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

=== 3.2.4 Global Synthesis on Multiple Climate-induced Drivers ===

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In the 21st century, ocean and coastal ecosystems are projected to face conditions unprecedented over past centuries to millennia ( ''high confidence'' ) ( [[#3.2|Section 3.2]] ; WGI AR6 Chapters 4, 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] ), with increased temperatures ( ''virtually certain'' ) and frequency and severity of MHWs ( ''very high confidence'' ), stronger upper-ocean stratification ( ''high confidence'' ), continued rise in GMSL throughout the 21st century ( ''high confidence'' ) and increased frequency of extreme sea levels ( ''high confidence'' ), further acidification ( ''virtually certain'' ), oxygen decline ( ''high confidence'' ) and decreased surface nitrate inventories ( ''medium confidence'' ).

The rates and magnitudes of these changes largely depend on the extent of future emissions ( ''very high confidence'' ), with surface ocean warming and acidification ( ''very likely range'' ) at +3.47°C ± 1.28°C and −0.44 pH units ± 0.008 pH units in 2080–2099 (relative to 1870–1899) for SSP5-8.5 compared with +1.42°C ± 0.53°C and −0.16 pH units ± 0.003 pH units for SSP1-2.6 (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ).

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==== 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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==== 3.2.4.2 Time of Emergence ====

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Anthropogenic changes in climate-induced drivers assessed here exhibit vastly distinct times of emergence, which is the time scale over which an anthropogenic signal related to climate change is statistically detected to emerge from the background noise of natural climate for a specific region ( [[#Christensen--2007|Christensen et al., 2007]] ; [[#Hawkins--2012|Hawkins and Sutton, 2012]] ). SROCC concluded that for ocean properties, the time of emergence ranges from under a decade (e.g., surface ocean pH) to over a century (e.g., net primary production; see [[#3.4.3.3.4|Section 3.4.3.3.4]] for time of emergence of biological properties; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

The literature assessed in SROCC mainly focused on surface ocean properties and gradual mean changes. Since then, the time of emergence has also been investigated for subsurface properties, ocean extreme events and particularly vulnerable regions, such as the Arctic Ocean ( [[#Hameau--2019|Hameau et al., 2019]] ; [[#Oliver--2019|Oliver et al., 2019]] ; [[#Burger--2020|Burger et al., 2020]] ; [[#Landrum--2020|Landrum and Holland, 2020]] ; [[#Schlunegger--2020|Schlunegger et al., 2020]] ), but subsequent assessments are ''low confidence'' due to ''limited evidence'' . Below the surface, changes in temperature typically emerge from internal variability prior to changes in oxygen; however, in about a third of the global thermocline, deoxygenation emerges prior to warming ( [[#Hameau--2019|Hameau et al., 2019]] ). Permanent MHW states, defined as when SST exceeds the MHW threshold continuously over a full calendar year, will emerge during the 21st century in many parts of the surface ocean ( [[#Oliver--2019|Oliver et al., 2019]] ). Ocean acidification extremes have already emerged from background natural internal variability during the 20th century in most of the surface ocean ( [[#Burger--2020|Burger et al., 2020]] ). In the Arctic, anthropogenic sea ice changes have already emerged from the background internal variability, and anthropogenic alteration of air temperatures will emerge in the early- to mid-21st century ( [[#Landrum--2020|Landrum and Holland, 2020]] ).

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==== 3.2.4.4 Perspectives from Paleoclimatology Data ====

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Paleoclimatology observations are useful to assess multiple hazards of environmental change while excluding direct anthropogenic impacts ( [[#3.4.3.3|Section 3.4.3.3]] ). Ancient intervals of rapid climate warming that occurred between 300 and 50 million years ago (Ma) were triggered by the release of greenhouse gases ( ''high confidence'' ). The sources of greenhouse gases varied but include volcanic degassing from continental flood basalts and methane hydrates stored in marine sediments and soils ( [[#Foster--2018|Foster et al., 2018]] ). Six extreme ancient hyperthermal events are known from the last 300 Ma, when tropical SSTs reached 1.5°C–10°C warmer than pre-industrial conditions, and with substantial impacts on ancient life (Cross-Chapter Box PALEO in Chapter 1). Warming and deoxygenation in the oceans were closely associated in hyperthermal events ( ''high confidence'' ), with anoxia reaching the photic zone and abyssal depths ( [[#Kaiho--2014|Kaiho et al., 2014]] ; [[#Müller--2017|Müller et al., 2017]] ; [[#Penn--2018|Penn et al., 2018]] ; [[#Weissert--2019|Weissert, 2019]] ), whereas ocean acidification has not been demonstrated consistently ( ''medium confidence'' ) ( [[#Hönisch--2012|Hönisch et al., 2012]] ; [[#Penman--2014|Penman et al., 2014]] ; [[#Clarkson--2015|Clarkson et al., 2015]] ; [[#Harper--2020a|Harper et al., 2020a]] ; [[#Jurikova--2020|Jurikova et al., 2020]] ; [[#Müller--2020|Müller et al., 2020]] ).

Greenhouse gases also contributed substantially to shaping the longer-term climate trends over the past 50 million years, although changes in continental configuration and ocean circulation as well as planetary orbital cycles were equally important (WGI AR6 Cross-Chapter Box 2.1 in Chapter 2; [[#Westerhold--2020|Westerhold et al., 2020]] ; [[#Gulev--2021|Gulev et al., 2021]] ). There is little evidence for ocean acidification in the past 2.6 Ma ( ''low confidence'' ) ( [[#Hönisch--2012|Hönisch et al., 2012]] ), but ocean ventilation was highly sensitive to even modest warming such as observed in the past 10,000 years ( ''medium confidence'' ) ( [[#Jaccard--2012|Jaccard and Galbraith, 2012]] ; [[#Lembke-Jene--2018|Lembke-Jene et al., 2018]] ).

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