Editing IPCC:AR6/WGI/TS (section)

=== TS.2.4 The Ocean ===

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'''Observations, models and paleo-evidence indicate that recently observed changes in the ocean are unprecedented for centuries to millennia ( ''high confidence'' ). Over the past four to six decades, it is ''virtually certain'' that the global ocean has warmed, with human influence ''extremely likely'' the main driver since the 1970s, making climate change irreversible over centuries to millennia ( ''medium confidence'' ). It is ''virtually certain'' that upper ocean salinity contrasts have increased since the 1950s and ''extremely likely'' that human influence has contributed. It is ''virtually certain'' that upper ocean stratification has increased since 1970 and that sea water pH has declined globally over the last 40 years, with human influence being the main driver of the observed surface open ocean acidification ( ''virtually certain'' ). A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ), and surface ocean pH as low as recent times is uncommon in the last 2 million years ( ''medium confidence'' ). There is ''high confidence'' that marine heatwaves have become more frequent in the 20th century, and most of those since 2006 have been attributed to anthropogenic warming ( ''very likely'' ) . There is ''high confidence'' that oxygen levels have dropped in many regions since the mid 20th century and that the geographic range of many marine organisms has changed over the last two decades.''' '''The amount of ocean warming observed since 1971 will ''likely'' at least double by 2100 under a low warming scenario (SSP1-2.6) and will increase by 4–8 times under a high warming scenario (SSP5-8.5). Stratification ( ''virtually certain'' ), acidification ( ''virtually certain'' ), deoxygenation ( ''high confidence'' ) and marine heatwave frequency ( ''high confidence'' ) will continue to increase in the 21st century. While there is ''low confidence'' in 20th century AMOC change, it is ''very likely'' that AMOC will decline over the 21st century (Figure TS.11). Links to chapters 2.3, 3.5, 3.6, 4.3.2, 5.3, 7.2, 9.2, Box 9.2, 12.4'''
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[[File:6647f473cbdb47481186bde6a877fdad IPCC_AR6_WGI_TS_Figure_11.png]]

'''Figure TS.11 |''' '''Past and future ocean and ice-sheet changes.''' ''The intent of this figure is to show that observed and projected time series of many ocean and cryosphere indicators are consistent.'' Observed and simulated historical changes and projected future changes under varying greenhouse gas emissions scenarios. Simulated and projected ocean changes are shown as Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble mean, and 5–95% range (shading) is provided for scenarios SSP1-2.6 and SSP3-7.0 (except in panel a where the range is provided for scenario SSP1-2.6 and SSP5-8.5). Mean and 5–95% range in 2100 are shown as vertical bars on the right-hand side of each panel. (a) Change in multiplication factor in surface ocean marine heatwave days relative to 1995–2014 (defined as days exceeding the 99th percentile in sea surface temperature (SST) from 1995–2014 distribution). Assessed observational change span 1982–2019 from AVHRR satellite SST. (b) Atlantic Meridional Overturning Circulation (AMOC) transport relative to 1995–2014 (defined as maximum transport at 26°N). Assessed observational change spans 2004–2018 from the RAPID array smoothed with a 12-month running mean (shading around the mean shows the 12-month running standard deviation around the mean). (c) Global mean percent change in ocean oxygen (100–600 m depth), relative to 1995–2014. Assessed observational trends and ''very likely'' range are from the SROCC assessment, and span 1970–2010 centred on 2005. (d) Global mean surface pH. Assessed observational change spans 1985–2019, from the CMEMS SOCAT-based reconstruction (shading around the global mean shows the 90% confidence interval). (e), (f) : Ice sheet mass changes. Projected ice-sheet changes are shown as median, 5–95% range (light shading), and 17–83% range (dark shading) of cumulative mass loss and sea level equivalent from ISMIP6 emulation under SSP1-2.6 and SSP5-8.5 (shading and bold line), with individual emulated projections as thin lines. Median (dot), 17–83% range (thick vertical bar), and 5–95% range (thin vertical bar) in 2100 are shown as vertical bars on the right-hand side of each panel, from ISMIP6, ISMIP6 emulation, and LARMIP-2. Observation-based estimates: For Greenland (e), for 1972–2018 (Mouginot), for 1992–2016 (Bamber), for 1992–2020 (IMBIE) and total estimated mass loss range for 1840–1972 (Box). For Antarctica (f), estimates based on satellite data combined with simulated surface mass balance and glacial isostatic adjustment for 1992–2020 (IMBIE), 1992–2016 (Bamber), and 1979–2017 (Rignot). Left inset maps: mean Greenland elevation changes 2010–2017 derived from CryoSat-2 radar altimetry (e) and mean Antarctica elevation changes 1978–2017 derived from restored analogue radar records (f). Right inset maps: ISMIP6 model mean (2093–2100) projected changes under the MIROC5 climate model for the RCP8.5 scenario. Links to chapters 2.3.3; 2.3.4; 3.5.4; 4.3.2; 5.3.2; 5.3.3; 5.6.3; 9.2.3; 9.4.1; 9.4.2; Box 9.2; Box 9.2, Figure 1; Figures 9.10, 9.17 and 9.18

It is ''virtually certain'' that the global ocean has warmed since at least 1971, representing about 90% of the increase in the global energy inventory (Section TS.3.1). The ocean is currently warming faster than at any other time since at least the last deglacial transition ( ''medium confidence'' ), with warming extending to depths well below 2000 m ( ''very high confidence'' ). It is ''extremely likely'' that human influence was the main driver of this recent ocean warming. Ocean warming will continue over the 21st century ( ''virtually certain'' ), and will ''likely'' continue until at least to 2300 even for low CO <sub>2</sub> emissions scenarios. Ocean warming is irreversible over centuries to millennia ( ''medium confidence'' ), but the magnitude of warming is scenario-dependent from about the mid-21st century ( ''medium confidence'' ). The warming will not be globally uniform, with heat primarily stored in Southern Ocean water-masses and weaker warming in the subpolar North Atlantic ( ''high confidence'' ). Limitations in the understanding of feedback mechanisms limit our confidence in future ocean warming close to Antarctica and how this will affect sea ice and ice shelves. Links to chapters 2.3.3, 3.5.1, 4.7.2, 7.2.2, 9.2.2, 9.2.3, 9.2.4, 9.3.2, 9.6.1, Cross-Chapter Box 9.1

Global mean SST has increased since the beginning of the 20th century by 0.88 [0.68 to 1.01] °C, and it is ''virtually certain'' it will continue to increase throughout the 21st century, with increasing hazards to marine ecosystems ( ''medium confidence'' ). Marine heatwaves have become more frequent over the 20th century ( ''high confidence'' ), approximately doubling in frequency ( ''high confidence'' ) and becoming more intense and longer since the 1980s ( ''medium confidence'' ). Most of the marine heatwaves over 2006–2015 have been attributed to anthropogenic warming ( ''very likely'' ) . Marine heatwaves will continue to increase in frequency, with a ''likely'' global increase of 2–9 times in 2081–2100 compared to 1995–2014 under SSP1-2.6, and 3–15 times under SSP5-8.5 (Figure TS.11a), with the largest changes in the tropical and Arctic ocean. Links to chapters 2.3.1, Cross-Chapter Box 2.3, 9.2.1, Box 9.2, 12.4.8

Observed upper-ocean stratification (0–200 m) has increased globally since at least 1970 ''('' ''virtually certain'' '')'' . Based on recent refined analyses of the available observations, there is ''high confidence'' that it increased by 4.9 ± 1.5% from 1970–2018, which is about twice as much as assessed in SROCC, and will continue to increase throughout the 21st century at a rate depending on the emissions scenario ( ''virtually certain'' ). Links to chapters 2.3.3, 9.2.1

It is ''virtually certain'' that since 1950 near-surface high-salinity regions have become more saline, while low-salinity regions have become fresher, with ''medium confidence'' that this is linked to an intensification of the hydrological cycle (Box TS.6). It is ''extremely likely'' that human influence has contributed to this salinity change and that the large-scale pattern will grow in amplitude over the 21st century ( ''medium confidence'' ). Links to chapters 2.3.3, 3.5.2, 9.2.2, 12.4.8

The AMOC was relatively stable during the past 8000 years ( ''medium confidence'' ). There is ''low confidence'' in the quantification of AMOC changes in the 20th century because of ''low agreement'' in quantitative reconstructed and simulated trends, missing key processes in both models and measurements used for formulating proxies, and new model evaluations. Direct observational records since the mid-2000s are too short to determine the relative contributions of internal variability, natural forcing and anthropogenic forcing to AMOC change ( ''high confidence'' ). An AMOC decline over the 21st century is ''very likely'' for all SSP scenarios (Figure TS.11b); a possible abrupt decline is assessed further in Box TS.3. Links to chapters 2.3.3, 3.5.4, 4.3.2, 8.6.1, 9.2.3, Cross-Chapter Box 12.3

There is ''high confidence'' that many ocean currents will change in the 21st century in response to changes in wind stress. There is ''low confidence'' in 21st century change of Southern Ocean circulation, despite ''high confidence'' that it is sensitive to changes in wind patterns and increased ice-shelf melt. Western boundary currents and subtropical gyres have shifted poleward since 1993 ( ''medium confidence'' ). Subtropical gyres, the East Australian Current Extension, the Agulhas Current, and the Brazil Current are projected to intensify in the 21st century in response to changes in wind stress, while the Gulf Stream and the Indonesian Throughflow are projected to weaken ( ''medium confidence'' ). All of the four main eastern boundary upwelling systems are projected to weaken at low latitudes and intensify at high latitudes in the 21st century ( ''high confidence'' ). Links to chapters 2.3.3, 9.2.3

It is ''virtually certain'' that surface pH has declined globally over the last 40 years and that the main driver is uptake of anthropogenic CO <sub>2</sub> . Ocean acidification and associated reductions in the saturation state of calcium carbonate – a constituent of skeletons or shells of a variety of marine organisms – is expected to increase in the 21st century under all emissions scenarios ( ''high confidence'' ). A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ), and surface ocean pH as low as recent times is uncommon in the last 2 million years ( ''medium confidence'' ). There is ''very high confidence'' that present-day surface pH values are unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at least that time. Over the past 2–3 decades, a pH decline in the ocean interior has been observed in all ocean basins ( ''high confidence'' ) (Figure TS.11d). Links to chapters 2.3.3, 2.3.4, 3.6.2, 4.3.2, 5.3.2, 5.3.3, 5.6.3, 12.4.8

Open-ocean deoxygenation and expansion of oxygen minimum zones have been observed in many areas of the global ocean since the mid 20th century ( ''high confidence'' ), in part due to human influence ( ''medium confidence'' ). Deoxygenation is projected to continue to increase with ocean warming ( ''high confidence'' ) (Figure TS.11c). Higher climate sensitivity and reduced ocean ventilation in CMIP6 compared to CMIP5 results in substantially greater projections of subsurface (100–600 m) oxygen decline than reported in SROCC for the period 2080–2099. Links to chapters 2.3.3, 2.3.4, Cross-Chapter Box 2.4, 3.6.2, 5.3.3, 12.4.8

Over at least the last two decades, the geographic range of many marine organisms has shifted towards the poles and towards greater depths ( ''high confidence'' ), indicative of shifts towards cooler waters. The range of a smaller subset of organisms has shifted equatorward and to shallower depths ( ''high confidence'' ). Phenological metrics associated with the life cycles of many organisms have also changed over the last two decades or longer ( ''high confidence'' ). Since the changes in the geographical range of organisms and their phenological metrics have been observed to differ with species and location, there is the possibility of disruption to major marine ecosystems. Links to chapters 2.3.4

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