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

== 9.1 Introduction ==

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This chapter provides a holistic assessment of the physical processes underlying global and regional changes in the ocean, cryosphere and sea level, as well as improved understanding of observed, attributed and projected future changes since the IPCC Fifth Assessment Report (AR5) and the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; see outline in Figure 9.1). The ocean and cryosphere (defined as the frozen components of the Earth system such as sea ice, ice sheets, glaciers, permafrost and snow) exchange heat and freshwater with the atmosphere and each other (Figure 9.2). In a warming climate, the combined effects of thermal expansion of seawater and melting of the terrestrial cryosphere result in global mean sea level rise (Box 9.1).

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[[File:64407deea65ca92502fd39afd6f95c62 IPCC_AR6_WGI_Figure_9_1.png]]

'''Figure''' '''9.1 |''' '''Visual guide to Chapter 9.''' Sections dealing with the cryosphere are highlighted with a snowflake.

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[[File:9609c0c07190b8172cffac20ae22874d IPCC_AR6_WGI_Figure_9_2.png]]

'''Figure 9.2''' '''|''' '''Components of ocean, cryosphere and sea level assessed in this chapter. (a)''' Schematic of processes (mCDW=modified Circumpolar Deep Water, GIA=Glacial Isostatic Adjustment). White arrows indicate ocean circulation. Pinning points indicate where the grounding line is most stable and ice-sheet retreat will slow. '''(b)''' Geographic distribution of ocean and cryosphere components (numbers indicate glacierized regions ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] )). See Figures 9.20 and 9.21 for labels. Sea ice shaded to indicate the annual mean concentration. Green ocean colours indicate larger surface current speed. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

Ocean acidification and deoxygenation are covered in Chapter 5, and regional changes to the ocean and cryosphere are covered in [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] and the Atlas. Ecosystem range shifts and climate risk for marine biodiversity associated with ocean change are assessed in AR6 Working Group II (WGII). The notion of ‘climate velocity’ often used in impact studies, which is defined as the speed and direction at which a climate variable moves across a corresponding spatial field, is underpinned by the assessment of changes in the physical characteristics of the ocean provided in this chapter.

There are two major advances of this chapter compared with AR5 and SROCC facilitated by community efforts. The first is the temporal and spatial increase in observations of both the ocean and the cryosphere ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.1.1|Section 1.5.1.1]] ). In particular, extended observations have allowed for improved assessment of past change and closure of both the energy and sea level budgets in a consistent way (Cross-Chapter Box 9.1) and the sea level budget for the last century ( [[#9.6.1.1|Section 9.6.1.1]] ). Higher resolution observations have revealed the details of the Atlantic Meridional Overturning Circulation (AMOC; [[#9.2.3.1|Section 9.2.3.1]] ) and globally resolved glacier changes for the first time ( [[#9.5.1.1|Section 9.5.1.1]] ). Improved methodology has resulted in a doubling of the assessed level of observed increase in global ocean 0–200 m stratification compared to SROCC assessment ( [[#9.2.1.3|Section 9.2.1.3]] ).

The second advance is the use of a hierarchy of models and emulators to update projections of oceanic, cryospheric and sea level change arising from Coupled Model Intercomparison Project Phase 6 (CMIP6) and related projects ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.4.3|Section 1.5.4.3]] , Table 1.3, and Annex II). <sup>[[#footnote-003|2]]</sup> The CMIP6 included an ice-sheet modelling intercomparison for the first time. Particular modelling advances relevant to this chapter are the increase in ocean resolution in the High Resolution Model Intercomparison Project (HighResMIP) and Ocean Model Intercomparison Project phase 2 (OMIP-2) experiments (Sections 1.5.3.1 and 9.2), projections of future glacier (GlacierMIP) and ice sheet (ISMIP6) and Linear Antarctic Response Model Intercomparison Project (LARMIP-2) response from multi-model studies (Sections 9.5.1 and 9.4, and Box 9.3), and new methods to synthesize ocean and cryosphere models into sea level projections for all Shared Socieo-economic Pathway scenarios (SSPs; Sections 1.6.1, 9.4.1.3, 9.4.2.5 and 9.6.3, and Cross-Chapter Box 1.4) and warming levels (Sections 9.6.3 and 1.6.2, and Cross-Chapter Box 11.1). In particular, sea level projections and the individual contributions ( [[#9.6.3.3|Section 9.6.3.3]] ) are consistent with equilibrium climate sensitivity and surface temperature assessments across this Report (Box 4.1 and Cross-Chapter Box 7.1).

There are other advances in scientific understanding. In the cryosphere, this chapter assesses how fast-responding elements (sea ice, permafrost and snow; Sections 9.3, 9.5.2 and 9.5.3) track warming levels across observations and projections independent of scenario, process understanding of uncertainty in Antarctic Ice Sheet projections ( [[#9.4.2|Section 9.4.2]] and Box 9.4) and new insight into thresholds for Arctic sea ice ( [[#9.3.1.1|Section 9.3.1.1]] ) and Greenland and West Antarctic ice sheets (Sections 9.4.1.4 and 9.4.2.6). In the ocean, process understanding of ocean heat uptake ( [[#9.2.2.1|Section 9.2.2.1]] and Cross-Chapter Box 5.3) and observed changes in ocean stratification ( [[#9.2.1.3|Section 9.2.1.3]] ) have implications for ocean biogeochemistry are also important.

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Box 9.1 | Key Processes Driving Sea Level Change

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Sea level change arises from processes acting on a range of spatial and temporal scales, in the ocean, cryosphere, solid Earth, atmosphere and on land (Figure 9.2). '''Relative sea level (RSL) change''' is the change in local mean sea surface height relative to the sea floor, as measured by instruments that are fixed to the Earth’s surface (e.g., tide gauges). This reference frame is used when considering coastal impacts, hazards and adaptation needs. In contrast, '''geocentric sea level change''' is the change in local mean sea surface height with respect to the terrestrial reference frame, and is the sea level change observed with instruments from space. This box provides a brief summary of sea level processes using standard terminology ( [[#Gregory--2019|Gregory et al., 2019]] ).

'''Global processes'''

'''Global mean sea level change''' (Sections 9.6 and 2.3.3.3) is the change in volume of the ocean divided by the ocean surface area. It is the sum of changes in ocean density (‘global mean thermosteric sea level change’) and changes in the ocean mass as a result of changes in the cryosphere or land-water storage (‘barystatic sea level change’).

'''Steric sea level change''' is caused by changes in the ocean density and is composed of ‘thermosteric sea level change’ and ‘halosteric sea level change’. '''Thermosteric sea level''' '''change''' (also referred to as ‘thermal expansion’) occurs as a result of changes in ocean temperature: increasing temperature reduces ocean density and increases the volume per unit of mass. '''Halosteric sea level change''' occurs as a result of salinity variations: higher salinity leads to higher density and decreases the volume per unit of mass. Although both processes can be relevant on regional to local scales, thermosteric changes contribute to global mean sea level change, whereas global mean halosteric change is negligible ( [[#Gregory--2019|Gregory et al., 2019]] ). There is ''high confidence'' in the understanding of processes causing thermosteric sea level change ( [[#9.2.4.1|Section 9.2.4.1]] ).

'''The Greenland and Antarctic ice sheets''' are the largest reservoirs of frozen freshwater and therefore potentially the largest contributors to sea level rise. Fluctuations in ice-sheet volume arise from the imbalance between accumulation (either at the ice-sheet surface or on the underside of ice shelves) and loss from sublimation, surface and basal melting, and iceberg calving. Ice sheets discharge the majority of their mass through marine-terminating ice streams that are in some cases buttressed by floating ice shelves. Changes in the thickness and extent of the ice shelves due to melt from below, calving, or disintegration, as a result of surface meltwater penetrating crevasses, can affect the flow of the inland ice streams. There is ''medium confidence'' in ice-sheet processes but ''low confidence'' in their forcing (ocean changes and ice-shelf collapse) and in instability processes (Sections 9.4.1 and 9.4.2). <sup>[[#footnote-002|3]]</sup>

'''Glaciers''' contribute to sea level change via an imbalance between mass gain and mass loss processes, which leads to adjustments in the glacier geometry over an extended period of time, called the response time. The response time may range from a few years to a few hundred years. The glacial meltwater does not all flow immediately into the ocean: it can refreeze, feed rivers (where it may be extracted for domestic use), evaporate, or be stored in (proglacial) lakes or closed basins. There is ''medium'' to ''high confidence'' in the understanding of processes leading to sea level contributions from glaciers ( [[#9.5.1|Section 9.5.1]] ).

'''Land-water storage''' includes surface water, soil moisture, groundwater storage and snow, but excludes water stored in glaciers and ice sheets. Changes in land-water storage can be caused either by direct human intervention in the water cycle (e.g., storage of water in reservoirs by building dams in rivers, groundwater extraction for consumption and irrigation, or deforestation) or by climate variations (e.g., changes in the amount of water in internally drained lakes and wetlands, the canopy, the soil, the permafrost and

the snowpack). Land-water storage changes caused by climate variations may be indirectly affected by anthropogenic influences. It is difficult to assign a single confidence level to land-water storage as understanding can vary from ''low confidence'' in groundwater recharge processes to ''high confidence'' in water storage via snowpack changes (Sections 8.2.3 and 8.3.1.7).

'''Regional and local processes'''

'''Ocean dynamic sea level''' '''change''' refers to the change in mean sea level relative to the geoid and is associated with the circulation and density-driven changes in the ocean. Ocean dynamic sea level change varies regionally but by definition has a zero global mean. It includes the depression of the sea surface by atmospheric pressure. There is ''medium confidence'' in the understanding of ocean processes leading to dynamic sea level change ( [[#9.2.4.2|Section 9.2.4.2]] ).

'''Changes in Earth gravity, Earth rotation and viscoelastic solid Earth deformation''' '''(GRD)''' – result from the redistribution of mass between terrestrial ice and water reservoirs and the ocean. Contemporary terrestrial mass loss leads to elastic solid Earth uplift and a nearby RSL fall. (For a single source of terrestrial mass loss, this is within about 2000 km; for multiple sources, the distance depends on the interaction of the different RSL patterns.) Farther away (around more than 7000 km for a single source of terrestrial mass loss), RSL rises more than the global average, due to first-order gravitational effects. Earth deformation associated with adding water to the ocean and a shift of the Earth’s rotation axis towards the source of terrestrial mass loss leads to second-order effects that increase spatial variability of the pattern globally. GRD effects due to the redistribution of ocean water within the ocean itself are referred to as '''self-attraction and loading effects''' . There is ''high confidence'' in the understanding of GRD processes.

'''Glacial isostatic adjustment''' is ongoing GRD in response to past changes in the distribution of ice and water on Earth’s surface. On a time scale of decades to tens of millennia following mass redistribution, Earth’s mantle flows viscously as it evolves toward isostatic equilibrium, causing solid Earth movement and geoid changes, which can result in regional to local sea level variations. There is ''medium confidence'' in the understanding of glacial isostatic adjustment processes.

'''Vertical land motion''' is the change in height of the land surface or the sea floor and can have several causes in addition to elastic deformation associated with contemporary GRD and viscoelastic deformation associated with glacial isostatic adjustment. Subsidence (sinking of the land surface or sea floor) can occur through compaction of alluvial sediments in deltaic regions, removal of fluids such as gas, oil, and water, or drainage of peatlands. Tectonic deformation of the Earth’s crust can occur as a result of earthquakes and volcanic eruptions. There is ''medium confidence'' in the understanding of vertical land motion processes.

'''Extreme sea level''' is an exceptionally low or high local sea surface height arising from combined short-term phenomena (e.g., storm surges, tides and waves). RSL changes affect extreme sea levels directly by shifting the mean water levels, and indirectly by modulating the depth for propagation of tides, waves and/or surges. Extreme sea levels can be influenced by changes in the frequency, tracks, or strength of weather systems, or anthropogenic changes such as dredging. '''Extreme still water level''' refers to the combined contribution of RSL change, tides and storm surges. Wind-generated waves also contribute to coastal sea level. '''Extreme total water level''' is the extreme still water level plus wave setup (time-mean sea level elevation due to wave energy dissipation). When considering coastal impacts, swash (vertical displacement up the shore-face induced by individual waves) is also important and included in '''Extreme coastal water level''' . There is ''low'' to ''medium confidence'' in the understanding of extreme sea level processes (Sections 9.6.4 and 12.4).

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