SOTC: Contribution of the Cryosphere to Changes in Sea Level

Global sea level rose by about 120 meters during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century when the instrumental record of sea level change shows evidence for an onset of sea level rise. In the mid-19th century, the rate of sea level rise probably started exceeding the mean rate from the previous 2,000 years. Satellite altimetry observations, available since the early 1990s, provide more accurate sea level data with nearly global coverage and indicate that since 1993 sea level has been rising at a rate of about 3 millimeters per year. Sea level has risen since the early 1970s due to a combination of ocean thermal expansion and glacier mass loss (IPCC 2007 and IPCC 2013).

Contribution from the cryosphere

Which of the topics discussed so far in State of the Cryosphere have the potential to contribute to a rising sea level during a warming climate? Several — but some more than others.

  • The seasonal snow cover melts during spring and summer and much of that water flows into rivers which eventually reach the sea. However, this is a process with a seasonal hydrologic cycle. With no net increase in seasonal snowfall over time, and no significant increase has occurred in recent decades, melting snow is not a factor that contributes to annual net sea level rise.
  • Sea ice and ice shelves are already located in the ocean and thus do not have any further significant influence on sea level after they melt.
  • As permafrost thaws, and the ice within the soil melts, an additional amount of liquid water becomes available but how much of this water actually reaches streams and rivers, and eventually the sea, is not well known at this time.
  • The most significant contributors to sea level within the current climate are glaciers, but ice sheets in Greenland and Antarctica hold the potential to eventually dwarf other cryospheric contributors to sea level rise.

Current conditions: contribution from melting glaciers

Global sea level is currently rising as a result of both ocean thermal expansion and glacier melt, with each accounting for about half of the observed sea level rise, and each caused by recent increases in global mean temperature. For the period 1961-2003, the observed sea level rise due to thermal expansion was 0.42 millimeters per year and 0.69 millimeters per year due to total glacier melt (small glaciers, ice caps, ice sheets) (IPCC 2007). Between 1993 and 2003, the contribution to sea level rise increased for both sources to 1.60 millimeters per year and 1.19 millimeters per year respectively (IPCC 2007).

Antarctica and Greenland, the world's largest ice sheets, make up the vast majority of the Earth's ice. If these ice sheets melted entirely, sea level would rise by more than 70 meters. These ice sheets had long been believed to be in equilibrium, but more recent studies indicate growing ice sheet imbalance, with more mass leaving the ice sheet than is replaced by snowfall, especially in West Antarctica. See Ice Sheets for more information.

In contrast to the polar regions, the network of lower latitude small glaciers and ice caps, although making up only about four percent of the total land ice area or about 760,000 square kilometers, may have provided as much as 60 percent of the total glacier contribution to sea level change since 1990s (Meier et al. 2007).

Contribution to Sea Level by Glaciers, Ice Caps, and Ice SheetsSea level rise contributors: Comparison of volume (white), area (grey) and percent contribution to sea level rise (red) by small glaciers and ice caps, and the Greenland and Antarctic Ice Sheets. Image courtesy (Meier et al. 2007).

How glaciers' contribution to sea level is computed

Global mass balance data are transformed to sea-level equivalent by first multiplying the ice thickness (meters) lost to melting by the density of ice (about 900 kilograms per cubic meter), to obtain a water equivalent thickness, and then multiplying by the surface area of these "small" glaciers (about 760,000 square kilometers). This provides an annual average mass balance of approximately -0.273 meters for the period 1961 to 2005. When dividing the mass balance value by the surface area of the oceans (361.6 million square kilometers), the final result is 0.58 millimeters of sea level rise per year. The Glacier Contribution to Sea Level graph demonstrates how the contribution from melting glaciers began increasing at a faster rate starting in the 1990s. This is in agreement with high-latitude air temperature records. The IPCC (2013) stated that it was "very likely" (at least 90 percent confidence) that the mean annual global rate of ocean level increase was 1.5 to 1.9 millimeters between 1901 and 2010, 1.7 to 2.3 millimeters between 1971 and 2010, and 2.8 to 3.6 millimeters between 1993 and 2010.

Small Glacier/Ice Cap Contribution to Sea Level, Air TemperatureSmall glacier/ice cap contribution: The cumulative contribution to sea level from small glaciers and ice caps (red) plotted with the annual global surface air temperature anomaly (blue). Image courtesy Mark Dyurgerov, Institute of Arctic and Alpine Research, University of Colorado, Boulder.

The IPCC Fifth Assessment Report contained significant uncertainty in its projections for glacier contributions to sea level rise over the course of the 21st century. An update (Radić et al. 2014) incorporated a newly released inventory showing the outline of nearly every glacier on Earth, 14 global climate models, and two emission scenarios: RCP4.5 (intermediate emissions) and RCP8.5 (high emissions). The intermediate-emission scenario indicated a sea level rise of 155 ± 41 millimeters, and the high-emission scenario indicated a rise of 216 ± 44 millimeters, though the authors conceded that large uncertainties remain.

Sea level projections from Radic et al. 2014Sea level rise projections: These graphs show projected glacier volume loss  and corresponding sea level equivalent over the 21st century. Graphs courtesy (Radić et al. 2014).

The IPCC Fifth Assessment Report relied on a few hundred glaciers to make its predictions. A subsequent study (Zemp et al. 2019) drew from more than 19,000 glaciers worldwide. Inferring from observed volume changes in those glaciers, the study found that mountain glaciers outside of the polar regions, as well as peripheral glaciers fringing the Greenland and Antarctic Ice Sheets, contributed 27 ± 22 millimeters to global mean sea level rise from 1961 to 2016.

The Glacier Model Intercomparison Project (GlacierMIP) projected future sea level rise, for the year 2100 relative to the year 2015, driven by glacier mass loss outside the Greenland and Antarctic Ice Sheets (but also including glaciers along the peripheries of those ice sheets). GlacierMIP used six global glacier models using four emission scenarios: RCP2.6, RCP4.5, RCP6.0, and RCP8.5. Averaged results from the models ranged from 94 ± 25 millimeters for RCP2.6, to 200 ± 44 millimeters for RCP8.5. (Hock et al. 2019).

See Mountain Glaciers for more information.

Potential contribution of ice sheets

Destabilization of Antarctic and Greenland ice sheets has the potential to raise global sea level, ultimately dwarfing contributions from mountain glaciers, although sea level would not rise uniformly everywhere.

Sea level rise diagramsContributions from Greenland and Antarctica: These maps show the effects of relative sea level change when ice is lost from Greenland (left) and West Antarctica (right). More severe levels of sea level rise appear in darker shades of orange and red. The green line indicates sea level matching current levels. Ice loss from West Antarctica amplifies sea level rise along the east and west coastlines of North America. Image courtesy (Bamber and Riva 2010).

No matter what contributes to global sea level rise, individual locations will experience different changes in sea level due to local factors. A significant factor in many areas involves ice that melted long ago. A glacier or ice sheet exerts pressure on the land directly underneath it, and can push adjacent land areas up, the same way gently pressing one part of a balloon will cause another part of that balloon to bulge. Remove your hand, and the balloon will revert to its old shape. Some areas, such as Juneau, Alaska, are actually rising because they are still adjusting to the retreat of glaciers after the Little Ice Age. Multiple locations along the U.S. East Coast, however, are slowly sinking because of the retreat of the Laurentide Ice Sheet, which previously exerted so much pressure on land to the north.

Going forward, ice loss in Greenland and Antarctica will have varying effects on Earth's oceans. That is because the ice sheet itself has mass that exerts a gravitational force on the surrounding ocean. A loss of mass from the ice sheet causes nearby ocean levels to fall as the mass and gravity of the ice sheet decreases. However, since overall sea level would rise, the sea level increase in areas far from the ice sheet would be higher than the global average. Consequently, ice sheet contribution to sea level rise—even if it were the same amount—would have different impacts, depending on whether the contribution came from Greenland or Antarctica (Bamber and Riva 2010).

Sea level rise diagramsProjected sea level rise: These maps show projected sea level rise from all sources (not just ice sheet loss or contributions from the cryosphere) under RCP 8.5 in 2050, 2100, and 2300. Note that each map has a different scale, so red represents 60 centimeters of sea level rise in 2050, and 1,500 centimeters of sea level rise in 2300. Image courtesy (Kopp et al. 2017).

How much ice sheets will contribute to sea level rise in the coming decades and centuries will depend in a large part on human activity. Representative Concentration Pathways (RCPs) are scenarios for rates and magnitudes of climate change driven by greenhouse gas emissions. RCP 2.6 assumes low greenhouse gas emissions; RCP 8.5 assumes high greenhouse gas emissions; RCP 4.5 assumes greenhouse gas emissions in between 2.6 and 8.5. A 2014 study estimated global sea level rise—from all sources, not just ice-sheet melt—at 90 percent probability for the 21st century: 0.3 to 0.8 meters under RCP 2.6, 0.4 to 0.9 meters under RCP 4.5, and 0.5 to 1.2 meters under RCP 8.5.

Although thermal expansion has been projected to contribute the most to sea level rise, the potential of large contributions from the Antarctic Ice Sheet has added significant uncertainty to predictions. This is the factor responsible for the most of the spread in projected sea level rise by the late 21st century (Kopp et al. 2014). A subsequent study incorporated physical processes on ice shelves that might accelerate ice sheet loss and sea level rise. One process was hydrofracturing, a process where surface water slices through the ice due to its higher density. Another process was marine ice cliff instability, arising from the relative weakness of ice. Ice cliffs more than about 90 meters tall are inherently unstable because ice at earthly temperatures is too weak to support the edifice. Incorporating these processes in some models leads to higher projections of global mean sea level rise by the year 2100: 0.26 to 0.98 meters under RCP 2.6, and 0.93 to 2.43 meters under RCP 8.5. The greater spread between RCPs indicated a greater role for emissions in ice sheet contribution to sea level rise (Kopp et al. 2017).

These studies underscore an important point: Being far away from a melting ice sheet is no source of safety. In fact, the opposite is true. Because of changes in Earth's gravity field resulting from ice sheet mass loss, ocean sea level will actually drop near the areas of melt and rise elsewhere. Miami, Tokyo, Shanghai, and Los Angeles are just a few of the coastal cities that can expect higher sea levels due to faraway ice-sheet melt (Larour et al. 2017).

A somewhat encouraging finding relates to Thwaites Glacier. Draining the central West Antarctic Ice Sheet, Thwaites Glacier has the potential to contribute significantly to sea level rise, and this glacier has doubled its ice-loss rate since the 1990s. An upside of ice loss, though, is that reduction in the overlying weight causes the bedrock to rebound, slowing further glacier flow. Benefits of rebounding bedrock won't become apparent in the coming century, but by the year 2350, the rebounding rock could reduce the glacier's contribution to rising sea level by more than 25 percent (Larour et al. 2019, Steig 2019).

See Ice Sheets for more information.

Last updated: 24 June 2019