Ocean Heat and Salinity
At the ocean surface, temperature has on average increased by 0.88 [0.68–1.01] °C from 1850-1900 to 2011-2020, with 0.60 [0.44–0.74] °C of this warming having occurred since 1980. The ocean surface temperature is projected to increase from 1995–2014 to 2081–2100 on average by 0.86 [0.43–1.47, likely range] °C in SSP1-2.6 and by 2.89 [2.01–4.07, 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. At least 83% of the ocean surface will very likely warm over the 21st century in all SSP scenarios.
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-emission scenarios because of the slow circulation of the deep ocean. Ocean heat content has increased from 1971 to 2018 by [0.28–0.55] yottajoules and will likely increase until 2100 by 2 to 4 times that amount under SSP1-2.6 and 4 to 8 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 and that the warming is irreversible over centuries to millennia. On annual to decadal time scales, the redistribution of heat by the ocean circulation dominates spatial patterns of temperature change. 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.
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. Since the 1980s, they have approximately doubled in frequency and have become more intense and longer. This trend will continue, with marine heatwaves at global scale becoming 4 [2–9, likely range] times more frequent in 2081–2100 compared to 1995–2014 under SSP1-2.6, and 8 [3–15, likely range] times more frequent under SSP5-8.5. The largest changes will occur in the tropical ocean and the Arctic.
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 high-latitude surface freshening. 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 the SROCC, with a 4.9 ± 1.5% increase from 1970 to 2018 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).
The Atlantic Meridional Overturning Circulation (AMOC) will very likely decline over the 21st century for all SSP scenarios. There is medium confidence that the decline will not involve an abrupt collapse before 2100. For the 20th century, there is low confidence in reconstructed and modelled AMOC changes because of their low agreement in quantitative trends. The low confidence also arises from new observations that indicate missing key processes in both models and measurements used for formulating proxies and from new evaluations of modelled AMOC variability. This results in low confidence in quantitative projections of AMOC decline in the 21st century, despite the high confidence in the future decline as a qualitative feature based on process understanding.
Southern Ocean circulation and associated temperature changes in Antarctic ice-shelf cavities are sensitive to changes in wind patterns and increased ice-shelf melt. However, limitations in understanding feedback mechanisms involving the ocean, atmosphere and cryosphere, which are not fully represented in the current generation of climate models, generally limit our confidence in future projections of the Southern Ocean and of its forcing on Antarctic sea ice and ice shelves.
Many ocean currents will change in the 21st century as a response to changes in wind stress associated with anthropogenic warming. Western boundary currents have shifted poleward since 1993 consistent with a poleward shift of the subtropical gyres. Of the four eastern boundary upwelling systems, only the California current system has experienced some large-scale upwelling-favourable wind intensification since the 1980s. In the 21st century, consistent with projected changes in the surface winds, the East Australian Current Extension and Agulhas Current Extension will intensify, while the Gulf Stream and Indonesian Throughflow will weaken. Eastern boundary upwelling systems will change, with a dipole spatial pattern within each system of reduction at low latitude and enhancement at high latitude.
The Arctic Ocean will likely become practically sea ice–free during the seasonal sea ice minimum for the first time before 2050 in all considered SSP scenarios. There is no tipping point for this loss of Arctic summer sea ice. The practically ice-free state is projected to occur more often with higher greenhouse gas concentrations and will become the new normal for high-emission scenarios by the end of this century. Based on observational evidence, Coupled Model Intercomparison Project Phase 6 (CMIP6) models and conceptual understanding, the substantial satellite-observed decrease of Arctic sea ice area over the period 1979–2019 is well described as a linear function of global mean surface temperature, and thus of cumulative anthropogenic CO2 emissions, with superimposed internal variability. According to both process understanding and CMIP6 simulations, a practically sea ice–free state will likely be observed in some years before additional (post-2020) cumulative anthropogenic CO2 45 emissions reach 1000 GtCO2.
For Antarctic sea ice, regionally opposing trends and large interannual variability result in no significant trend in satellite-observed sea ice area from 1979 to 2020 in both winter and summer. The regionally opposing trends result primarily from changing regional wind forcing. There is low confidence in model simulations of past and future Antarctic sea ice evolution due to deficiencies of process representation, in particular at the regional level.
The Greenland Ice Sheet has lost 4890 [4140–5640] Gt mass over the period 1992–2020, equivalent to 13.5 [11.4–15.6] mm global mean sea level rise. The mass-loss rate was on average 39 [–3 to 80] Gt yr–1 over the period 1992–1999, 175 [131 to 220] Gt yr–1 over the period 2000–2009 and 243 [197 to 290] Gt yr–1 over the period 2010–2019. This mass loss is driven by both discharge and surface melt, with the latter increasingly becoming the dominating component of mass loss with high interannual variability in the last decade. The largest mass losses occurred in the Northwest and the Southeast of Greenland.
The Antarctic Ice Sheet has lost 2670 [1800–3540] Gt mass over the period 1992–2020, equivalent to 7.4 [5.0–9.8] mm global mean sea level rise. The mass-loss rate was on average 49 [–2 to 100] Gt yr–1 over the period 1992–1999, 70 [22 to 119] Gt yr–1 over the period 2000–2009 and 148 [94 to 202] Gt yr–1 over the period 2010–2019. Mass losses from West Antarctic outlet glaciers outpaced mass gain from increased snow accumulation on the continent and dominated the ice sheet mass losses since 1992. These mass losses from the West Antarctic outlet glaciers were mainly induced by ice shelf basal melt and locally by ice shelf disintegration preceded by strong surface melt. Parts of the East Antarctic Ice Sheet have lost mass in the last two decades.
Both the Greenland Ice Sheet (virtually certain) and the Antarctic Ice Sheet (likely) will continue to lose mass throughout this century under all considered SSP scenarios. The related contribution to global mean sea level rise until 2100 from the Greenland Ice Sheet will likely be 0.01–0.10 m under SSP 1-2.6, 0.04–0.13 m under SSP2-4.5 and 0.09–0.18 m under SSP5-8.5, while the Antarctic Ice Sheet will likely contribute 0.03–0.27 m under SSP1-2.6, 0.03–0.29 m under SSP2-4.5 and 0.03–0.34 m under SSP5-8.5. The loss of ice from Greenland will become increasingly dominated by surface melt, as marine margins retreat and the ocean-forced dynamic response of ice-sheet margins diminishes. In the Antarctic, dynamic losses driven by ocean warming and ice shelf disintegration will likely continue to outpace increasing snowfall this century. Beyond 2100, total mass loss from both ice sheets will be greater under high-emission scenarios than under low-emission scenarios. The assessed likely ranges consider those ice-sheet processes in whose representation in current models we have at least medium confidence, including surface mass balance and grounding-line retreat in the absence of instabilities. Under high-emission scenarios, poorly understood processes related to Marine Ice Sheet Instability and Marine Ice Cliff Instability, characterized by deep uncertainty, have the potential to strongly increase Antarctic mass loss on century to multi-century time scales.
Glaciers lost 6200 [4600–7800] Gt of mass (17.1 [12.7–21.5] mm global mean sea level equivalent) over the period 1993 to 2019 and will continue losing mass under all SSP scenarios. During the decade 2010 to 2019, glaciers lost more mass than in any other decade since the beginning of the observational record. For all regions with long-term observations, glacier mass in the decade 2010 to 2019 is the smallest since at least the beginning of the 20th century. Because of their lagged response, glaciers will continue to lose mass at least for several decades even if global temperature is stabilized. Glaciers will lose 29,000 [9,000–49,000] Gt and 58,000 [28,000–88,000] Gt over the period 2015–2100 for RCP2.6 and RCP8.5, respectively, which represents 18 [5–31] % and 36 [16–56] % of their early-21st-century mass, respectively.
Increases in permafrost temperature have been observed over the past three to four decades throughout the permafrost regions, and further global warming will lead to near surface permafrost volume loss. Complete permafrost thaw in recent decades is a common phenomenon in discontinuous and sporadic permafrost regions. Permafrost warmed globally by 0.29 [0.17–0.41, likely range] °C between 2007 and 2016. An increase in the active layer thickness is a pan-Arctic phenomenon, subject to strong heterogeneity in surface conditions. The volume of perennially frozen soil within the upper 3 m of the ground will decrease by about 25% per 1°C of global surface air temperature change (up to 4°C above pre industrial temperature).
Northern Hemisphere spring snow cover extent has been decreasing since and there is high confidence that this trend extends back to 1950. Further decrease of Northern Hemisphere seasonal snow cover extent is virtually certain under further global warming. The observed sensitivity of Northern Hemisphere snow cover extent to Northern Hemisphere land surface air temperature for 1981–2010 is –1.9 [–2.8 to –1.0, likely range] million km2 per 1°C throughout the snow season. It is virtually certain that Northern Hemisphere snow cover extent will continue to decrease as global climate continues to warm, and process understanding strongly suggests that this also applies to Southern Hemisphere seasonal snow cover. Northern Hemisphere spring snow cover extent will decrease by about 8% per 1°C of global surface air temperature change (up to 4°C above pre-industrial temperature).
Global mean sea level (GMSL) rose faster in the 20th century than in any prior century over the last three millennia, with a 0.20 [0.15–0.25] m rise over the period 1901 to 2018. GMSL rise has accelerated since the late 1960s, with an average rate of 2.3 [1.6–3.1] mm yr-1 over the period 1971–2018 increasing to 3.7 [3.2–4.2] mm yr-1 over the period 2006–2018. New observation-based estimates published since SROCC lead to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components. While ocean thermal expansion (38%) and mass loss from glaciers (41%) dominate the total change from 1901 to 2018, ice sheet mass loss has increased and accounts for about 35% of the sea level increase during the period 2006–2018.
At the basin scale, sea levels rose fastest in the Western Pacific and slowest in the Eastern Pacific over the period 1993–2018. 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. The anthropogenic signal in regional sea level change will emerge in most regions by 2100.
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 and will be the main driver of a substantial increase in the frequency of extreme still water levels over the next century. 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. 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. 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. 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.
It is virtually certain that global mean sea level will continue to rise through 2100, because all assessed contributors to global mean sea level 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 1995–2014 GMSL will rise by 2050 between 0.18 [0.15–0.23, likely range] m (SSP1-1.9) and 0.23 [0.20–0.30, likely range] m (SSP5-8.5), and by 2100 between 0.38 [0.28–0.55, likely range] m (SSP1-1.9) and 0.77 [0.63–1.02, 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.
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 characterised 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 meter of sea level rise by 2100.
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. 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 [0.4–0.9, likely range] m (SSP1-1.9) and 1.4 [1.0–1.9, 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.
Cryospheric Changes and Sea Level Rise at Specific Levels of Global
At sustained warming levels between 1.5°C and 2°C, the Arctic Ocean will become practically sea ice–free in September in some years; the ice sheets will continue to lose mass, but will not fully disintegrate on time scales of multiple centuries; there is limited evidence that the Greenland and West Antarctic Ice Sheets will be lost almost completely and irreversibly over multiple millennia; about 50–60% of current glacier mass excluding the two ice sheets and the glaciers peripheral to the Antarctic Ice Sheet will remain, predominantly in the polar regions; Northern hemisphere spring snow cover extent will decrease by up to 20% relative to 1995– 51 2014; the permafrost volume in the top 3 m will decrease by up to 50% relative to 1995–2014. Committed GMSL rise over 2000 years will be about 2-6 m with 2°C of peak warming (medium agreement, limited evidence).
At sustained warming levels between 2°C and 3°C, the Arctic Ocean will be practically sea ice–free throughout September in most years; there is limited evidence that the Greenland and West Antarctic Ice Sheets will be lost almost completely and irreversibly over multiple millennia; both the probability of their complete loss and the rate of mass loss will increase with higher temperatures; about 50–60% of current glacier mass outside Antarctica will be lost; Northern hemisphere spring snow cover extent will decrease by up to 30% relative to 1995–2014; permafrost volume in the top 3 m will decrease by up to 75% relative to 1995–2014. Committed GMSL rise over 2000 years will be about 4-10 m with 3°C of peak warming (medium agreement, limited evidence).
At sustained warming levels between 3°C and 5°C, the Arctic Ocean will become practically sea ice–free throughout several months in most years; near-complete loss of the Greenland Ice Sheet and complete loss of the West Antarctic Ice Sheet will occur irreversibly over multiple millennia; substantial parts or all of Wilkes Subglacial Basin in East Antarctica will be lost over multiple millennia; 60–75% of current glacier mass outside Antarctica will disappear; nearly all glacier mass in low latitudes, Central Europe, Caucasus, Western Canada and USA, 20 North Asia, Scandinavia and New Zealand will likely disappear; Northern Hemisphere spring snow cover extent will decrease by up to 50% relative to 1995–2014; permafrost volume in the top 3 m will decrease by up to 90% compared to 1995–2014. Committed GMSL rise over 2000 years will be about 12–16 m with 4°C of peak warming and 19–22 m with 5°C of peak warming (medium agreement, limited evidence).