Editing IPCC:AR6/WGI/TS (section)

===== TS.4.3.2.9 Ocean =====

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'''The Indian Ocean, western equatorial Pacific Ocean and western boundary currents have warmed faster than the global average ( ''very high confidence'' ), with the largest changes in the frequency of marine heatwaves (MHWs) projected in the western tropical Pacific and the Arctic Ocean ( ''medium confidence'' ). The Pacific and Southern Ocean are projected to freshen and the Atlantic to become more saline ( ''medium confidence'' ). Anthropogenic warming is ''very likely'' to further decrease ocean oxygen concentrations, and this deoxygenation is expected to persist for thousands of years ( ''medium confidence'' ). Arctic sea ice losses are projected to continue, leading to a practically ice-free Arctic in September by the end of the 21st century under high CO <sub>2</sub> emissions scenarios ( ''high confidence'' ). Links to chapters 2.3, 5.3, 9.2, 9.3, Box 9.2, 12.3.6, 12.4.8'''
In addition to the main changes summarized above and in Section TS.4.3.1, further details are given below.

'''Ocean surface temperature:''' The Southern Ocean, the eastern equatorial Pacific, and the North Atlantic Ocean have warmed more slowly than the global average or slightly cooled. Global warming of 2°C above 1850–1900 levels would result in the exceedance of numerous hazard thresholds for pathogens, seagrasses, mangroves, kelp forests, rocky shores, coral reefs and other marine ecosystems ( ''medium confidence'' ). Links to chapters 9.2.13, 12.4.8

'''Marine heatwaves:''' Moderate increases in MHW frequency are projected for mid-latitudes, and only small increases are projected for the Southern Ocean ( ''medium confidence'' ). Under the SSP5-8.5 scenario, permanent MHWs (more than 360 days per year) are projected to occur in the 21st century in parts of the tropical ocean, the Arctic Ocean, and around 45°S; however, the occurrence of such permanent MHWs can be largely avoided under the SSP1-2.6 scenario. Links to chapters Box 9.2, 12.4.8

'''Ocean acidity:''' With the rising CO <sub>2</sub> concentration, the ocean surface pH has declined globally over the past four decades ( ''virtually certain'' ). Links to chapters 2.3.3.5, 5.3.3.2, 12.4.8

'''Ocean salinity:''' At the basin scale, it is ''very likely'' that the Pacific and the Southern Ocean have freshened while the Atlantic has become more saline. Links to chapters 2.3.3.2, 9.2.2.2, 12.4.8

'''Dissolved oxygen:''' In recent decades, low oxygen zones in ocean ecosystems have expanded. Links to chapters 2.3.4.2, 5.3.3.2, 12.4.8

'''Sea ice:''' Arctic perennial sea ice is being replaced by thin, seasonal ice, with earlier spring melt and delayed fall freeze up. There is no clear trend in the Antarctic sea ice area over the past few decades and ''low confidence'' in its future change. Links to chapters 2.3.2.1.1, 9.3.1.1, 12.4.8, 12.4.9

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