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

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

<div id="h3-1-siblings" class="h3-siblings"></div>

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]] ).

<div id="_idContainer013" class="Figure"></div>

[[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]] ).

<div id="_idContainer015" class="Figure"></div>

[[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)]] .

<div id="3.2.2.2" class="h3-container"></div>

<span id="sea-level-rise-and-extreme-sea-levels"></span>