Roger G. Barry
Professor of Geography and Director, World Data Center (WDC) for Glaciology, Boulder
Sea level change is driven by: the thermal expansion/contraction of sea water in response to warming/cooling of the ocean; additional runoff to the oceans from the melting of ice sheets and glaciers; and changes in ground and surface water storage associated with the building of dams, consumptive use for agriculture, industrial production and human consumption. Satellite altimetry data available since the early 1990's provides nearly global observations of sea level rising at a rate of about 3 mm per year. For the period 1993-2003, the observed sea level rise due to thermal expansion was 1.6 mm /yr and 1.2 mm/yr due to total land ice melt.
It is estimated that sea level rose by ~1.7 mm/yr during the twentieth century. Thermal expansion accounts for about half of this increase and land ice melt, mostly from mountain glaciers, about 20 to 25 percent. The contributions from changes in ice volume in Greenland and the Antarctic are uncertain, even as to sign of the change for Antarctica. (The Antarctic and Greenland ice sheets hold the majority of the earth's ice: an amount equivalent to 70 m of sea level). There is also considerable uncertainty in the sign of the change attributed to terrestrial water storage in reservoirs and aquifers.
The largest contributions of melt water are from shrinking mountain glaciers and ice caps. Over the period 1901–1962, contributions to sea level rise from glaciers in southern Alaska-northwestern Canada, central Asia including the Himalayas, and the southern Andes were estimated to be about 34, 16 and 12 percent of the total land ice contribution to sea level rise, respectively (Haeberli, 1998). A further 10 percent may have derived from the Arctic islands of North America and Eurasia, with the remainder coming from glaciers in other regions. This regional balance of glacier meltwater contributions to sea level change appears to have shifted: Dyurgerov and Meier (1997) report a loss of 512 km-squared area of ice from glaciers in Svalbard and 832 km-squared area from the Arctic islands over the period 1961 through 1993. These amounts represent 43 percent of the total ice loss in the northern hemisphere over the 32-year period. (Note that 400,000 km-squared area of ice represents 1m of global sea level.). Satellite altimetry data, available since the early 1990s, provide more accurate sea level data with nearly global coverage. They indicate that since 1993 sea level has been rising at a rate of about 3 mm per year. For the period 1993-2003, the observed sea level rise due to thermal expansion was 1.6 mm /yr and 1.2 mm/yr due to total land ice melt.
The mass balance of 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. The Greenland Ice Sheet, however, may be contributing about 30 percent of all glacier melt to rising sea level. Mid- latitude glaciers and ice caps, although making up only about four percent of the total land ice area, may have provided as much as 60 percent of the total glacier contribution to sea level change since the 1990s (Meier et., 2007).The total contribution from small glaciers and ice caps by the year 2100 is expected to be 240 +/- 128 mm, which represents an average annual increase of more than 2.0 millimeters per year.
Dowdeswell, Julian A., et al. 1997. The mass balance of circum-Arctic glaciers and recent climate change. Quaternary Research 48 :1–14;
Dyurgerov, M. B., Meier, M. F. 1997. Mass balance of mountain and subpolar glaciers: a new global assessment for 1961–1990. Arctic and Alpine Research 29(4) :379–391
Dyurgerov, M .B. and M. F. Meier. 1997. Year-to-year fluctuation of global mass balance of small glaciers and their contribution to sea level changes. Arctic and Alpine Research 29(4): 392–401.
Haeberli, W. 1998. Historical evolution and operational aspects of worldwide glacier monitoring. In Into the Second Century of Worldwide Glacier Monitoring: Prospects and Strategies, Haeberli, Hoelzle, and Suter, eds. UNESCO, p. 35–51.
Intergovernmental Panel on Climate Change (IPCC). 2007. IPCC Fourth Assessment Report - Climate Change 2007: The Physical Science Basis. Cambridge: Cambridge University Press.
Meier, M.F., M.B. Dyurgerov, U.K. Rick, S. O'Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson, and A.F. Glazovsky. 2007. Glaciers dominate eustatic sea-level rise in the 21st century. Science 317: 1064-1067.