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

== Box 3.4 Warm-Water (Tropical) Coral Reefs in a 1.5°C Warmer World ==

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Warm-water coral reefs face very high risks (Figure 3.18) from climate change. A world in which global warming is restricted to 1.5°C above pre-industrial levels would be a better place for coral reefs than that of a 2°C warmer world, in which coral reefs would mostly disappear (Donner et al., 2005; Hoegh-Guldberg et al., 2014; Schleussner et al., 2016b; van Hooidonk et al., 2016; Frieler et al., 2017; Hughes et al., 2017a) <sup>[[#fn:r704|704]]</sup> . Even with warming up until today (GMST for decade 2006–2015: 0.87°C; Chapter 1), a substantial proportion of coral reefs have experienced large-scale mortalities that have lead to much reduced coral populations (Hoegh-Guldberg et al., 2014) <sup>[[#fn:r705|705]]</sup> . In the last three years alone (2016–2018), large coral reef systems such as the Great Barrier Reef (Australia) have lost as much as 50% of their shallow water corals (Hughes et al., 2017b) <sup>[[#fn:r706|706]]</sup> .

Coral-dominated reefs are found along coastlines between latitudes 30°S and 30°N, where they provide habitat for over a million species (Reaka-Kudla, 1997) <sup>[[#fn:r707|707]]</sup> and food, income, coastal protection, cultural context and many other services for millions of people in tropical coastal areas (Burke et al., 2011; Cinner et al., 2012; Kennedy et al., 2013; Pendleton et al., 2016) <sup>[[#fn:r708|708]]</sup> . Ultimately, coral reefs are underpinned by a mutualistic symbiosis between reef-building corals and dinoflagellates from the genus ''Symbiodinium'' (Hoegh-Guldberg et al., 2017) <sup>[[#fn:r709|709]]</sup> . Warm-water coral reefs are found down to depths of 150 m and are dependent on light, making them distinct from the cold deep-water reef systems that extend down to depths of 2000 m or more. The difficulty in accessing deep-water reefs also means that the literature on the impacts of climate change on these systems is very limited by comparison to those on warm-water coral reefs (Hoegh-Guldberg et al., 2017) <sup>[[#fn:r710|710]]</sup> . Consequently, this Box focuses on the impacts of climate change on warm-water (tropical) coral reefs, particularly with respect to their prospects under average global surface temperatures of 1.5°C and 2°C above the pre-industrial period.

The distribution and abundance of coral reefs has decreased by approximately 50% over the past 30 years (Gardner et al., 2005; Bruno and Selig, 2007; De’ath et al., 2012) <sup>[[#fn:r711|711]]</sup> as a result of pollution, storms, overfishing and unsustainable coastal development (Burke et al., 2011; Halpern et al., 2015; Cheal et al., 2017) <sup>[[#fn:r712|712]]</sup> . More recently, climate change (i.e., heat stress; Hoegh-Guldberg, 1999; Baker et al., 2008; Spalding and Brown, 2015; Hughes et al., 2017b) <sup>[[#fn:r713|713]]</sup> has emerged as the greatest threat to coral reefs, with temperatures of just 1°C above the long-term summer maximum for an area (reference period 1985–1993) over 4–6 weeks being enough to cause mass coral bleaching (loss of the symbionts) and mortality ( ''very high confidence'' ) (WGII AR5, Box 18-2; Cramer et al., 2014) <sup>[[#fn:r714|714]]</sup> . Ocean warming and acidification can also slow growth and calcification, making corals less competitive compared to other benthic organisms such as macroalgae or seaweeds (Dove et al., 2013; Reyes-Nivia et al., 2013, 2014) <sup>[[#fn:r715|715]]</sup> . As corals disappear, so do fish and many other reef-dependent species, which directly impacts industries such as tourism and fisheries, as well as the livelihoods for many, often disadvantaged, coastal people (Wilson et al., 2006; Graham, 2014; Graham et al., 2015; Cinner et al., 2016; <sup>[[#fn:r716|716]]</sup> Pendleton et al., 2016) <sup>[[#fn:r717|717]]</sup> . These impacts are exacerbated by increasingly intense storms (Section 3.3.6), which physically destroy coral communities and hence reefs (Cheal et al., 2017) <sup>[[#fn:r718|718]]</sup> , and by ocean acidification (Sections 3.3.10 and 3.4.4.5), which can weaken coral skeletons, contribute to disease, and slow the recovery of coral communities after mortality events ( ''low to medium confidence'' ) (Gardner et al., 2005; Dove et al., 2013; Kennedy et al., 2013; Webster et al., 2013; Hoegh-Guldberg, 2014b; Anthony, 2016) <sup>[[#fn:r719|719]]</sup> . Ocean acidification also leads to enhanced activity by decalcifying organisms such as excavating sponges (Kline et al., 2012; Dove et al., 2013; Fang et al., 2013, 2014; Reyes-Nivia et al., 2013, 2014) <sup>[[#fn:r720|720]]</sup> .

The predictions of back-to-back bleaching events (Hoegh-Guldberg, 1999) <sup>[[#fn:r721|721]]</sup> have become the reality in the summers of 2016–2017 (e.g., Hughes et al., 2017b) <sup>[[#fn:r722|722]]</sup> , as have projections of declining coral abundance ( ''high confidence'' ). Models have also become increasingly capable and are currently predicting the large-scale loss of coral reefs by mid-century under even low-emissions scenarios (Hoegh-Guldberg, 1999; Donner et al., 2005; Donner, 2009; van Hooidonk and Huber, 2012; Frieler et al., 2013; Hoegh-Guldberg et al., 2014; van Hooidonk et al., 2016) <sup>[[#fn:r723|723]]</sup> . Even achieving emissions reduction targets consistent with the ambitious goal of 1.5°C of global warming under the Paris Agreement will result in the further loss of 70–90% of reef-building corals compared to today, with 99% of corals being lost under warming of 2°C or more above the pre-industrial period (Frieler et al., 2013; Hoegh-Guldberg, 2014b; Hoegh-Guldberg et al., 2014; Schleussner et al., 2016b; Hughes et al., 2017a) <sup>[[#fn:r724|724]]</sup> .

The assumptions underpinning these assessments are considered to be highly conservative. In some cases, ‘optimistic’ assumptions in models include rapid thermal adaptation by corals of 0.2°C–1°C per decade (Donner et al., 2005) <sup>[[#fn:r725|725]]</sup> or 0.4°C per decade (Schleussner et al., 2016b) <sup>[[#fn:r726|726]]</sup> , as well as very rapid recovery rates from impacts (e.g., five years in the case of Schleussner et al., 2016b) <sup>[[#fn:r727|727]]</sup> . Adaptation to climate change at these high rates, has not been documented, and recovery from mass mortality tends to take much longer (&gt;15 years; Baker et al., 2008) <sup>[[#fn:r728|728]]</sup> . Probability analysis also indicates that the underlying increases in sea temperatures that drive coral bleaching and mortality are 25% less ''likely'' under 1.5°C when compared to 2°C (King et al., 2017) <sup>[[#fn:r729|729]]</sup> . Spatial differences between the rates of heating suggest the possibility of temporary climate refugia (Caldeira, 2013; van Hooidonk et al., 2013; Cacciapaglia and van Woesik, 2015; Keppel and Kavousi, 2015) <sup>[[#fn:r730|730]]</sup> , which may play an important role in terms of the regeneration of coral reefs, especially if these refuges are protected from risks unrelated to climate change. Locations at higher latitudes are reporting the arrival of reef-building corals, which may be valuable in terms of the role of limited refugia and coral reef structures but will have low biodiversity ''(high'' ''confidence'' ) when compared to present-day tropical reefs (Kersting et al., 2017) <sup>[[#fn:r731|731]]</sup> . Similarly, deep-water (30–150 m) or mesophotic coral reefs (Bongaerts et al., 2010; Holstein et al., 2016) <sup>[[#fn:r732|732]]</sup> may play an important role because they avoid shallow water extremes (i.e., heat and storms) to some extent, although the ability of these ecosystems to assist in repopulating damaged shallow water areas may be limited (Bongaerts et al., 2017) <sup>[[#fn:r733|733]]</sup> .

Given the sensitivity of corals to heat stress, even short periods of overshoot (i.e., decades) are expected to be extremely damaging to coral reefs. Losing 70–90% of today’s coral reefs, however, will remove resources and increase poverty levels across the world’s tropical coastlines, highlighting the key issue of equity for the millions of people that depend on these valuable ecosystems (Cross-Chapter Box 6; Spalding et al., 2014; Halpern et al., 2015) <sup>[[#fn:r734|734]]</sup> . Anticipating these challenges to food and livelihoods for coastal communities will become increasingly important, as will adaptation options, such as the diversification of livelihoods and the development of new sustainable industries, to reduce the dependency of coastal communities on threatened ecosystems such as coral reefs (Cinner et al., 2012, 2016; Pendleton et al., 2016) <sup>[[#fn:r735|735]]</sup> . At the same time, coastal communities will need to pre-empt changes to other services provided by coral reefs such as coastal protection (Kennedy et al., 2013; Hoegh-Guldberg et al., 2014; Pörtner et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r736|736]]</sup> . Other threats and challenges to coastal living, such as sea level rise, will amplify challenges from declining coral reefs, specially for SIDS and low-lying tropical nations. Given the scale and cost of these interventions, implementing them earlier rather than later would be expedient.

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