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

==== 3.2.4.1 Compound Changes in the 21st century ====

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Earth system models project distinct regional evolutions of the different CIDs over the 21st century ( ''very high confidence'' ) (Figures 3.5, 3.6, 3.7; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Tropical and subtropical oceans are characterised by projected warming and acidification, accompanied by declining nitrate concentrations in equatorial upwelling regions. The North Atlantic is characterised by a high exposure to acidification and declining nitrate concentrations. The North Pacific is characterised by high sensitivity to compound changes, with high rates of warming, acidification, deoxygenation and nutrient depletion. In contrast, the development of compound hazards is limited in the Southern Ocean, where rates of warming and nutrient depletion are lower. The Arctic Ocean is characterised by the highest rates of acidification and warming, strong nutrient depletion, and it will ''likely'' become practically sea ice free in the September mean for the first time before the year 2050 in all SSP scenarios ( ''high confidence'' ) (Figures 3.5, 3.6, 3.7; Sections 3.2.2, 3.2.3).

In general, the projected changes in climate-induced drivers are less in absolute terms in the deep-sea (mesopelagic and bathypelagic domains and deep-sea habitats) than in the surface ocean and in shallow-water habitats (e.g., kelp ecosystems, warm-water corals) ( ''very high confidence'' ) (Figures 3.6, 3.7; [[#Mora--2013|Mora et al., 2013]] ; [[#Sweetman--2017|Sweetman et al., 2017]] ). The mesopelagic domain will be nevertheless exposed to high rates of deoxygenation (Figure 3.6) and high climate velocities (Figure 3.4; [[#3.2.2.1|Section 3.2.2.1]] ), as well as impacted by the shoaling of aragonite or calcite saturation horizon ( [[#3.2.3|Section 3.2.3.2]] ). Significant differences in projected trends between the SSPs show that mitigation strategies will limit exposure of deep-sea ecosystems to potential warming, acidification and deoxygenation during the 21st century ( ''very high confidence'' ) (Figure 3.6; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

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'''Figure 3.6 |''' '''Projected trends across open-ocean systems.''' Projected annual and global (a) average warming, (b) acidification, (c) changes in dissolved oxygen concentrations and (d) changes in nitrate (NO 3 ) concentrations for four open-ocean systems, including the epipelagic (0–200 m depth), mesopelagic (200–1000 m), bathypelagic (&gt;1000 m) domains and deep benthic waters (&gt;200 m). All projections are based on Coupled Model Intercomparison Project 6 models and for three Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP2-4.5 and SSP5-8.5 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Anomalies in the near-term (2020–2041), mid-term (2041–2060) and long-term (2081–2100) are all relative to 1985–2014. Error bars represent ''very likely'' ranges.

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'''Figure 3.7 |''' '''Projected trends across coastal-ocean ecosystems.''' Projected '''(a)''' warming, '''(b)''' acidification, '''(c)''' changes in dissolved oxygen concentrations, '''(d)''' changes in nitrate (NO 3 ) concentrations and '''(e)''' changes in summer sea ice cover fraction (September and north of 66°N for the Northern Polar Oceans, and March and south of 66°S for the Southern Polar Ocean) for five coastal-ocean ecosystems. All projected trends are for the surface ocean, except oxygen concentration changes that are computed for the subsurface ocean (100–600 m depth) for the upwelling ecosystems and the polar seas. All projections are based on Coupled Model Intercomparison Project 6 (CMIP6) models and for three Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP2-4.5 and SSP5-8.5 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Anomalies in the near term (2020–2041), mid term (2041–2060) and long term (2081–2100) are all relative to 1985–2014. Error bars represent ''very likely'' ranges. Coastal seas are defined on a 1° × 1° grid when bathymetry is less than 200 m deep. Distribution of warm-water corals is from UNEP-WCMC et al. (2018). Distribution of kelp ecosystems is from [[#OBIS--2020|OBIS (2020)]] . Upwelling areas are defined according to [[#Rykaczewski--2015|Rykaczewski et al. (2015)]] .

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