Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.4.8 Open and Deep Ocean ===

<div id="h2-15-siblings" class="h2-siblings"></div>

Oceans face challenges from anthropogenic perturbations to the global Earth system, which cause increasing ocean warming, carbon dioxide-induced acidification and oxygen loss ( [[#Bindoff--2019|Bindoff et al., 2019]] ). Climate change will affect the major oceanic CIDs described in [[#12.2|Section 12.2]] : mean ocean temperature, marine heatwave, ocean acidity, ocean salinity, and dissolved oxygen (O <sub>2</sub> ), as well as severe wind storm and sea ice. These changes result in a shifting profile of hazards relevant to impact and risk assessments ( [[#12.3|Section 12.3]] ). New evidence, the SROCC (IPCC 2019b) assessments and advances in the new CMIP6 climate simulations reinforce confidence in projected changes in climatic impact-drivers in the global oceans. As the ocean has taken up about 90% of the global warming for the period 1971–2018 ( [[IPCC:Wg1:Chapter:Chapter-7#7.2.2.2|Section 7.2.2.2]] ), the emergence of the sea surface temperature increase signal has already been observed in global oceans over the last century ( [[#Hawkins--2020|Hawkins et al., 2020]] ). The signal in sea ice extent decrease has already emerged in the Arctic Ocean ( [[#Landrum--2020|Landrum and Holland, 2020]] ), while ocean acidification and low oxygen have also already emerged in many oceanic regions and will emerge in all global oceans by 2050 under RCP8.5 ( [[#12.5.2|Section 12.5.2]] and Table 12.10). This section assesses key climatic impact-drivers that can be linked with sectoral and regional vulnerability and exposure in open and deep oceans, drawing from previous Chapters (Chapters 2, 3, 4, 5 and 9).

<div id="_idContainer106" class="Basic-Text-Frame"></div>

'''Table 12.10''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in open and deep ocean regions, representing their aggregate characteristic changes for mid-century for scenarios RCP4.5, SSP2-4.5, SRES A1B, or above within each AR6 region (defined in Chapter 1), approximately corresponding (for CIDs that are independent of sea level rise) to global warming levels between 2°C and 2.4°C (see [[#12.4|Section 12.4]] for more details of the assessment method).''' The table also includes the assessment of observed or projected time-of-emergence of the CID change signal from the natural interannual variability if found with at least ''medium confidence'' in [[#12.5.2|Section 12.5.2]] . Asterisks indicate regions that extend across both sides of the map.

[[File:29b4e958d29bffef109fcd88081100a0 IPCC_AR6_WGI_Chapter12_Table_12_10.jpg]]

'''Mean ocean temperature:''' It is ''very likely'' that global mean sea surface temperature (SST) has increased by 0.88 [0.68 to 1.01] °C from 1850–1900 to 2011–2020, and 0.60 [0.44 to 0.74] °C from 1980 to 2020 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1.6|Section 2.3.1.1.6]] and Table 2.4). There is ''very high confidence'' that the Indian Ocean, the western equatorial Pacific Ocean, and western boundary currents have warmed faster than the global average, while the Southern Ocean, the eastern equatorial Pacific, and the North Atlantic Ocean have warmed more slowly or slightly cooled ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.1.1|Section 9.2.1.1]] ). It is ''virtually certain'' that global mean SST will continue to increase in the 21st century at a rate depending on future emissions scenario, with CMIP6 projections indicating an increase of 0.86°C ( ''likely'' range 0.43°C–1.47°C) under SSP1-2.6 and 2.89°C (2.01°C–4.07°C) under SSP5-8.5, by 2081–2100, relative to 1995–2014 ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.1.1|Section 9.2.1.1]] ). Global warming of 2°C above pre-industrial levels is projected to increase SST, resulting in the exceedance of numerous hazard thresholds for pathogens, seagrasses, mangroves, kelp forests, rocky shores, coral reefs and other marine ecosystems ( ''medium confidence'' ) ( [[#Poloczanska--2013a|Poloczanska et al., 2013a]] , b, 2016; [[#Liu--2014|Liu et al., 2014]] ; [[#Pörtner--2014|Pörtner et al., 2014]] ; [[#Graham--2015|Graham et al., 2015]] ; [[#Schoepf--2015|Schoepf et al., 2015]] ; [[#Gobler--2017|Gobler et al., 2017]] ; [[#Henson--2017|Henson et al., 2017]] ; [[#Hoegh-Guldberg--2017|Hoegh-Guldberg et al., 2017]] ; [[#Krueger--2017|Krueger et al., 2017]] ; [[#Hughes--2018a|Hughes et al., 2018a]] , b; [[#Perry--2018|Perry et al., 2018]] ). It is ''virtually certain'' that upper-ocean stratification has increased at a rate of 4.9 ± 1.5% during 1970–2018 and that this will continue to increase in the 21st century ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.1.3|Section 9.2.1.3]] ), potentially leading to reduced nutrient supply and total productivity ( ''low confidence'' ) ( [[#Moore--2018|Moore et al., 2018]] ).

'''Marine heatwave:''' Marine heatwaves (MHWs) have increased in frequency over the 20th century, with an approximate doubling since the 1980s ( ''high confidence'' ), and their intensity and duration have also increased ( ''medium confidence'' ) (Box 9.2). Projections show that this increasing trend ''likely'' continues with 2–9 times more frequent MHWs (at global scale) projected by 2081–2100, relative to 1995–2014 under SSP1-2.6, and 4–18 times more frequent MWHs under SSP5-8.5. The largest changes in MHW frequency are ''likely'' to occur in the tropical ocean and the Arctic, while there is ''medium confidence'' of moderate increases in the mid-latitudes, and of small increases in the Southern Ocean (Box 9.2). Permanent MHWs (more than 360 days per year, relative to the historical climate conditions) are projected to occur in the 21st century in parts of the tropical ocean, in the Arctic Ocean, and around latitude 45°S, under SSP5-8.5 (Box 9.2). The occurrence of such permanent MHWs can be largely avoided under the SSP1-2.6 scenario (Box 9.2). MHWs can have devastating and long-term impacts on ecosystems ( [[#Oliver--2018|Oliver et al., 2018]] ), making them an emerging hazard for marine ecosystems ( [[#Frölicher--2018|Frölicher and Laufkötter, 2018]] ; [[#Smale--2019|Smale et al., 2019]] ). A series of MHWs that occurred in 2010–2011 had consequences for seagrass in western Australia ( [[#Wernberg--2013|Wernberg et al., 2013]] ; [[#Arias-Ortiz--2018|Arias-Ortiz et al., 2018]] ), and for the lobster fishery in the Gulf of Maine ( [[#Pershing--2018|Pershing et al., 2018]] ). The MHWs that occured western Australia in 2015/2016 led to the third-highest mass coral bleaching globally ( [[#Le%20Nohaïc--2017|Le Nohaïc et al., 2017]] ).

'''Ocean acidity:''' With the increasing CO <sub>2</sub> concentration, the global mean ocean surface pH is decreasing and is now the lowest it has been for at least a thousand years ( ''very high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.5|Section 2.3.3.5]] ). It is ''very'' ''likely'' that, since the 1980s, ocean surface pH has changed at a rate of –0.016 to –0.019 per decade in the subtropical open oceans, at –0.010 to –0.026 per decade in the tropical Pacific, and at –0.003 to –0.026 per decade in open subpolar and polar zones (Sections 2.3.3.5 and 5.3.3.2). Over the period 1870–1899 to 2080–2099, ocean surface pH is projected to decline by –0.16 ± 0.002 under SSP1–2.6, and by –0.44 ± 0.005 under SSP5-8.5 (Sections 4.3.2.5 and 5.3.4.1). Declining ocean pH will exacerbate negative impacts on marine species ( ''medium confidence'' ) ( [[#Albright--2016|Albright et al., 2016]] ; [[#Kwiatkowski--2016|Kwiatkowski et al., 2016]] ; [[#Watson--2017|Watson et al., 2017]] ).

'''Ocean salinity:''' Salinity contrasts have increased since the 1950s, near the ocean surface ( ''virtually certain'' ) and in the subsurface ( ''very likely'' ) , with high-salinity regions becoming more saline and low-salinity regions becoming fresher ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ). At the basin scale, it is ''very likely'' that the Pacific and the Southern Oceans have freshened and that the Atlantic has become more saline ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ). The SROCC (IPCC 2019b) assessment that a general pattern of fresh ocean regions getting fresher and salty ocean regions getting saltier will continue in the 21st century is confirmed in [[IPCC:Wg1:Chapter:Chapter-9#9.2.2.2|Section 9.2.2.2]] .

At the regional scale, by 2100 the average Arctic surface salinity is projected to decrease by 1.5 ± 1.1 psu (practical salinity units), and the liquid freshwater column in the Arctic Ocean is projected to increase by 5.4 ± 3.8 m under RCP8.5, ( [[#Shu--2018|Shu et al., 2018]] ). In the Indian Ocean, sea surface salinity is projected to decrease by between 0.49 and 0.75 psu by 2080, compared to 2015, under RCP2.6 and RCP2.6, respectively ( [[#Akhiljith--2019|Akhiljith et al., 2019]] ). Projections for the North and South Atlantic oceans indicate increasing salinity in the upper layer (0–500 m) under both RCP4.5 and RCP8.5, due to the decreasing freshwater input from the equator and increasing net evaporation ( [[#Skliris--2020|Skliris et al., 2020]] ). There is ''medium confidence'' that fresh ocean regions (Pacific, Southern and Indian oceans) will get fresher and salty ocean regions (Atlantic Ocean) will get saltier over the 21st century ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.2.2|Section 9.2.2.2]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Ocean warming and high-latitude surface freshening is projected to continue to increase upper-ocean stratification in the 21st century ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.1.3|Section 9.2.1.3]] ).

'''Dissolved oxygen:''' Since the middle of the 20th century, oxygen concentrations of open and coastal waters have been declining, and such deoxygenation affects biological and biogeochemical processes in the ocean ( [[#Schmidtko--2017|Schmidtko et al., 2017]] ). In recent decades, low-oxygen zones in ocean ecosystems have expanded, and projections indicate an acceleration with global warming ( ''medium confidence'' ) ( [[#Diaz--2008|Diaz and Rosenberg, 2008]] ; [[#Gilly--2013|Gilly et al., 2013]] ; [[#Gobler--2014|Gobler et al., 2014]] ). A 2% loss (4.8 ± 2.1 pmoles O <sub>2</sub> ) in total dissolved oxygen in the upper ocean layer (100–600 m) has been observed during 1970–2010 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.4.2|Section 2.3.4.2]] ), with the highest oxygen loss of up to 30 mol m <sup>–2</sup> per decade in the equatorial and North Pacific, the Southern Ocean and the South Atlantic Ocean ( [[IPCC:Wg1:Chapter:Chapter-5#5.3.3.2|Section 5.3.3.2]] ). Global mean ocean oxygen concentration is projected to decrease by 6.36 ± 2.92 mmol m <sup>–3</sup> under SSP1-2.6 and by 13.27 ± 5.28 mmol m <sup>–3</sup> under SSP5-8.5 in the subsurface (100–600 m) by 2080–2099, compared to 1870–1899, which is respectively 71% and 40% greater than previous estimates based on CMIP5 models ( [[IPCC:Wg1:Chapter:Chapter-5#5.3.3.2|Section 5.3.3.2]] ). In the benthic ocean, projected future losses of dissolved oxygen concentration by 2080–2099, compared to 1870–1899, are −5.14 ± 2.04 mmol m <sup>−3</sup> under SSP1-2.6 and −6.04 ± 2.19 mmol m <sup>−3</sup> under SSP5-8.5 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). [[IPCC:Wg1:Chapter:Chapter-5#5.3.3.2|Section 5.3.3.2]] assessed ''very likely'' global decreases in ocean oxygen concentrations although there is ''medium confidence'' in specific regional declines that are expected to expand both anoxic and hypoxic zones, with such reductions of oxygen expected to persist for thousands of years ( [[#Yamamoto--2015|Yamamoto et al., 2015]] ; [[#Frölicher--2020|Frölicher et al., 2020]] ).

'''Sea ice:''' The Arctic sea ice area for September has decreased from 6.23 to 3.76 million km <sup>2</sup> and for March from 14.52 to 13.42 million km <sup>2</sup> between 1979–1988 and 2010–19 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.1|Section 2.3.2.1.1]] ). There is ''high confidence'' that sea ice in the Arctic will further decrease in the future under all emissions scenarios ( [[IPCC:Wg1:Chapter:Chapter-9#9.3.1.1|Section 9.3.1.1]] ). In contrast, there is no clear observed trend in the Antarctic sea ice area over the past few decades and there is ''low confidence'' of future changes ( [[IPCC:Wg1:Chapter:Chapter-9#9.3.1.1|Section 9.3.1.1]] ). The duration of the summer ice season in the Arctic has increased by 5 to 20 weeks between 1979 and 2013, with a significant trend ranging from 5 to 17 days per decade for earlier spring retreat and from 5 to 25 days per decade for later autumn advance, with consequences for Arctic marine mammals (AMMs) due to sea ice habitat loss ( [[#Laidre--2015|Laidre et al., 2015]] ). The Arctic is projected to be ice-free more often during summer under 2°C global warming compared to 1.5°C global warming ( [[IPCC:Wg1:Chapter:Chapter-9#9.3.1.1|Section 9.3.1.1]] ; see also Sections 12.4.9 and 4.4.2.1), opening new shipping lanes for international commerce ( [[#Valsson--2011|Valsson and Ulfarsson, 2011]] ) and lengthening the season for offshore resource extraction ( [[#Schaeffer--2012|Schaeffer et al., 2012]] ). Iceberg numbers are expected to increase as a result of global warming, forming an elevated hazard to shipping and offshore facilities ( [[#Bigg--2018|Bigg et al., 2018]] ).

'''It is''' virtually certain '''that global mean SST will continue to increase throughout the 21st century, resulting in the exceedance of numerous climatic impact-driver thresholds relevant to marine ecosystems''' ( medium confidence '''). Marine heatwave days are projected to increase in global oceans, with a larger increase in the tropical ocean and Arctic Ocean''' ( high confidence '''). It is''' virtually certain '''that upper-ocean stratification will continue to increase in the 21st century. Future ocean warming will''' very likely '''assist the development of both anoxic and hypoxic zones, with such reductions of oxygen expected to persist for thousands of years. Future projections also indicate freshening of the Pacific, Southern and Indian oceans and a saltier Atlantic Ocean''' ( medium confidence ''').'''

The assessed direction of change in climatic impact-drivers for open and deep ocean regions and associated confidence levels are illustrated in Table 12.10, following the AR6 WGI ocean reference regions (Figure Atlas.2b).

<div id="12.4.9" class="h2-container"></div>

<span id="polar-terrestrial-regions"></span>