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

=== 3.4.4 Reversibility and Impacts of Temporary Overshoot of 1.5°C or 2°C Warming ===

<div id="h2-13-siblings" class="h2-siblings"></div>

Scenarios limiting warming to the 1.5°C and 2°C limits in the Paris Agreement can involve temporarily exceeding those warming levels before declining again (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-4#4.6.2|Section 4.6.2.1]] ; [[#Lee--2021|Lee et al., 2021]] ). The effect of such ‘overshoot’ on marine and coastal ecosystems depends on the reversibility of both the response of climate-induced drivers and the response of organisms and ecosystems to the climate impact-drivers during the overshoot period. WGI AR6 assessed that temporary overshoot of a 2°C warming threshold has irreversible effects on global mean sea level and also effects on ocean heat content that persist beyond 2100 (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-4#4.6.2|Section 4.6.2.1]] ; [[#Lee--2021|Lee et al., 2021]] ). Model results indicate that sea surface temperatures ( ''high confidence'' ), Arctic sea ice ( ''high confidence'' ), surface ocean acidification ( ''very high confidence'' ) and surface ocean deoxygenation ( ''very high confidence'' ) are reversible within years to decades if net emissions reach zero or below (WGI AR6 Table 4.10; [[#Lee--2021|Lee et al., 2021]] ). Although changes in these surface ocean variables are reversible, habitat-forming ecosystems, including coral reefs and kelp forests, may undergo irreversible phase shifts with &gt;1.5°C warming (Sections 3.4.2.1, 3.4.2.3), and are thus at high risk this century in 1.5°C or 2°C scenarios involving overshoot ( [[#Tachiiri--2019|Tachiiri et al., 2019]] ). In an overshoot scenario in which CO 2 returns to 2040 levels by 2100 (SSP5-3.4-OS; [[#O’Neill--2016|O’Neill et al., 2016]] ), SST and Arctic sea ice do not fully return by 2100 to levels prior to the CO 2 peak ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-4#4.6.2|Section 4.6.2.1]] ; [[#Lee--2021|Lee et al., 2021]] ), suggesting that reversal of marine ecological impacts from 21st century climate impacts would extend into the 22nd century or beyond ( [[#McManus--2021|McManus et al., 2021]] ). Models also indicate that global sea level rise, as well as warming, ocean acidification and deoxygenation at depth, are irreversible for centuries or longer ( ''very high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-4#4.6.2|Section 4.6.2.1]] and Table 4.10; [[#Palter--2018|Palter et al., 2018]] ; [[#Li--2020c|Li et al., 2020c]] ; [[#Lee--2021|Lee et al., 2021]] ).

<div id="box-3.3" class="h2-container box-container"></div>

'''Box 3.3 | Deep-Sea Ecosystems'''
<div id="h2-30-siblings" class="h2-siblings"></div>

Deep-sea ecosystems include all waters below the 200-m isobath as well as the underlying benthos, and they provide habitats for highly diversified and specialised biota, which play a key role in the cycling of carbon and other nutrients (see Figure Box 3.3.1; [[#Thurber--2014|Thurber et al., 2014]] ; [[#Middelburg--2018|Middelburg, 2018]] ; [[#Snelgrove--2018|Snelgrove et al., 2018]] ). The deep sea covers &gt;63% of Earth’s surface (Costello and Cheung, 2010) and is exposed to climate-driven changes in abyssal, intermediate and surface waters that influence sinking fluxes of particulate organic matter ( ''high confidence'' ) (see Figure Box 3.3.1; Sections 3.1, 3.2.1, 3.2.2, 3.4.3.4; WGII AR5 Section 30.5.7; SROCC Sections 5.2.3, 5.2.4; [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). These ecosystems are also influenced by non-climate drivers, especially fisheries, oil and gas extraction ( [[#Thurber--2014|Thurber et al., 2014]] ; [[#Cordes--2016|Cordes et al., 2016]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ); cable laying ( [[#United%20Nations--2021|United Nations, 2021]] ); and mineral resource exploration ( [[#Hein--2021|Hein et al., 2021]] ); with proposed large-scale deep-sea mining a potential future source of impacts ( [[#Danovaro--2018|Danovaro, 2018]] ; [[#Levin--2020|Levin et al., 2020]] ).

Ocean warming alters biological processes in deep-sea ecosystems in ways that affect deep-sea habitat, biodiversity and material processing. Enhancement of microbial respiration by warming attenuates sinking POC, which has been associated with the globally projected declines in total seafloor biomass of −9.8 and −13.0% by 2081–2100 relative to 1995–2014 under SSP1-2.6 and SSP5-8.5, respectively ( ''limited evidence'' ) ( [[#3.4.3.4|Section 3.4.3.4]] ). Additionally, climate-change-driven oxygen loss ( [[#3.2.3|Section 3.2.3.2]] ; [[#Luna--2012|Luna et al., 2012]] ; [[#Belley--2016|Belley et al., 2016]] ) and geographic shifts in predator distributions ( [[#3.4.3.1|Section 3.4.3.1]] ) are anticipated to affect deep-sea biodiversity ( ''limited evidence, high agreement'' ) ( [[#Smith--2012|Smith et al., 2012]] ; [[#Morato--2020|Morato et al., 2020]] ). Complex responses of some bathyal crustacean assemblages to environmental change suggest an increase in phylogenetic diversity but limited decreases in abundances with temperature ( [[#Ashford--2019|Ashford et al., 2019]] ). Acute mortality of some reef-forming cold-water corals to laboratory-simulated warming ( [[#Lunden--2014|Lunden et al., 2014]] ) suggests that both long-term warming and the increase of MHWs in intermediate and deep waters ( [[#Elzahaby--2019|Elzahaby and Schaeffer, 2019]] ) could pose significant risk to associated ecosystems ( ''high confidenc'' e). Thermal tolerance thresholds (lethal and sub-lethal) of scleractinians in laboratory settings depend on their geographic position and capacity for thermal adaptation, as well as other factors including food, oxygen and pH ( ''medium to high confidence'' ) ( [[#Naumann--2013|Naumann et al., 2013]] ; [[#Hennige--2014|Hennige et al., 2014]] ; [[#Lunden--2014|Lunden et al., 2014]] ; [[#Naumann--2014|Naumann et al., 2014]] ; [[#Georgian--2016|Georgian et al., 2016]] ; [[#Gori--2016|Gori et al., 2016]] ; [[#Maier--2016|Maier et al., 2016]] ; [[#Büscher--2017|Büscher et al., 2017]] ).

The extension and intensification of deep-water acidification ( [[#3.2.3|Section 3.2.3.1]] ) has been identified as a further key risk to deep-water coral ecosystems ( ''medium confidence'' ) ( [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Literature since SROCC supports this assessment ( [[#Morato--2020|Morato et al., 2020]] ; [[#Puerta--2020|Puerta et al., 2020]] ), although scleractinians and gorgonians are found in regions undersaturated with respect to aragonite ( [[#Thresher--2011|Thresher et al., 2011]] ; [[#Fillinger--2013|Fillinger and Richter, 2013]] ; [[#Baco--2017|Baco et al., 2017]] ). Laboratory experiments on reef-forming scleractinians show variable results, with regional acclimation potential and population-genetic adaptations ( [[#Georgian--2016|Georgian et al., 2016]] ; [[#Kurman--2017|Kurman et al., 2017]] ). ''Desmophyllum pertusum'' 7 [[#footnote-000|1]] and ''Madrepora oculata'' maintain calcification in moderately low pH (7.75) and near-saturation of aragonite ( [[#Hennige--2014|Hennige et al., 2014]] ; [[#Maier--2016|Maier et al., 2016]] ; [[#Büscher--2017|Büscher et al., 2017]] ), but lower pH (7.6) and corrosive conditions lead to net dissolution of ''D. pertusum'' skeletons ( ''high confidence'' ) ( [[#Lunden--2014|Lunden et al., 2014]] ; [[#Kurman--2017|Kurman et al., 2017]] ; [[#Gómez--2018|Gómez et al., 2018]] ). Experiments suggest that ''D. dianthus'' is more sensitive to warming than acidification and when both are high, as projected under climate change. Warming appears to compensate for declines in calcification, with fitness also sensitive to food availability ( [[#Bramanti--2013|Bramanti et al., 2013]] ; [[#Movilla--2014|Movilla et al., 2014]] ; [[#Gori--2016|Gori et al., 2016]] ; [[#Baussant--2017|Baussant et al., 2017]] ; [[#Büscher--2017|Büscher et al., 2017]] ; [[#Schönberg--2017|Schönberg et al., 2017]] ; [[#Höfer--2018|Höfer et al., 2018]] ; [[#Maier--2019|Maier et al., 2019]] ).

In OMZ regions ( [[#3.2.3|Section 3.2.3.2]] ), benthic species distributions ( [[#Sperling--2016|Sperling et al., 2016]] ; [[#Levin--2018|Levin, 2018]] ; [[#Gallo--2020|Gallo et al., 2020]] ), abundance and composition of demersal fishes in canyons ( [[#De%20Leo--2012|De Leo et al., 2012]] ) and deep-pelagic zooplankton ( [[#Wishner--2018|Wishner et al., 2018]] ) follow oxygen gradients, indicating that deep-sea biodiversity and ecosystem structure will be impacted by extension of hypoxic areas ( ''medium confidence'' ). Fossil records show benthic population collapse and turnover when oxygen ranged from oxic to mildly or severely hypoxic (Cross-Chapter Box PALEO in Chapter 1; [[#Moffitt--2015|Moffitt et al., 2015]] ). Regional extirpations among cold-water corals in the paleorecord were associated with substantial declines in oxygen, coincident with abrupt warming and altered properties of intermediate water-masses ( [[#Wienberg--2018|Wienberg et al., 2018]] ; [[#Hebbeln--2019|Hebbeln et al., 2019]] ). Despite mortality and functional impacts from low oxygen concentrations observed in aquaria ( [[#Lunden--2014|Lunden et al., 2014]] ), recent observations of the deep-water coral ''D. pertusum'' suggest adaptive capacity to hypoxia among specimens from OMZ regions that are highly productive ( ''low confidence'' ) ( [[#Hanz--2019|Hanz et al., 2019]] ; [[#Hebbeln--2020|Hebbeln et al., 2020]] ).

Chemosynthetic ecosystems could be particularly prone to oxygen decline ( ''low to medium confidence'' ). Projected OMZ expansion in the vicinity of seep communities could favour sulphide-tolerant species, as suggested from fossil records ( [[#Moffitt--2015|Moffitt et al., 2015]] ), but this will exclude large symbiont-bearing foundation species of methane-seep ecosystems ( [[#Fischer--2012|Fischer et al., 2012]] ; [[#Sweetman--2017|Sweetman et al., 2017]] ). Projected warming, or shifts in warm-current circulation along continental margins, could enhance dissociation of buried methane hydrates ( [[#Phrampus--2012|Phrampus and Hornbach, 2012]] ; [[#Phrampus--2014|Phrampus et al., 2014]] ), either increasing anaerobic methane oxidation ( [[#Boetius--2013|Boetius and Wenzhöfer, 2013]] ), which benefits seep communities, or increasing gas fluxes, which would decrease anaerobic methane oxidation rates and exclude chemosynthetic fauna.

Environmental niche models ( [[#FAO--2019|FAO, 2019]] ; [[#Morato--2020|Morato et al., 2020]] ; [[#Puerta--2020|Puerta et al., 2020]] ) project that under RCP8.5, &gt;50% of present-day scleractinian habitats in the North Atlantic Ocean will become unsuitable by 2100, with greater impacts on ''D. pertusum'' than on ''D. dianthus'' or ''M. oculata'' . For gorgonians, corresponding habitat loss is ''likely'' &gt;80%. Much less is known about the environmental niches of deep-sea sponges, preventing a similar assessment ( [[#Kazanidis--2019|Kazanidis et al., 2019]] ; [[#Puerta--2020|Puerta et al., 2020]] ).

Climate-driven impacts further limit the resilience of deep-sea ecosystems to impacts from human activities ( ''high confidence'' ) ( [[#Levin--2015|Levin and Le Bris, 2015]] ; [[#Rogers--2015|Rogers, 2015]] ; [[#Sweetman--2017|Sweetman et al., 2017]] ). However, assessing cumulative climatic and non-climatic impacts is challenging for these data-poor environments ( [[#Ashford--2018|Ashford et al., 2018]] ; [[#Levin--2018|Levin, 2018]] ; [[#Armstrong--2019|Armstrong et al., 2019]] ; [[#Heffernan--2019|Heffernan, 2019]] ; [[#Kazanidis--2020|Kazanidis et al., 2020]] ; [[#Orejas--2020|Orejas et al., 2020]] ), where lack of knowledge increases the possibility of overlooking ecosystem vulnerabilities and risks ( [[#Levin--2021|Levin, 2021]] ). A paucity of information about the natural variability and historical trends of these habitats prevents robust assessment of adaptive capacities and potential vulnerabilities to extreme events ( [[#Aguzzi--2019|Aguzzi et al., 2019]] ; [[#Levin--2019|Levin et al., 2019]] ; [[#Chapron--2020|Chapron et al., 2020]] ; [[#Danovaro--2020|Danovaro et al., 2020]] ; [[#Le%20Bris--2020|Le Bris and Levin, 2020]] ; [[#Levin--2021|Levin, 2021]] ). The spatial resolution of CMIP5 models is too coarse to robustly project changes in mesoscale circulation at the seafloor ( [[#Sulpis--2019|Sulpis et al., 2019]] ), on which deep-sea ecosystems depend for organic material supplies and dispersal of planktonic and planktotrophic larvae ( ''high confidence'' ) ( [[#Fox--2016|Fox et al., 2016]] ; [[#Mitarai--2016|Mitarai et al., 2016]] ; [[#Dunn--2018|Dunn et al., 2018]] ). Higher-resolution modelling from CMIP6 ( [[#Orr--2017|Orr et al., 2017]] ), multi-annual and high-frequency records of ocean bottom-water properties ( [[#Meinen--2020|Meinen et al., 2020]] ), and better understanding and accounting of biogeochemical mechanisms of organic carbon transport to the ocean interior is expected to improve this capacity ( [[#Boyd--2019|Boyd et al., 2019]] ; [[#Séférian--2020|Séférian et al., 2020]] ).

[[File:0eef17c2358561813f4bb48823289f96 IPCC_AR6_WGII_Figure_3_Box_3_3_1.png]]

'''Figure Box 3.3.1 |''' '''The combination of climate-induced drivers in different deep-ocean ecosystems.''' (Key physical and biological drivers of change in the deep-sea and benthic habitats with specific vulnerabilities are discussed in [[#3.4.3.3|Section 3.4.3.3]] .)

<div id="footnote-000" class="_idFootnote"></div>

[[#footnote-000-backlink|1]] 7 Previously named ''Lophelia pertusa''

<div id="3.5" class="h1-container"></div>

<span id="vulnerability-resilience-and-adaptive-capacity-in-marine-socialecological-systems-including-impacts-on-ecosystem-services"></span>