The 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. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 millimeters per year. 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. Climate models based on the current rate of increase in greenhouse gases, however, indicate that sea level may rise at about 4 millimeters per year reaching 0.22 to 0.44 meters above 1990 levels by the period 2090-2099 (IPCC 2007).
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.
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. However, current estimates indicate that mass balance for the Antarctic ice sheet is in approximate equilibrium and may represent only about 10 percent of the current contribution to sea level rise coming from glaciers. However, some localized areas of the Antarctic have recently shown significant negative balance, e.g., Pine Island and Thwaites Glaciers, and glaciers on the Antarctic Peninsula. There is still much uncertainty about accumulation rates in Antarctica, especially on the East Antarctic Plateau. The Greenland Ice Sheet may be contributing about 30 percent of all glacier melt to rising sea level. Furthermore, recent observations show evidence for increased ice flow rates in some regions of the Greenland Ice Sheet, suggesting that ice dynamics may be a key factor in the response of coastal glaciers and ice sheets to climate change and their role in sea level rise.
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).
Sea 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. During the period 1960-1990, glaciers contributed 0.37 +/- 0.16 millimeters per year to sea level while during 1990-2004, the contribution increased to 0.77+/-0.22 millimeters per year (IPCC 2007). However, the latest predictions suggest possibly an even greater contribution by small glaciers and icecaps. Meier et al. (2007) conclude that with the current acceleration of glacier contribution to sea level rise, the total contribution from small glaciers and ice caps by the year 2100 is expected to be 240 +/- 128 millimeters, which represents an average annual increase of more than 2.0 millimeters per year.
Small 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.
Last updated: 12 March 2009