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

=== 3.2.2 Physical Changes ===

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==== 3.2.2.1 Ocean Warming, Climate Velocities and Marine Heatwaves ====

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Global mean SST has increased since the beginning of the 20th century by 0.88°C ( ''very likely'' range: 0.68–1.01°C), and it is ''virtually certain'' that the global ocean has warmed since at least 1971 (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). A key characteristic of ocean temperature change relevant for ecosystems is climate velocity, a measure of the speed and direction at which isotherms move under climate change ( [[#Burrows--2011|Burrows et al., 2011]] ), which gives the rate at which species must migrate to maintain constant climate conditions. It has been shown to be a useful and simple predictor of species distribution shifts in marine ecosystems ( [[#Chen--2011|Chen et al., 2011]] ; [[#Pinsky--2013|Pinsky et al., 2013]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ). Median climate velocity in the surface ocean has been 21.7 km per decade since 1960, with higher values in the Arctic/sub-Arctic and within 15° of the Equator (Figure 3.3; [[#Burrows--2011|Burrows et al., 2011]] ). While climate velocity has been slower in the mesopelagic layer (200–1000 m) than in the epipelagic layer (0–200 m) over the past 50 years, it has been shown to be faster in the bathypelagic (1000–4000 m) and abyssopelagic (&gt;4000 m) layers (Figure 3.4; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ), suggesting that deep-ocean species could be as exposed to effects of warming as species in the surface ocean ( [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ).

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[[File:708c67e23788e6c29827206356733693 IPCC_AR6_WGII_Figure_3_003.png]]

'''Figure 3.3 |''' '''Observed surface ocean warming, surface climate velocity and reconstructed changes in marine heatwaves (MHWs) over the past 100 years.''' (a) Sea surface temperature trend (degrees Celsius per century) over 1925–2016 from Hadley Centre Sea Ice and Sea Surface Temperature 1.1 (HadISST1.1; (b) surface climate velocity (kilometres per decade) over 1925–2016 computed from HadISST1.1 and (c) change in total MHW days for the surface ocean over 1925–1954 to 1987–2016 based on monthly proxies. (Data from [[#Oliver--2018|Oliver et al., 2018]] ).

Marine heatwaves (MHWs) are periods of extreme seawater temperature relative to the long-term mean seasonal cycle, that persist for days to months, and that may carry severe consequences for marine ecosystems and their services (WGI AR6 Box 9.2; [[#Hobday--2016a|Hobday et al., 2016a]] ; [[#Smale--2019|Smale et al., 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). MHWs became more frequent over the 20th century ( ''high confidence'' ) and into the beginning of the 21st century, approximately doubling in frequency ( ''high confidence'' ) and becoming more intense and longer since the 1980s ( ''medium confidence'' ) (WGI AR6 Box 9.2; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). These trends in MHWs are explained by an increase in ocean mean temperatures ( [[#Oliver--2018|Oliver et al., 2018]] ), and human influence has ''very likely'' contributed to 84–90% of them since at least 2006 (WGI AR6 Box 9.2; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). The probability of occurrence (as well as duration and intensity) of the largest and most impactful MHWs that have occurred in the past 30 years has increased more than 20-fold due to anthropogenic climate change ( [[#Laufkötter--2020|Laufkötter et al., 2020]] ).

Ocean warming will continue over the 21st century ( ''virtually certain'' ), with the rate of global ocean warming starting to be scenario-dependent from about the mid-21st century ( ''medium confidence'' ). At the ocean surface, it is ''virtually certain'' that SST will continue to increase throughout the 21st century, with increasing hazards to many marine ecosystems (WGI AR6 Box 9.2; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ) ''.'' The future global mean SST increase projected by CMIP6 models for the period 1995–2014 to 2081–2100 is 0.86°C ( ''very likely'' range: 0.43–1.47°C) under SSP1-2.6, 1.51°C (1.02–2.19°C) under SSP2-4.5, 2.19°C (1.56–3.30°C) under SSP3-7.0 and 2.89°C (2.01–4.07°C) under SSP5-8.5 (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Stronger surface warming occurs in parts of the tropics, in the North Pacific, and in the Arctic Ocean, where SST increases by &gt;4°C in 2080–2099 under SSP5-8.5 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). The CMIP6 climate models also project ocean warming at the seafloor, with the magnitude of projected changes being less than that of surface waters but having larger uncertainties ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). The projected end-of-the-century warming in CMIP6 as reported here is greater than assessed with Coupled Model Intercomparison Project 5 (CMIP5) models in AR5 and in SROCC for similar radiative forcing scenarios (Figure 3.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ), because of greater climate sensitivity in the CMIP6 model ensemble than in CMIP5 (WGI AR6 Chapter 4; [[#Forster--2020|Forster et al., 2020]] ; [[#Lee--2021|Lee et al., 2021]] ).

Marine heatwaves will continue to increase in frequency, with a ''likely'' global increase of 2–9 times in 2081–2100 compared with 1995–2014 under SSP1-2.6, and 3–15 times under SSP5-8.5, with the largest increases in tropical and Arctic oceans (WGI AR6 Box 9.2; [[#Frölicher--2018|Frölicher et al., 2018]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

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[[File:47f328cb87e020e334c82d4c176308d0 IPCC_AR6_WGII_Figure_3_004.png]]

'''Figure 3.4 |''' '''Historical and projected climate velocity.''' Climate velocities (in kilometres per decade) are shown for the '''(a,d,g)''' historical period (1965–2014), and the last 50 years of the 21st century (2051–2100), under '''(b,e,h)''' SSP1-2.6 and '''(c,f,i)''' SSP5-8.5. Also shown are the epipelagic (0–200 m), mesopelagic (200–1000 m) and bathypelagic (1000–4000 m) domains. Updated figure from [[#Brito-Morales--2020|Brito-Morales et al. (2020)]] , with Coupled Model Intercomparison Project 6 models used in [[#Kwiatkowski--2020|Kwiatkowski et al. (2020)]] .

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==== 3.2.2.2 Sea Level Rise and Extreme Sea Levels ====

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Global mean sea level (GMSL) (Cross-Chapter Box SLR in Chapter 3) has risen by about 0.20 m since 1901 and continues to accelerate (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ; [[#Church--2011|Church and White, 2011]] ; [[#Jevrejeva--2014|Jevrejeva et al., 2014]] ; [[#Hay--2015|Hay et al., 2015]] ; [[#Kopp--2016|Kopp et al., 2016]] ; [[#Dangendorf--2017|Dangendorf et al., 2017]] ; WCRP Global Sea Level Budget Group, 2018; [[#Kemp--2018|Kemp et al., 2018]] ; [[#Ablain--2019|Ablain et al., 2019]] ; [[#Gulev--2021|Gulev et al., 2021]] ).

Most coastal ecosystems (mangroves, seagrasses, salt marshes, shallow coral reefs, rocky shores and sandy beaches) are affected by changes in relative sea level (RSL, the change in the mean sea level relative to the land; [[#3.4.2|Section 3.4.2]] ). Regional rates of RSL rise differ from the global mean due to a range of factors, including local subsidence driven by anthropogenic activities such as groundwater and hydrocarbon extraction (WGI AR6 Box 9.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). In many deltaic regions, anthropogenic subsidence is currently the dominant driver of RSL rise (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.6.3|Section 9.6.3.2]] ; [[#Tessler--2018|Tessler et al., 2018]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). RSL rise is driving a global increase in the frequency of extreme sea levels ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.6.4.1|Section 9.6.4.1]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

GMSL rise through the middle of the 21st century exhibits limited dependence on emissions scenario; between 1995–2014 and 2050, GMSL is ''likely'' to rise by 0.15–0.23 m under SSP1-1.9 and 0.20–0.30 m under SSP5-8.5 (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.6.3|Section 9.6.3]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Beyond 2050, GMSL and RSL projections are increasingly sensitive to the differences among emission scenarios. Considering only processes in which there is at least ''medium confidence'' (e.g., thermal expansion, land-water storage, land-ice surface mass balance and some ice-sheet dynamic processes), GMSL between 1995–2014 and 2100 is ''likely'' to rise by 0.28–0.55 m under SSP1-1.9, 0.33–0.61 m under SSP1-2.6, 0.44–0.76 m under SSP2-4.5, 0.55–0.90 m under SSP3-7.0 and 0.63–1.02 m under SSP5-8.5 (Figure 3.5). Under high-emission scenarios, ice-sheet processes in which there is ''low confidence'' and ''deep uncertainty'' might contribute more than one additional metre to GMSL rise by 2100 (WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

Rising mean RSL will continue to drive an increase in the frequency of extreme sea levels ( ''high confidence'' ). The expected frequency of the current 1-in-100-year extreme sea level is projected to increase by a median of 20–30 times across tide-gauge sites by 2050, regardless of emission scenario ( ''medium confidence'' ). In addition, extreme-sea-level frequency may be affected by changes in tropical cyclone climatology ( ''low confidence'' ), wave climatology ( ''low confidence'' ) and tides ( ''high confidence'' ) associated with climate change and sea level change (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.6.4.2|Section 9.6.4.2]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

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==== 3.2.2.3 Changes in Ocean Circulation, Stratification and Coastal Upwelling ====

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Ocean circulation and its variations are key to the evolution of the physical, chemical and biological properties of the ocean. Vertical mixing and upwelling are critical factors affecting the supply of nutrients to the sunlit ocean and hence the magnitude of primary productivity. Ocean currents not only transport heat, salt, carbon and nutrients, but they also control the dispersion of many organisms and the connectivity between distant populations.

Ocean stratification is an important factor controlling biogeochemical cycles and affecting marine ecosystems. WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ) assessed that it is ''virtually certain'' that stratification in the upper 200 m of the ocean has been increasing since 1970. Recent evidence has strengthened estimates of the rate of change ( [[#Yamaguchi--2019|Yamaguchi and Suga, 2019]] ; [[#Li--2020a|Li et al., 2020a]] ; [[#Sallée--2021|Sallée et al., 2021]] ), with an estimated increase of 1.0 ± 0.3% ( ''very likely'' range) per decade over the period 1970–2018 ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), higher than assessed in SROCC. It is ''very likely'' that stratification in the upper few hundred metres of the ocean will increase substantially in the 21st century in all ocean basins, driven by intensified surface warming and near-surface freshening at high latitudes (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ; [[#Capotondi--2012|Capotondi et al., 2012]] ; [[#Fu--2016|Fu et al., 2016]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

Contrasting changes among the major eastern boundary coastal upwelling systems (EBUS) were identified in AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ). While SROCC assessed with ''high confidence'' that three (Benguela, Peru-Humboldt, California) out of the four major EBUS have experienced upwelling-favourable wind intensification in the past 60 years ( [[#Sydeman--2014|Sydeman et al., 2014]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ), WGI AR6 revisited this assessment based on evidence showing ''low agreement'' between studies that have investigated trends over past decades ( [[#Varela--2015|Varela et al., 2015]] ). WGI AR6 assessed that only the California Current system has undergone large-scale upwelling-favourable wind intensification since the 1980s ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.5]] ; [[#García-Reyes--2010|García-Reyes and Largier, 2010]] ; [[#Seo--2012|Seo et al., 2012]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

While no consistent pattern of contemporary changes in upwelling-favourable winds emerges from observation-based studies, numerical and theoretical work projects that summertime winds near poleward boundaries of upwelling zones will intensify, while winds near equatorward boundaries will weaken ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.3.5]] ; [[#García-Reyes--2015|García-Reyes et al., 2015]] ; [[#Rykaczewski--2015|Rykaczewski et al., 2015]] ; [[#Wang--2015|Wang et al., 2015]] ; [[#Aguirre--2019|Aguirre et al., 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Nevertheless, projected future annual cumulative upwelling wind changes at most locations and seasons remain within ±10–20% of present-day values ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.3.5]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

Continuous observation of the Atlantic meridional overturning circulation (AMOC) has improved the understanding of its variability ( [[#Frajka-Williams--2019|Frajka-Williams et al., 2019]] ), but there is ''low confidence'' in the quantification of AMOC changes in the 20th century because of ''low agreement'' in quantitative reconstructed and simulated trends (WGI AR6 Sections 2.3.3, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Direct observational records since the mid-2000s remain too short to determine the relative contributions of internal variability, natural forcing and anthropogenic forcing to AMOC change ( ''high confidence'' ) (WGI AR6 Sections 2.3.3, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Over the 21st century, AMOC will ''very likely'' decline for all SSP scenarios but will not involve an abrupt collapse before 2100 (WGI AR6 Sections 4.3.2, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] ).

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==== 3.2.2.4 Sea Ice Changes ====

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Sea ice is a key driver of polar marine life, hosting unique ecosystems and affecting diverse marine organisms and food webs through its impact on light penetration and supplies of nutrients and organic matter ( [[#Arrigo--2014|Arrigo, 2014]] ). Since the late 1970s, Arctic sea ice area has decreased for all months, with an estimated decrease of 2 million km 2 (or 25%) for summer sea ice (averaged for August, September and October) in 2010–2019 as compared with 1979–1988 (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.3.1|Section 9.3.1.1]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). For Antarctic sea ice there is no significant global trend in satellite-observed sea ice area from 1979 to 2020 in either winter or summer, due to regionally opposing trends and large internal variability (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.3.2|Section 9.3.2.1]] ; [[#Maksym--2019|Maksym, 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

CMIP6 simulations project that the Arctic Ocean will ''likely'' become practically sea ice free (area below 1 million km 2 ) for the first time before 2050 and in the seasonal sea ice minimum in each of the four emission scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5 and SSP5-8.5 (Figure 3.7; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.3.2|Section 9.3.2.2]] ; Notze and SIMIP Community, 2020; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Antarctic sea ice area is also projected to decrease during the 21st century, but due to mismatches between model simulations and observations, combined with a lack of understanding of reasons for substantial inter-model spread, there is ''low confidence'' in model projections of future Antarctic sea ice changes, particularly at the regional level (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.3.2|Section 9.3.2.2]] ; [[#Roach--2020|Roach et al., 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

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