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

=== Sea Level ===

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'''Global mean sea level (GMSL) rose faster in the 20th century than in any prior century over the last three millennia''' ( ''high confidence'' '''), with a 0.20 [0.15 to 0.25] m rise over the period 190''' '''1–2''' '''018''' ( ''high confidence'' '''). GMSL rise has accelerated since the late 1960s, with an average rate of 2.3 [1.6 to 3.1] mm yr''' –1 '''over the period 197''' '''1–2''' '''018 increasing to 3.7 [3.2 to 4.2] mm yr''' –1 '''over the period 200''' '''6–2''' '''018''' ( ''high confidence'' ''').''' New observation-based estimates published since SROCC lead to an assessed sea level rise over the period 1901–2018 that is consistent with the sum of individual components. Ocean thermal expansion (38%) and mass loss from glaciers (41%) dominate the total change from 1901 to 2018. The contribution of Greenland and Antarctica to GMSL rise was four times larger during 2010 '''–''' 2019 than during 1992 '''–''' 1999 ( ''high confidence'' ). Because of the increased ice-sheet mass loss, the total loss of land ice (glaciers and ice sheets) was the largest contributor to global mean sea level rise over the period 2006–2018 ( ''high confidence'' ). {2.3.3, 9.6.1, 9.6.2, Cross-Chapter Box 9.1, Table 9.A.1, Box 7.2}

'''At the basin scale, sea levels rose fastest in the Western Pacific and slowest in the Eastern Pacific over the period 199''' '''3–2''' '''018''' ( ''medium confidence'' ''').''' Regional differences in sea level arise from: ocean dynamics; changes in Earth gravity, rotation and deformation due to land ice and land-water changes; and vertical land motion. Temporal variability in ocean dynamics dominates regional patterns on annual to decadal time scales ( ''high confidence'' ). The anthropogenic signal in regional sea level change will emerge in most regions by 2100 ( ''medium confidence'' ). {9.2.4, 9.6.1}

'''Regional sea level change has been the main driver of changes in extreme still water levels across the quasi-global tide gauge network over the 20th century''' ( ''high confidence'' ''') and will be the main driver of a substantial increase in the frequency of extreme still water levels over the next century''' ( ''medium confidence'' ''').''' Observations show that high-tide flooding events that occurred five times per year during the period 1960–1980 occurred, on average, more than eight times per year during the period 1995–2014 ( ''high confidence'' ). Under the assumption that other contributors to extreme sea levels remain constant (e.g., stationary tides, storm-surge, and wave climate), extreme sea levels that occurred once per century in the recent past will occur annually or more frequently at about 19–31% of tide gauges by 2050 and at about 60% (SSP1-2.6) to 82% (SSP5-8.5) of tide gauges by 2100 ( ''medium confidence'' ). In total, such extreme sea levels will occur about 20 to 30 times more frequently by 2050 and 160 to 530 times more frequently by 2100 compared to the recent past, as inferred from the median amplification factors for SSP1-2.6, SSP2-4.5, and SSP5-8.5 ( ''medium confidence'' ). Over the 21st century, the majority of coastal locations will experience a median projected regional sea level rise within ±20% of the median projected GMSL change ( ''medium confidence'' ). {9.6.3, 9.6.4}

'''It is''' ''virtually certain'' '''that GMSL will continue to rise until at least 2100, because all assessed contributors to GMSL are''' ''likely'' '''to''' ''virtually certain'' '''to continue contributing throughout this century. Considering only processes for which projections can be made with at least''' ''medium confidence'' ''', relative to the period 199''' '''5–2''' '''014, GMSL will rise by 2050 between 0.18 [0.15 to 0.23,''' ''likely'' '''range] m (SSP1-1.9) and 0.23 [0.20 to 0.29,''' ''likely'' '''range] m (SSP5-8.5), and by 2100 between 0.38 [0.28 to 0.55,''' ''likely'' '''range] m (SSP1-1.9) and 0.77 [0.63 to 1.01,''' ''likely'' '''range] m (SSP5-8.5)''' '''.''' This GMSL rise is primarily caused by thermal expansion and mass loss from glaciers and ice sheets, with minor contributions from changes in land-water storage. These ''likely'' range projections do not include those ice-sheet-related processes that are characterized by deep uncertainty. {9.6.3}

'''Higher amounts of GMSL rise before 2100 could be caused by earlier-than-projected disintegration of marine ice shelves, the abrupt, widespread onset of marine ice sheet instability and marine ice cliff instability around Antarctica, and faster-than-projected changes in the surface mass balance and discharge from Greenland.''' These processes are characterized by deep uncertainty arising from limited process understanding, limited availability of evaluation data, uncertainties in their external forcing and high sensitivity to uncertain boundary conditions and parameters. In a low-likelihood, high-impact storyline, under high emissions such processes could in combination contribute more than one additional metre of sea level rise by 2100. {9.6.3, Box 9.4}

'''Beyond 2100, GMSL will continue to rise for centuries due to continuing deep-ocean heat uptake and mass loss of the Greenland and Antarctic ice sheets, and will remain elevated for thousands of years''' ( ''high confidence'' ''').''' Considering only processes for which projections can be made with at least ''medium confidence'' and assuming no increase in ice-mass flux after 2100, relative to the period 1995–2014, by 2150, GMSL will rise between 0.6 [0.4 to 0.9, ''likely'' range] m (SSP1-1.9) and 1.4 [1.0 to 1.9, ''likely'' range] m (SSP5-8.5). By 2300, GMSL will rise between 0.3 m and 3.1 m under SSP1-2.6, between 1.7 m and 6.8 m under SSP5-8.5 in the absence of marine ice cliff instability, and by up to 16 m under SSP5-8.5 considering marine ice cliff instability ( ''low confidence'' ). {9.6.3}

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