Editing IPCC:AR6/WGII/Cross-Chapter-Paper-1 (section)

=== CCP1.2.4 Marine ===

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The 43 hotspots in marine ecosystems cover 46,600,000 km 2 , representing 9% of the ocean area (Table CCP1.1; Figure CCP1.2). They include coral reef ecosystems, kelp forests, seagrass meadows, polar and upwelling zones (Figures CCP1.1 3; CCP1.1 4).

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[[File:374dd1148a2f8894380e9bc4486a4168 IPCC_AR6_WGII_Figure_CCP1_014.png]]

'''Figure CCP1.1 4 |''' '''Species in island coral and rocky reef biodiversity hotspots.''' Photos by Galice Hoarau (top four Sulawesi), and Mark Costello (other nine).

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'''Figure CCP1.1 3 |''' '''High-latitude marine biodiversity hotspots.''' Northeast Atlantic temperate seagrass beds, soft-corals, and kelp forests in Norway (photos by Galice Hoarau). South African fynbos and Agulhas current and Antarctic Peninsula and Weddell Sea (photos by Denis Costello). In the Americas, the Humboldt Current Chile and Chesapeake Bay (photos by Mark Costello).

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==== CCP1.2.4.1 Observed Impacts ====

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Observed impacts attributable to climate change are strongly biased geographically, with most data from the temperate Northern Hemisphere, followed by subtropical to temperate Australia and few long-term data in the tropics ( [[#Poloczanska--2013|Poloczanska et al., 2013]] ; [[#Poloczanska--2016|Poloczanska et al., 2016]] ). Marine heatwaves have increased over the past century, causing mass mortalities in the hotspots of the Mediterranean (H216), Great Barrier Reef (H236), western and southern Australia (H227, 228), northwest Atlantic (H207) and northeast Pacific (H197) ( ''high confidence'' ) ( [[#Hobday--2018|Hobday et al., 2018]] ; [[#Oliver--2018|Oliver et al., 2018]] ). The shift of thousands of species from equatorial latitudes since the 1950s has been attributed to climate warming ( ''medium confidence'' ) ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ).

Climate change-related hazards, particularly marine heat events, have caused widespread coral bleaching and mass mortalities as the time between consecutive bleaching events decreases ( ''high confidence'' ) ( [[#IPCC--2018|IPCC, 2018]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Coral reefs in some Indian Ocean hotspots (H230, 234) already exhibit net loss of coral reefs ( ''low confidence'' ) ( [[#Perry--2018|Perry et al., 2018]] ). While coral bleaching is a visible symptom of heat stress, warming has also induced restructuring of associated fish and invertebrate communities in the Great Barrier Reef (H236) ( ''medium confidence'' ) ( [[#Stuart-Smith--2018|Stuart-Smith et al., 2018]] ).

Although the number of coral species that are both exposed and vulnerable to climate hazards is greatest in the central Indo-Pacific, the proportion of corals at risk is greater in the lower diversity Caribbean hotspots (H209) ( ''medium confidence'' ) ( [[#Foden--2013|Foden et al., 2013]] ). Some reef corals are able to acclimate to heatwaves ( ''low confidence'' ) ( [[#DeCarlo--2019|DeCarlo et al., 2019]] ), and some have expanded their latitudinal ranges polewards ( ''high confidence'' ), up to 14 km yr –1 in the northwest Pacific ( [[#Yamano--2011|Yamano et al., 2011]] ). Although future latitudinal expansions may be limited by winter light availability ( [[#Muir--2015|Muir et al., 2015]] ), new coral reefs are already emerging in Japan ( [[#Kumagai--2018|Kumagai et al., 2018]] ).

The Mediterranean Sea hotspot (H216) is negatively affected by climate change ( ''high confidence'' ) (Cross-Chapter Paper 4). Species entering via the Suez Canal from the Red Sea (H220) are facilitated by warming and lead to profound community changes ( ''high confidence'' ) ( [[#Yeruham--2015|Yeruham et al., 2015]] ; [[#Rilov--2016|Rilov, 2016]] ; [[#Vasilakopoulos--2017|Vasilakopoulos et al., 2017]] ; [[#Givan--2018|Givan et al., 2018]] ; [[#Bianchi--2019|Bianchi et al., 2019]] ). In contrast, the more open coastal seas of the Atlantic and Pacific coasts of North America have had increasing species richness since the 1970s ( [[#Batt--2017|Batt et al., 2017]] ).

Kelp forests are in decline in mid-latitudes due to warming and associated increased herbivory ( ''medium confidence'' ) ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.3|Section 3.4.2.3]] , Chapter 11). South and southeastern (H228), and southwestern (H227) Australia have experienced a climate-related decline of kelp forests ( [[#Wernberg--2011|Wernberg et al., 2011]] ; [[#Vergés--2016|Vergés et al., 2016]] ; [[#Wernberg--2016|Wernberg et al., 2016]] ). West Australia (H227) has been affected by extreme climate events characterised by the replacement of kelp and sessile invertebrates by algal turfs and warm-water fish species ( [[#Wernberg--2013|Wernberg et al., 2013]] ; [[#Wernberg--2016|Wernberg et al., 2016]] ). Australia’s Great Barrier Reef (H236), kelp forests, seagrass meadows and mangroves (due to drought), have suffered mortalities due to climate change ( ''medium confidence'' ) ( [[#Babcock--2019|Babcock et al., 2019]] ). Climate warming driven changes in seaweed assemblages have been reported not only in Australia, but in the marine biodiversity hotspots of Atlantic Canada, Japan, Mediterranean, New Zealand ( [[#Laffoley--2016|Laffoley and Baxter, 2016]] ; [[#Thomsen--2019|Thomsen et al., 2019]] ; [[#Thomsen--2019|Thomsen and South, 2019]] ) and California (H207, 231, 216, 238, 199) ( [[#Arafeh-Dalmau--2019|Arafeh-Dalmau et al., 2019]] ; [[#McPherson--2021|McPherson et al., 2021]] ). However, while climate change is having measurable effects on kelp, the dominant effects on kelp projected to 2025 are fishing, through its effects on herbivores and predators ( ''medium confidence'' ) ( [[#Steneck--2002|Steneck et al., 2002]] ). Although fishing affected Atlantic cod in the Barents Sea (H214) and Gulf of Maine (H207) biodiversity hotspots, it was also affected by climate change, but negatively and positively, respectively ( [[#Kjesbu--2014|Kjesbu et al., 2014]] ; [[#Pershing--2015|Pershing et al., 2015]] ).

Range expansions out of the Nansei Shoto (H231) hotspot south of Japan has led to the replacement of temperate kelp forests by tropical coral and herbivorous fishes on Japanese coasts ( [[#Kumagai--2018|Kumagai et al., 2018]] ). The Yellow Sea (H230) is one of the most exploited marine hotspots, with decreasing ecosystem services compounded by climate change but there is ''low confidence'' for climate change contributing substantially to ecological degradation ( [[#Wang--2016|Wang et al., 2016]] ; [[#Song--2019|Song and Duan, 2019]] ).

Upwelling systems are best known for bringing nutrients to the surface. These stimulate phytoplankton blooms, which in turn support important fisheries ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.1|Section 3.4.2.1]] 1). However, this deep water also tends to be low in oxygen, which can be further depleted by respiration and surface warming. Prolonged marine heatwaves in the Californian Current hotspot (H197) drove major shifts in the geographic range of birds, mammals, fish, crustaceans, molluscs and other species, and toxic algal blooms ( [[#Sanford--2019|Sanford et al., 2019]] ).

In both the Antarctic (H213) and Arctic (H196, 214), the loss of ice impacts on the behaviour and foraging ability of marine mammals and birds ( [[#Doney--2012|Doney et al., 2012]] ). The retreat of sea ice in the Bering Sea (H196) hotspot has been followed by a reorganisation of the seabed and fish communities, a northward shift in species, and greater species’ biomass and richness ( [[#Mueter--2008|Mueter and Litzow, 2008]] ; [[#Grebmeier--2018|Grebmeier et al., 2018]] ). In the Eurasian Arctic (H214), species richness has similarly been increasing ( [[#Węsławski--2011|Węsławski et al., 2011]] ; [[#Kortsch--2012|Kortsch et al., 2012]] ; [[#Certain--2015|Certain and Planque, 2015]] ; [[#Fossheim--2015|Fossheim et al., 2015]] ; [[#Węsławski--2018|Węsławski et al., 2018]] ), as has phytoplankton productivity ( [[#Arrigo--2008|Arrigo et al., 2008]] ). The distribution of krill has already contracted with ocean warming in the Southern Ocean ( ''medium confidence'' ) ( [[#Cox--2018|Cox et al., 2018]] ; [[#Atkinson--2019|Atkinson et al., 2019]] ).

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==== CCP1.2.4.2 Projected Impacts ====

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Tropical extirpations, already underway ( [[IPCC:Wg2:Chapter:Chapter-1#1.2|Section 1.2.4.1]] ), are projected to reduce hotspot diversity especially in the Coral Triangle (H226, 232, 234), Maldives (H224) and, to a lesser extent, in the Caribbean (H200, H210) ( [[#Jones--2015|Jones and Cheung, 2015]] ; [[#García%20Molinos--2016|García Molinos et al., 2016]] ) and Persian Gulf (H219) ( [[#Wabnitz--2018|Wabnitz et al., 2018]] ). Paleo evidence supports projections of tropical biodiversity loss under high global warming ( ''high confidence'' ) ( [[#Kiessling--2012|Kiessling et al., 2012]] ; [[#Yasuhara--2020|Yasuhara et al., 2020]] ).

Warm-water coral reefs are expected to decline with 1.5°C warming ''(very high confidence'' ) ( [[#King--2017|King et al., 2017]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ) leading to systems with reduced biodiversity and structural complexity ( ''high confidence'' ) (Chapters 3; 11; Box 11.2). In the Coral Triangle, marine heatwaves are projected to have the same effect as an added mean annual 0.5°C sea surface temperature increase ( [[#McManus--2020|McManus et al., 2020]] ). While some corals are expected to survive in deep ‘mesophotic’ reefs ( [[#Laverick--2019|Laverick and Rogers, 2019]] ), the shallow coral reefs of today will not last the century if climate warming continues without mitigation ( ''high confidence'' ) ( [[#Hughes--2018a|Hughes et al., 2018a]] ; [[#Hughes--2018b|Hughes et al., 2018b]] ; [[#IPCC--2018|IPCC, 2018]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Hughes--2019b|Hughes et al., 2019b]] ).

In the Mediterranean, ocean acidification has been projected to lead to increases of fleshy algae at the expense of calcifying algae ( [[#Zunino--2017|Zunino et al., 2017]] ). However, seagrass has been projected to decline ( [[#Chefaoui--2018|Chefaoui et al., 2018]] ) and increase ( [[#Zunino--2017|Zunino et al., 2017]] ) in the Mediterranean Sea hotspot (H216). Kelp forests are expected to decline in the northwest Atlantic (Grand Banks, H207), whereas gains and losses are projected to be approximately balanced in the Northeast Atlantic Shelf (H215) under Representative Concentration Pathway (RCP) 8.5 ( [[#Assis--2018|Assis et al., 2018]] ; [[#Wilson--2019|Wilson et al., 2019]] ), but may lead to impoverished benthic assemblages ( [[#Teagle--2018|Teagle and Smale, 2018]] ).

Projected climate caused changes in biodiversity in coastal upwelling regions are uncertain. While productivity in the California Current (H197) system is projected to increase with future climate change, nonlinear plankton responses and uncertain interactions with food web dynamics hinder predictions of ecosystem responses ( [[#Xiu--2018|Xiu et al., 2018]] ). In addition, this hotspot is projected to suffer from ocean acidification by 2050 ( [[#Gruber--2012|Gruber et al., 2012]] ).

Around Antarctica (H213), almost half of all species are endemic ( [[#Costello--2010|Costello et al., 2010]] ), and warming during this century is projected to cause a reduction in suitable thermal environment for 79% of its species (RCP8.5) ( ''low confidence'' ) ( [[#Basher--2016|Basher and Costello, 2016]] ; [[#Griffiths--2017|Griffiths et al., 2017]] ). The previously mentioned declines in Southern Ocean krill due to climate change contribute to projected declines in baleen whales there ( [[#Tulloch--2019|Tulloch et al., 2019]] ).

Species richness in the northern polar hotspots is expected to increase substantially ( ''high confidence'' ) ( [[#Cheung--2015|Cheung et al., 2015]] ). However, population sizes of presently occurring native species are expected to decline, especially in the Barents Sea (H214) ( [[#Koenigstein--2018|Koenigstein et al., 2018]] ). Ocean acidification is projected to continue globally, and while its impact is uncertain and projected to be less than the effect of warming, it may lead to changes in marine food webs due to varying effects on marine species ( [[#Terhaar--2020|Terhaar et al., 2020]] ). Hotspots in temperate latitudes are projected to have assemblages modified by immigration from the tropics and emigration to polar waters. Where land barriers and other geographical limits to range shifts occur, limited dispersal and habitat fragmentation may also limit the capacity of some species to track climate velocities, such as in the Baltic Sea (H215) ( [[#Jonsson--2018|Jonsson et al., 2018]] ), Mediterranean Sea (H216) ( [[#Burrows--2014|Burrows et al., 2014]] ; [[#Arafeh-Dalmau--2021|Arafeh-Dalmau et al., 2021]] ) and Antarctica (H213) ( ''medium confidence'' ) ( [[#Cristofari--2018|Cristofari et al., 2018]] ).

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