Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.4.10 Framework organisms (tropical corals, mangroves and seagrass) ====

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Marine organisms (‘ecosystem engineers’), such as seagrass, kelp, oysters, salt marsh species, mangroves and corals, build physical structures or frameworks (i.e., sea grass meadows, kelp forests, oyster reefs, salt marshes, mangrove forests and coral reefs) which form the habitat for a large number of species (Gutiérrez et al., 2012) <sup>[[#fn:r640|640]]</sup> . These organisms in turn provide food, livelihoods, cultural significance, and services such as coastal protection to human communities (Bell et al., 2011, 2018; Cinner et al., 2012; Arkema et al., 2013; Nurse et al., 2014; Wong et al., 2014; Barbier, 2015; Bell and Taylor, 2015; Hoegh-Guldberg et al., 2015; Mycoo, 2017; Pecl et al., 2017) <sup>[[#fn:r641|641]]</sup> .

Risks of climate change impacts for seagrass and mangrove ecosystems were recently assessed by an expert group led by Short et al. (2016) <sup>[[#fn:r642|642]]</sup> . Impacts of climate change were assessed to be similar across a range of submerged and emerged plants. Submerged plants such as sea-grass were affected mostly by temperature extremes (Arias-Ortiz et al., 2018) <sup>[[#fn:r643|643]]</sup> , and indirectly by turbidity, while emergent communities such as mangroves and salt marshes were most susceptible to sea level variability and temperature extremes, which is consistent with other evidence (Di Nitto et al., 2014; Sierra-Correa and Cantera Kintz, 2015; Osorio et al., 2016; Sasmito et al., 2016) <sup>[[#fn:r644|644]]</sup> , especially in the context of human activities that reduce sediment supply (Lovelock et al., 2015) <sup>[[#fn:r645|645]]</sup> or interrupt the shoreward movement of mangroves though the construction of coastal infrastructure. This in turn leads to ‘coastal squeeze’ where coastal ecosystems are trapped between changing ocean conditions and coastal infrastructure (Mills et al., 2016) <sup>[[#fn:r646|646]]</sup> . Projections of the future distribution of seagrasses suggest a poleward shift, which raises concerns that low-latitude seagrass communities may contract as a result of increasing stress levels (Valle et al., 2014) <sup>[[#fn:r647|647]]</sup> .

Climate change (e.g., sea level rise, heat stress, storms) presents risk for coastal ecosystems such as seagrass ( ''high confidence'' ) and reef-building corals ( ''very high confidence'' ) (Figure 3.18, Supplementary Material 3.SM.3.2), with evidence of increasing concern since AR5 and the conclusion that tropical corals may be even more vulnerable to climate change than indicated in assessments made in 2014 (Hoegh-Guldberg et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r648|648]]</sup> . The current assessment also considered the heatwave-related loss of 50% of shallow-water corals across hundreds of kilometres of the world’s largest continuous coral reef system, the Great Barrier Reef. These large-scale impacts, plus the observation of back-to-back bleaching events on the Great Barrier Reef (predicted two decades ago, Hoegh-Guldberg, 1999) <sup>[[#fn:r649|649]]</sup> and arriving sooner than predicted (Hughes et al., 2017b, 2018) <sup>[[#fn:r650|650]]</sup> , suggest that the research community may have underestimated climate risks for coral reefs (Figure 3.18). The general assessment of climate risks for mangroves prior to this special report was that they face greater risks from deforestation and unsustainable coastal development than from climate change (Alongi, 2008; Hoegh-Guldberg et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r651|651]]</sup> . Recent large-scale die-offs (Duke et al., 2017; Lovelock et al., 2017) <sup>[[#fn:r652|652]]</sup> , however, suggest that risks from climate change may have been underestimated for mangroves as well. With the events of the last past three years in mind, risks are now considered to be undetectable to moderate (i.e., moderate risks now start at 1.3°C as opposed to 1.8°C; ''medium confidence'' ). Consequently, when average global warming reaches 1.3°C above pre-industrial levels, the risk of climate change to mangroves are projected to be ''moderate'' (Figure 3.18) while tropical coral reefs will have reached a high level of risk as examplified by increasing damage from heat stress since the early 1980s. At global warming of 1.8°C above pre-industrial levels, seagrasses are projected to reach moderate to high levels of risk (e.g., damage resulting from sea level rise, erosion, extreme temperatures, and storms), while risks to mangroves from climate change are projected to remain moderate (e.g., not keeping up with sea level rise, and more frequent heat stress mortality) although there is ''low certainty'' as to when or if this important ecosystem is ''likely'' to transition to higher levels of additional risk from climate change (Figure 3.18).

Warm water (tropical) coral reefs are projected to reach a very high risk of impact at 1.2°C (Figure 3.18), with most available evidence suggesting that coral-dominated ecosystems will be non-existent at this temperature or higher ( ''high confidence'' ). At this point, coral abundance will be near zero at many locations and storms will contribute to ‘flattening’ the three-dimensional structure of reefs without recovery, as already observed for some coral reefs (Alvarez-Filip et al., 2009) <sup>[[#fn:r653|653]]</sup> . The impacts of warming, coupled with ocean acidification, are expected to undermine the ability of tropical coral reefs to provide habitat for thousand of species, which together provide a range of ecosystem services (e.g., food, livelihoods, coastal protection, cultural services) that are important for millions of people ( ''high confidence'' ) (Burke et al., 2011) <sup>[[#fn:r654|654]]</sup> .

Strategies for reducing the impact of climate change on framework organisms include reducing stresses not directly related to climate change (e.g., coastal pollution, overfishing and destructive coastal development) in order to increase their ecological resilience in the face of accelerating climate change impacts (World Bank, 2013; Ellison, 2014; Anthony et al., 2015; Sierra-Correa and Cantera Kintz, 2015; Kroon et al., 2016; O’Leary et al., 2017) <sup>[[#fn:r655|655]]</sup> , as well as protecting locations where organisms may be more robust (Palumbi et al., 2014) <sup>[[#fn:r656|656]]</sup> or less exposed to climate change (Bongaerts et al., 2010; van Hooidonk et al., 2013; Beyer et al., 2018) <sup>[[#fn:r657|657]]</sup> . This might involve cooler areas due to upwelling, or involve deep-water locations that experience less extreme conditions and impacts. Given the potential value of such locations for promoting the survival of coral communities under climate change, efforts to prevent their loss resulting from other stresses are important (Bongaerts et al., 2010, 2017; Chollett et al., 2010, 2014; Chollett and Mumby, 2013; Fine et al., 2013; van Hooidonk et al., 2013; Cacciapaglia and van Woesik, 2015; Beyer et al., 2018) <sup>[[#fn:r658|658]]</sup> . A full understanding of the role of refugia in reducing the loss of ecosystems has yet to be developed ( ''low to medium confidence'' ). There is also interest in ''ex situ'' conservation approaches involving the restoration of corals via aquaculture (Shafir et al., 2006; Rinkevich, 2014) <sup>[[#fn:r659|659]]</sup> or the use of ‘assisted evolution’ to help corals adapt to changing sea temperatures (van Oppen et al., 2015, 2017) <sup>[[#fn:r660|660]]</sup> , although there are numerous challenges that must be surpassed if these approaches are to be cost-effective responses to preserving coral reefs under rapid climate change ( ''low confidence'' ) (Hoegh-Guldberg, 2012, 2014a; Bayraktarov et al., 2016) <sup>[[#fn:r661|661]]</sup> .

High levels of adaptation are expected to be required to prevent impacts on food security and livelihoods in coastal populations ( ''medium confidence'' ). Integrating coastal infrastructure with changing ecosystems such as mangroves, seagrasses and salt marsh, may offer adaptation strategies as they shift shoreward as sea levels rise ( ''high confidence'' ). Maintaining the sediment supply to coastal areas would also assist mangroves in keeping pace with sea level rise (Shearman et al., 2013; Lovelock et al., 2015; Sasmito et al., 2016) <sup>[[#fn:r662|662]]</sup> . For this reason, habitat for mangroves can be strongly affected by human actions such as building dams which reduce the sediment supply and hence the ability of mangroves to escape ‘drowning’ as sea level rises (Lovelock et al., 2015) <sup>[[#fn:r663|663]]</sup> . In addition, integrated coastal zone management should recognize the importance and economic expediency of using natural ecosystems such as mangroves and tropical coral reefs to protect coastal human communities (Arkema et al., 2013; Temmerman et al., 2013; Ferrario et al., 2014; Hinkel et al., 2014; Elliff and Silva, 2017) <sup>[[#fn:r664|664]]</sup> . Adaptation options include developing alternative livelihoods and food sources, ecosystem-based management/adaptation such as ecosystem restoration, and constructing coastal infrastructure that reduces the impacts of rising seas and intensifying storms (Rinkevich, 2015; Weatherdon et al., 2016; Asiedu et al., 2017a; Feller et al., 2017) <sup>[[#fn:r665|665]]</sup> . Clearly, these options need to be carefully assessed in terms of feasibility, cost and scalability, as well as in the light of the coastal ecosystems involved (Bayraktarov et al., 2016) <sup>[[#fn:r666|666]]</sup> .

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