Editing IPCC:AR6/WGI/Chapter-9 (section)

=== Ocean Heat and Salinity ===

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'''At the ocean surface, temperature has, on average, increased by 0.88 [0.68 to 1.01] °C between 185''' '''0–1''' '''900 and 201''' '''1–2''' '''020, with 0.60 [0.44 to 0.74] °C of this warming having occurred since 1980. The ocean surface temperature is projected to increase between 1995 to 2014 and 2081 to 2100 on average by 0.86 [0.43 to 1.47,''' ''likely'' '''range] °C in SSP1-2.6 and by 2.89 [2.01 to 4.07,''' ''likely'' '''range] °C in SSP5-8.5.''' Since the 1950s, the fastest surface warming has occurred in the Indian Ocean and in western boundary currents, while ocean circulation has caused slow warming or surface cooling in the Southern Ocean, equatorial Pacific, North Atlantic, and coastal upwelling systems ( ''very high'' ''confidence'' ). At least 83% of the ocean surface will ''very likely'' warm over the 21st century in all Shared Socio-economic Pathways (SSP) scenarios. {2.3.3, 9.2.1}

'''The heat content of the global ocean has increased since at least 1970, and will continue to increase over the 21st century''' ( ''virtually certain'' '''). The associated warming will''' ''likely'' '''continue until at least 2300, even for low-emissions scenarios, because of the slow circulation of the deep ocean.''' Ocean heat content has increased from 1971 to 2018 by 0.396 [0.329 to 0.463, ''likely'' range] yottajoules and will ''likely'' increase until 2100 by two to four times that amount under SSP1-2.6 and four to eight times that amount under SSP5-8.5. The long time scale also implies that the amount of deep-ocean warming will only become scenario-dependent after about 2040 ( ''medium confidence'' ), and that the warming is irreversible over centuries to millennia ( ''very high confidence'' ). On annual to decadal time scales, the redistribution of heat by the ocean circulation dominates spatial patterns of temperature change ( ''high confiden'' ''ce'' ). At longer time scales, the spatial patterns are dominated by additional heat, primarily stored in water masses formed in the Southern Ocean, and by weaker warming in the North Atlantic where heat redistribution caused by changing circulation counteracts the additional heat input through the surface ( ''high confidence'' ). {9.2.2, 9.2.4, 9.6.1, Cross-Chapter Box 9.1}

'''Marine heatwaves – sustained periods of anomalously high near-surface temperatures that can lead to severe and persistent impacts on marine ecosystems – have become more frequent over the 20th century''' ( ''high confidence'' '''). Since the 1980s, they have approximately doubled in frequency''' ( ''high confidence'' ''') and have become more intense and longer''' ( ''medium confidence'' ''').''' This trend will continue, with marine heatwaves at global scale becoming four times [2 to 9, ''likely'' range] more frequent in 2081–2100 compared to 1995–2014 under SSP1-2.6, and eight times [3 to 15, ''likely'' range] more frequent under SSP5-8.5. The largest changes will occur in the tropical ocean and the Arctic ( ''medium confidence'' ). {Box 9.2}

'''The upper ocean has become more stably stratified since at least 1970 over the vast majority of the globe''' ( ''virtually certain'' '''), primarily due to surface-intensified warming and hi''' '''gh-lati''' '''tude surface freshening''' ( ''very high confidence'' ''').''' Changes in ocean stability affect vertical exchanges of surface waters with the deep ocean and large-scale ocean circulation. Based on recent refined analyses of the available observations, the global 0–200 m stratification is now assessed to have increased about twice as much as reported by SROCC, with a 4.9 ± 1.5% increase from 1970 to 2018 ( ''high confidence'' ) and even higher increases at the base of the surface mixed layer. Upper-ocean stratification will continue to increase throughout the 21st century ( ''virtually certain'' ). {9.2.1}

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