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

==== 3.4.2.1 Warm-Water Coral Reefs ====

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Warm-water coral reef ecosystems house one-quarter of the marine biodiversity and provide services in the form of food, income and shoreline protection to coastal communities around the world. These ecosystems are threatened by climate-induced and non-climate drivers, especially ocean warming, MHWs, ocean acidification, SLR, tropical cyclones, fisheries/overharvesting, land-based pollution, disease spread and destructive shoreline practices ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Hughes--2020|Hughes et al., 2020]] ). Warm-water coral reefs face near-term threats to their survival (Table 3.3), but research on observed and projected impacts is very advanced.

'''Table 3.3 |''' Summary of previous IPCC assessments of coral reefs

{| class="wikitable"
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! Observations

! Projections

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| ''AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Wong--2014|Wong et al., 2014]] )''

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| Coral reefs are one of the most vulnerable marine ecosystems ( ''high confidence'' ), and more than half of the world’s reefs are under medium or high risk of degradation.

Mass coral bleaching and mortality, triggered by positive temperature anomalies ( ''high confidence'' ), is the most widespread and conspicuous impact of climate change. Ocean acidification reduces biodiversity and the calcification rate of corals ( ''high confidence'' ) while at the same time increasing the rate of dissolution of the reef framework ( ''medium confidence'' ).

‘In summary, ocean warming is the primary cause of mass coral bleaching and mortality ( ''very high confidence'' ), which, together with ocean acidification, deteriorates the balance between coral reef construction and erosion ( ''high confidence'' ).’

| ‘Coral bleaching and mortality will increase in frequency and magnitude over the next decades ( ''very high confidence'' ).’ Analysis of the Coupled Model Intercomparison Project 5 ensemble projects the loss of coral reefs from most sites globally by 2050 under mid to high rates of warming ( ''very likely'' ).

‘Under the A1B CO 2 emission scenario, 99% of the reef locations will experience at least one severe bleaching event between 2090 and 2099, with ''limited evidence'' and ''low agreement'' that coral acclimation and/or adaptation will limit this trend.’

‘The onset of global dissolution [of coral reefs] is at an atmospheric CO 2 [concentration] of 560 ppm ( ''medium confidence'' ) and dissolution will be widespread in 2100’ (Representative Concentration Pathway, RCP8.5, ''medium confidence'' ).

‘A number of coral reefs could therefore keep up with the maximum rate of sea level rise (SLR) of 15.1 mm yr –1 projected for the end of the century [...], but lower net accretion [...] and increased turbidity will weaken this capability ( ''very high confidence'' ).’

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|
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| ''SR15 ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ; [[#IPCC--2019c|IPCC, 2019c]] )''

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| ‘Climate change [...] 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 [...] and mortality ( ''very high confidence'' ).’

Predictions of back-to-back bleaching events have become reality over 2015–2017 as have projections of declining coral abundance ( ''high confidence'' ).

| ‘Multiple lines of evidence indicate that the majority (70–90%) of warm water (tropical) coral reefs that exist today will disappear even if global warming is constrained to 1.5°C ( ''very high confidence'' ).’

Coral reefs, for example, 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'' ).

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|
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| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

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| ‘New evidence since AR5 and SR15 confirms the impacts of ocean warming and acidification on coral reefs ( ''high confidence'' ), enhancing reef dissolution and bioerosion ( ''high confidence'' ), affecting coral species distribution and leading to community changes ( ''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 ( ''low confidence'' ).’

‘Reefs are further exposed to other increased impacts, such as enhanced storm intensity, turbidity and increased runoff from the land ( ''high confidence'' ). Recovery of coral reefs resulting from repeated disturbance events is slow ( ''high confidence'' ) ''.'' Only few coral reef areas show some resilience to global change drivers ( ''low confidence'' ).’

| ‘Coral reefs will face very high risk at temperatures 1.5°C of global sea surface warming ( ''very high confidence'' ).’

‘Almost all coral reefs will degrade from their current state, even if global warming remains below 2°C ( ''very high confidence'' ), and the remaining shallow coral reef communities will differ in species composition and diversity from present reefs ( ''very high confidence'' ). This will greatly diminish the services they provide to society, such as food provision ( ''high confidence'' ), coastal protection ( ''high confidence'' ) and tourism ( ''medium confidence'' ).’

‘The very high vulnerability of coral reefs to warming, ocean acidification, increasing storm intensity and SLR under climate change, including enhanced bioerosion ( ''high confidence'' ), points to the importance of considering both mitigation and adaptation.’

|}

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[[File:5ffcc230559c2477d3fa6efd4f03d35a IPCC_AR6_WGII_Figure_3_012.png]]

'''Figure 3.12 |''' '''Summary assessment of observed hazards to coastal ecosystems and seas as assessed in Sectio''' '''n 3.''' '''4.2.'''

Global analyses published since AR5 show that mass coral bleaching events and disease outbreaks have increased due to more frequent and severe heat stress associated with ocean warming ( ''very high confidence, virtually certain'' ) ( [[#Donner--2017|Donner et al., 2017]] ; [[#Hughes--2018a|Hughes et al., 2018a]] ; [[#DeCarlo--2019|DeCarlo et al., 2019]] ; [[#Sully--2019|Sully et al., 2019]] ; [[#Tracy--2019|Tracy et al., 2019]] ). The mass coral bleaching, which occurred continuously across different parts of the tropics from 2014 to 2016, is considered the longest and most severe global coral bleaching event on record ( [[IPCC:Wg2:Chapter:Chapter-10#10.4.3|Section 10.4.3]] ; see Box 15.2; [[#Eakin--2019|Eakin et al., 2019]] ). The Great Barrier Reef underwent mass bleaching three times between 2016 and 2020 (see Box 11.2; [[#Pratchett--2021|Pratchett et al., 2021]] ), validating past model projections that some warm-water coral reefs would encounter bleaching-level heat stress multiple times per decade by the 2020s ( [[#Hoegh-Guldberg--1999|Hoegh-Guldberg, 1999]] ; [[#Donner--2009|Donner, 2009]] ).

Heat stress and mass bleaching events caused decreases in live coral cover ( ''virtually certain'' ) ( [[#Graham--2014|Graham et al., 2014]] ; [[#Hughes--2018b|Hughes et al., 2018b]] ), loss of sensitive species ( ''extremely likely'' ) ( [[#Donner--2019|Donner and Carilli, 2019]] ; [[#Lange--2019|Lange and Perry, 2019]] ; [[#Toth--2019|Toth et al., 2019]] ; [[#Courtney--2020|Courtney et al., 2020]] ), vulnerability to disease ( ''extremely likely'' ) ( [[#van%20Woesik--2017|van Woesik and Randall, 2017]] ; [[#Hadaidi--2018|Hadaidi et al., 2018]] ; [[#Brodnicke--2019|Brodnicke et al., 2019]] ; [[#Howells--2020|Howells et al., 2020]] ) and declines in coral recruitment in the tropics ( ''medium confidence'' ) ( [[#Hughes--2019|Hughes et al., 2019]] ; [[#Price--2019|Price et al., 2019]] ). Recent observations also suggest that excess nutrients can increase the susceptibility of corals to heat stress ( [[#DeCarlo--2020|DeCarlo et al., 2020]] ). Changes in coral community structure due to bleaching have caused declines in reef carbonate production ( ''high confidence'' ) ( [[#Perry--2017|Perry and Morgan, 2017]] ; [[#Lange--2019|Lange and Perry, 2019]] ; [[#Perry--2019|Perry and Alvarez-Filip, 2019]] ; [[#Courtney--2020|Courtney et al., 2020]] ; [[#van%20Woesik--2021|van Woesik and Cacciapaglia, 2021]] ) and in reef structural complexity ( ''high confidence, very likely'' ) ( [[#Couch--2017|Couch et al., 2017]] ; [[#Leggat--2019|Leggat et al., 2019]] ; [[#Magel--2019|Magel et al., 2019]] ), which increases water depth, reduces wave attenuation and increases coastal flood risk ( [[#Yates--2017|Yates et al., 2017]] ; [[#Beck--2018|Beck et al., 2018]] ). Corals may also lose reproductive synchrony through climate change ( [[#Shlesinger--2019|Shlesinger and Loya, 2019]] ), adding to their vulnerability. Bleaching and other drivers promote phase shifts to ecosystems dominated by macroalgae or other stress-tolerant species ( ''very high confidence'' ) ( [[#Graham--2015|Graham et al., 2015]] ; Stuart- [[#Smith--2018|Smith et al., 2018]] ), leading to changes in reef-fish species assemblages ( ''high confidence'' ) ( [[#Richardson--2018|Richardson et al., 2018]] ; [[#Robinson--2019a|Robinson et al., 2019a]] ; Stuart- [[#Smith--2021|Smith et al., 2021]] ).

Ocean acidification and associated declines in aragonite saturation state ( Ω aragonite ) decrease rates of calcification by corals and other calcifying reef organisms ( ''very high confidence'' ), reduce coral settlement ( ''medium confidence'' ) and increase bioerosion and dissolution of reef substrates ( ''high confidence'' ) ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Kline--2019|Kline et al., 2019]] ; [[#Pitts--2020|Pitts et al., 2020]] ). Warming can exacerbate the coral response to ocean acidification ( [[#Kornder--2018|Kornder et al., 2018]] ) and accelerate the decrease in coral skeletal density ( [[#Guo--2020|Guo et al., 2020]] ). In addition, reefs with lower coral cover and a higher proportion of slow-growing species, because of bleaching, are more sensitive to acidification (net dissolution occurs Ω aragonite = 2.3 for 100% coral cover, and Ω aragonite &gt;3.5 for 30% coral cover; [[#Kline--2019|Kline et al., 2019]] ). However, experimental evidence suggests that coral responses to ocean acidification are species specific ( ''medium confidence'' ) ( [[#Fabricius--2011|Fabricius et al., 2011]] ; [[#DeCarlo--2018|DeCarlo et al., 2018]] ; [[#Comeau--2019|Comeau et al., 2019]] ). Evidence from experiments suggests that crustose coralline algae, which contribute to reef structure and integrity and may be resistant to warming at the RCP8.5 level by 2100 ( [[#Cornwall--2019|Cornwall et al., 2019]] ), are also sensitive to declines in Ω aragonite ( ''high confidence'' ) ( [[#3.4.2.3|Section 3.4.2.3]] ; [[#Fabricius--2015|Fabricius et al., 2015]] ; [[#Smith--2020|Smith et al., 2020]] ). The integrated effect of acidification, bleaching, storms and other non-climate drivers on corals, coralline algae and other calcifiers can further compromise reef integrity and ecosystem services ( [[#Rivest--2017|Rivest et al., 2017]] ; [[#Cornwall--2018|Cornwall et al., 2018]] ; [[#Perry--2019|Perry and Alvarez-Filip, 2019]] ).

Since SROCC, there have been advances in experimental, field and modelling research on the projected response of coral cover and reef growth to bleaching and ocean acidification ( [[#Cziesielski--2019|Cziesielski et al., 2019]] ; [[#Morikawa--2019|Morikawa and Palumbi, 2019]] ; [[#Cornwall--2021|Cornwall et al., 2021]] ; [[#Klein--2021|Klein et al., 2021]] ; [[#Logan--2021|Logan et al., 2021]] ; [[#McManus--2021|McManus et al., 2021]] ), and on the effect of possible human interventions like assisted evolution on coral resilience ( [[#3.6.3.2.2|Section 3.6.3.2.2]] ; [[#Condie--2021|Condie et al., 2021]] ; [[#Hafezi--2021|Hafezi et al., 2021]] ; [[#Kleypas--2021|Kleypas et al., 2021]] ). New model projections incorporating physiological acclimation, larval dispersal and evolutionary processes find limited ability to adapt this century at rates of warming at or exceeding that in RCP4.5 ( ''high confidence, very likely'' ) ( [[#Bay--2017|Bay et al., 2017]] ; [[#Kubicek--2019|Kubicek et al., 2019]] ; [[#Matz--2020|Matz et al., 2020]] ; [[#McManus--2020|McManus et al., 2020]] ; [[#Logan--2021|Logan et al., 2021]] ; [[#McManus--2021|McManus et al., 2021]] ). For example, a global analysis ( [[#Logan--2021|Logan et al., 2021]] ) finds that increased thermal tolerance via evolution or switching to more stress-tolerant algal symbionts enable most (73–81%) coral to survive through 2100 under RCP2.6, but coral-dominated communities with a historical mix of coral taxa still disappear (0–8% coral survival) under RCP6.0 in simulations with adaptive mechanisms (Figure 3.13). Due to the impacts of warming, and to a lesser extent ocean acidification, global reef carbonate production is estimated to decline 71% by 2050 in SSP1-2.6, and the rate of SLR is estimated to exceed that of reef growth for 97% of reefs assessed, without adaptation by corals and their symbionts (WGI AR6 Table 9.9; [[#Cornwall--2021|Cornwall et al., 2021]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). The increased water depth due to coral loss and reef erosion, as well as reduced structural complexity, will limit wave attenuation and exacerbate the risk of flooding from SLR on reef-fringed shorelines and reef islands ( [[#Yates--2017|Yates et al., 2017]] ; [[#Beck--2018|Beck et al., 2018]] ; [[#Harris--2018|Harris et al., 2018]] ). Local coral reef fish species richness is projected to decline due to the impacts of warming on coral cover and diversity ( ''high confidence'' ), with declines up to 40% by 2060 in SSP5-8.5 ( [[#Strona--2021|Strona et al., 2021]] ).

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[[File:1b1407f3efeeed297db173219f1951da IPCC_AR6_WGII_Figure_3_013.png]]

'''Figure 3.13 |''' '''Coral reef futures, with and without adaptation.''' Graphs are based on a model of coral-symbiont evolutionary dynamics from [[#Logan--2021|Logan et al. (2021)]] , which simulates two coral types and symbiont populations for 1925 reef cells worldwide, from 1950 to 2100 drawn from simulations with National Oceanic and Atmospheric Administration–Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M) under four RCPs. Top panels show the simulated fraction of cells with healthy reefs, when both coral types are not in a state of severe bleaching or mortality, (i) without adaptive responses and (ii) with adaptive responses (symbiont evolution). Colours indicate maximum monthly sea surface temperature increase across all reef cells, versus a 1861–2010 baseline. Panels (a,b,c) depict snapshots of coral reef conditions at time points in the future, each with different levels of warming, drawn from the model-projected cover of the two coral types and from a literature assessment ( [[#3.4.2.1|Section 3.4.2.1]] ; [[#Hughes--2018b|Hughes et al., 2018b]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Darling--2019|Darling et al., 2019]] ; [[#Leggat--2019|Leggat et al., 2019]] ; [[#Cornwall--2021|Cornwall et al., 2021]] ).

These observed and projected impacts are supported by geological and paleo-ecological evidence showing a decline in coral reef extent and species richness under previous episodes of climate change and ocean acidification ( [[#Kiessling--2011|Kiessling and Simpson, 2011]] ; [[#Pandolfi--2011|Pandolfi et al., 2011]] ; [[#Kiessling--2012|Kiessling et al., 2012]] ; [[#Pandolfi--2014|Pandolfi and Kiessling, 2014]] ; [[#Kiessling--2015|Kiessling and Kocsis, 2015]] ). Major reef crises in the past 300 million years were governed by hyperthermal events ( ''medium confidence'' ) ( [[#3.2.4|Section 3.2.4.4]] ; Cross-Chapter Box PALEO in Chapter 1) longer in time scale than anthropogenic climate change, during which net coral reef accretion was more strongly affected than biodiversity ( ''medium confidence'' ).

In response to the global-scale decline in coral reefs and high future risk, recent literature focuses on finding thermal refuges and identifying uniquely resilient species, populations or reefs for targeted restoration and management ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ). Reefs exposed to internal waves ( [[#Storlazzi--2020|Storlazzi et al., 2020]] ), turbidity ( [[#Sully--2020|Sully and van Woesik, 2020]] ) or warm-season cloudiness ( [[#Gonzalez-Espinosa--2021|Gonzalez-Espinosa and Donner, 2021]] ) are expected to be less sensitive to thermal stress. Mesophotic reefs (30–150 m) have also been proposed as thermal refugia ( [[#Bongaerts--2010|Bongaerts et al., 2010]] ), although evidence from recent bleaching events, subsurface temperature records and species overlap is mixed ( [[#Frade--2018|Frade et al., 2018]] ; [[#Rocha--2018b|Rocha et al., 2018b]] ; [[#Eakin--2019|Eakin et al., 2019]] ; [[#Venegas--2019|Venegas et al., 2019]] ; [[#Wyatt--2020|Wyatt et al., 2020]] ). A study of 2584 reef sites across the Indian and Pacific oceans estimated that 17% had sufficient cover of framework-building corals to warrant protection, 54% required recovery efforts and 28% were on a path to net erosion ( [[#Darling--2019|Darling et al., 2019]] ). There is ''medium evidence'' for greater bleaching resistance among reefs subject to temperature variability or frequent heat stress ( [[#Barkley--2018|Barkley et al., 2018]] ; [[#Gintert--2018|Gintert et al., 2018]] ; [[#Hughes--2018a|Hughes et al., 2018a]] ; [[#Morikawa--2019|Morikawa and Palumbi, 2019]] ), but with trade-offs in terms of diversity and structural complexity ( [[#Donner--2019|Donner and Carilli, 2019]] ; [[#Magel--2019|Magel et al., 2019]] ). There is ''limited agreement'' about the persistence of thermal tolerance in response to severe heat stress ( [[#Le%20Nohaïc--2017|Le Nohaïc et al., 2017]] ; [[#DeCarlo--2019|DeCarlo et al., 2019]] ; [[#Fordyce--2019|Fordyce et al., 2019]] ; [[#Leggat--2019|Leggat et al., 2019]] ; [[#Schoepf--2020|Schoepf et al., 2020]] ). Recovery and restoration efforts that target heat-resistant coral populations and culture heat-tolerant algal symbionts have the greatest potential of effectiveness under future warming ( ''high confidence'' ) (see Box 5.5 in SROCC Chapter 5; [[#Bay--2017|Bay et al., 2017]] ; [[#Darling--2018|Darling and Côté, 2018]] ; [[#Baums--2019|Baums et al., 2019]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Howells--2021|Howells et al., 2021]] ); however, there is ''low confidence'' that enhanced thermal tolerance can be sustained over time ( [[#3.6.3.3.2|Section 3.6.3.3.2]] ; [[#Buerger--2020|Buerger et al., 2020]] ). The effectiveness of active restoration and other specific interventions (e.g., reef shading) are further assessed in [[#3.6.3.3.2|Section 3.6.3.3.2]] .

In summary, additional evidence since SROCC and SR15 (Table 3.3) finds that living coral and reef growth are declining due to warming and MHWs ( ''very high confidence'' ). Coral reefs are under threat of transitioning to net erosion with &gt;1.5°C of global warming ( ''high confidence'' ), with impacts expected to occur fastest in the Atlantic Ocean. The effectiveness of conservation efforts to sustain living coral area, coral diversity and reef growth is limited for the majority of the world’s reefs with &gt;1.5°C of global warming ( ''high confidence'' ) ( [[#3.6.3.3.2|Section 3.6.3.3.2]] ; [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ; [[#Bruno--2019|Bruno et al., 2019]] ; [[#Darling--2019|Darling et al., 2019]] ).

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