composition of coral reef-associated biota is exacerbated by changes in habitat conditions through increased sedimentation and nutrient concentrations from human coastal activities (Fabricius, 2005 <sup>[[#fn:r1047|1047]]</sup> ) ( ''high confidence'' ). Coral ecosystems in tropical small islands are also at high risk of being affected by extreme events, including storms, with their impacts exacerbated by SLR (Duvat et al., 2017 <sup>[[#fn:r1048|1048]]</sup> ; Harborne et al., 2017 <sup>[[#fn:r1049|1049]]</sup> ) ( ''high confidence'' ). Such risks on coral reef associated communities are substantially elevated when the level of these climatic and non-climatic hazards are above thresholds that may cause phase shifts in reef communities (McCook, 1999 <sup>[[#fn:r1050|1050]]</sup> ; Hughes et al., 2010 <sup>[[#fn:r1051|1051]]</sup> ; Graham et al., 2013 <sup>[[#fn:r1052|1052]]</sup> ; Hughes et al., 2018 <sup>[[#fn:r1053|1053]]</sup> ) ( ''high confidence'' ). A phase shift is characterised by an abrupt decrease in coral abundance or cover, with concurrent increase in the dominance of non-reef building organisms, such as algae and soft corals (Kleypas, 2019 <sup>[[#fn:r1054|1054]]</sup> ). Such phase shifts have already been observed in many coral reefs worldwide (Wernberg et al., 2016 <sup>[[#fn:r1055|1055]]</sup> ; Kleypas, 2019 <sup>[[#fn:r1056|1056]]</sup> ).

Notwithstanding the conclusion that coral reefs globally are projected to greatly decline at 2°C warming relative to pre-industrial level (Cacciapaglia and van Woesik, 2018 <sup>[[#fn:r1057|1057]]</sup> ; Dietz et al., 2018 <sup>[[#fn:r1058|1058]]</sup> ; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1059|1059]]</sup> ), climate impacts can be affected by variations in the sensitivity and adaptive capacity across coral species and coral reef ecosystems. Laboratory experiments show that some warm water corals possess the cellular, physiological or molecular machineries that could help them acclimatise or adapt to the effects of global change ( ''medium confidence'' ) (DeBiasse and Kelly, 2016 <sup>[[#fn:r1060|1060]]</sup> ; Gibbin et al., 2017 <sup>[[#fn:r1061|1061]]</sup> ; Wall et al., 2017 <sup>[[#fn:r1062|1062]]</sup> ; Camp et al., 2018 <sup>[[#fn:r1063|1063]]</sup> ; Donelson et al., 2018 <sup>[[#fn:r1064|1064]]</sup> ; Drake et al., 2018 <sup>[[#fn:r1065|1065]]</sup> ; Veilleux and Donelson, 2018 <sup>[[#fn:r1066|1066]]</sup> ; Hughes et al., 2019b <sup>[[#fn:r1067|1067]]</sup> ). For example, there are species or genotypes that show less impacts by either ocean acidification or increased temperatures (Cornwall et al., 2018 <sup>[[#fn:r1068|1068]]</sup> ; Gintert et al., 2018 <sup>[[#fn:r1069|1069]]</sup> ). Some corals and their symbionts might be able to use epigenetic (heritable [https://en.wikipedia.org/wiki/Phenotype phenotype] changes that do not involve alterations in the [https://en.wikipedia.org/wiki/DNA_sequence DNA sequence] s) mechanisms to reduce their sensitivity to temperature changes in their environment and to pass such traits to their offspring (Liew et al., 2017 <sup>[[#fn:r1070|1070]]</sup> ; Torda et al., 2017 <sup>[[#fn:r1071|1071]]</sup> ; Li et al., 2018b <sup>[[#fn:r1072|1072]]</sup> ; Liew et al., 2018 <sup>[[#fn:r1073|1073]]</sup> ). The variations in sensitivity and adaptive capacity of coral species to warming and ocean acidification contribute to changes in species composition of coral reefs as they are exposed to climatic and non-climatic hazards (Ingeman et al., 2019 <sup>[[#fn:r1074|1074]]</sup> ; Kleypas, 2019 <sup>[[#fn:r1075|1075]]</sup> ; Kubicek et al., 2019 <sup>[[#fn:r1076|1076]]</sup> ) ( ''high confidence'' ). However, it has not yet been established whether coral and coral associated biota adaptation may hold beyond 1.5°C warming. The onset of coral bleaching in the last decade has occurred at higher SSTs ( ∼ 0.5°C) than in the previous decade, suggesting that coral populations that remain after preceding bleaching events may have a higher thermal threshold (Sully et al., 2019 <sup>[[#fn:r1077|1077]]</sup> ) ( ''medium confidence'' ), potentially as a result of the increased dominance of species with lower sensitivity or higher adaptive capacity (Schulz et al., 2013 <sup>[[#fn:r1078|1078]]</sup> ; McClanahan et al., 2014 <sup>[[#fn:r1079|1079]]</sup> ; Mumby and van Woesik, 2014 <sup>[[#fn:r1080|1080]]</sup> ; Pandolfi, 2015 <sup>[[#fn:r1081|1081]]</sup> ; Folkersen, 2018 <sup>[[#fn:r1082|1082]]</sup> ) ( ''medium confidence'' ).

Coral reefs in deeper or mesophotic waters (found in tropical/subtropical regions at 30–150 m depth) may serve as refuges and sources for larval supply to those reefs exposed to disturbances (e.g., bleaching, storms, floods from land, sedimentation and tourism impacts) (Bridge et al., 2013 <sup>[[#fn:r1083|1083]]</sup> ; Thomas et al., 2015 <sup>[[#fn:r1084|1084]]</sup> ; Lindfield et al., 2016 <sup>[[#fn:r1085|1085]]</sup> ; Smith et al., 2016b <sup>[[#fn:r1086|1086]]</sup> ; Bongaerts et al., 2017 <sup>[[#fn:r1087|1087]]</sup> ). Reefs exposed to local oceanographic characteristics that reduce warming, such as upwelling, may similarly provide refuges and larval sources (Tkachenko and Soong, 2017 <sup>[[#fn:r1088|1088]]</sup> ). However, recent evidence suggests that mesophotic coral reefs are at higher risk than

previously indicated (Rocha et al., 2018 <sup>[[#fn:r1089|1089]]</sup> ). Monitoring of coral reefs worldwide shows that some areas in the eastern tropical Pacific Ocean (Smith et al., 2017 <sup>[[#fn:r1090|1090]]</sup> ), the Caribbean (Chollett and Mumby, 2013 <sup>[[#fn:r1091|1091]]</sup> ), the Red Sea (Fine et al., 2013 <sup>[[#fn:r1092|1092]]</sup> ; Osman et al., 2017 <sup>[[#fn:r1093|1093]]</sup> ), the Persian Gulf (Coles and Riegl, 2013 <sup>[[#fn:r1094|1094]]</sup> ) and the Great Barrier Reef, Australia (Hughes et al., 2010 <sup>[[#fn:r1095|1095]]</sup> ; Morgan et al., 2017 <sup>[[#fn:r1096|1096]]</sup> ) have recovered more rapidly after bleaching than the larger-scale average ( ''medium confidence'' ). There are regional differences in reef vulnerability when considering scales larger than 100 km or over latitudinal gradients (van Hooidonk et al., 2013 <sup>[[#fn:r1097|1097]]</sup> ; Heron et al., 2016 <sup>[[#fn:r1097|1097]]</sup> ; Langlais et al., 2017 <sup>[[#fn:r1099|1099]]</sup> ; McClenachan et al., 2017 <sup>[[#fn:r1100|1100]]</sup> ) ( ''high confidence'' ).

Based on findings from simulation modelling, SR15 concluded that “coral reefs are projected to decline by a further 70–90% at 1.5°C (very ''high confidence'' ) with larger losses (&gt;99%) at 2°C ( ''very high confidence'' )”. The variations in exposure, sensitivity and adaptive capacity between coral populations and regions are further projected to cause large changes in the composition and structure of the remaining coral reefs, with large regional differences (van Hooidonk et al., 2016 <sup>[[#fn:r1101|1101]]</sup> ; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1102|1102]]</sup> ; Kleypas, 2019 <sup>[[#fn:r1103|1103]]</sup> ; Kubicek et al., 2019 <sup>[[#fn:r1104|1104]]</sup> ; Sully et al., 2019 <sup>[[#fn:r1105|1105]]</sup> ).

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