Early Warning Signs of Global Warming: Glaciers Melting
Glaciers exist on all continents except Australia and at virtually all latitudes from the tropics to the poles. Mountain glaciers, such as those that exist at higher elevations in the mid-latitudes and tropics, are particularly sensitive indicators of climate change. The volume of ice in a glacier and correspondingly its surface area, thickness, and length is determined by the balance between inputs (accumulation of snow and ice) and outputs (melting and calving). As climate changes, the balance between inputs and outputs may change, resulting in a change in thickness and the advance or retreat of the glacier. Temperature, precipitation, humidity, wind speed, and other factors such as slope and the reflectivity of the glacier surface all affect the balance between inputs and outputs. Most glaciers in the world, however, are more sensitive to temperature than to other climatic factors (Fitzharris, 1996).
There is widespread evidence that glaciers are retreating in many mountain areas of the world. Since 1850 the glaciers of the European Alps have lost about 30 to 40% of their surface area and about half of their volume (Haeberli and Beniston, 1998). Similarly, glaciers in the New Zealand Southern Alps have lost 25% of their area over the last 100 years (Chinn, 1996), and glaciers in several regions of central Asia have been retreating since the 1950s (Fitzharris, 1996; Meier, 1998). For three glaciers in the US Pacific Northwest, the seven-year average rate of ice loss was higher for the period since 1989 than for any other period studied (Hodge et al., 1998). Glacial retreat is also prevalent in the higher elevations of the tropics. Glaciers on Mt. Kenya and Kilimanjaro have lost over 60% of their area in the last century (Hastenrath, 1991; Hastenrath and Greischar, 1997), and accelerated retreat has been reported for the Peruvian Andes (Mosley-Thompson, 1997).
By contrast, losses in the Arctic have been less pronounced (0-6% in area and 1-14% in volume), partly because those glaciers are much colder and the extra meltwater refreezes in the ice mass (M. Meier, pers. comm.). Although there is considerable variability at the regional and local scales and over shorter time periods, the overall global signal shows mass loss and retreat of glaciers during the last century. For the period 1884 1978, the mean global glacial retreat corresponds to a calculated warming of about 0.7°C per century (Oerlemans, 1994).
Global compilations indicate that the wastage of mountain glaciers during the last century has raised sea level by between 0.2 to 0.4 mm/yr, or roughly 20% of the observed change (e.g,. Warrick et al., 1996; Dyurgerov and Meier, 1997). By 2100, sea level is estimated to rise by an additional 46 to 58 cm, with an estimated total range of 20 to 86 cm (Warrick et al., 1996). Over half of the change will likely be from thermal expansion of ocean water, another 30% from melting of mountain glaciers, and 10% from melting of parts of the Greenland ice sheet (Gregory and Oerlemans, 1998). If future climate change is consistent with model projections, up to one quarter of the presently existing mountain glacier ice will disappear by 2050 (Fitzharris, 1996).
The shrinking of glaciers will likely have a significant socioeconomic impact in some mountain regions, though the exact local impacts remain uncertain and will vary. Regions that lose major parts of their glacier cover will experience alterations in hydrology. The glaciers will initially provide extra runoff from melting; but as the ice diminishes, the runoff will wane. Also, because revegetation of terrain is slower at high altitudes, deglaciated areas will be subject to erosion and decreased stability, heightening the need to protect buildings, roads, communication links, and other structures. For areas dependent on tourism, uncertain snow cover during peak winter sports seasons, natural hazards such as rock and ice falls, or loss of scenic beauty are of particular concern. An example of the type of scenario that could become more frequent occurred during the warm summer of 1998, when a ski area in the Tyrolian Alps was forced to close a lift after melting ice dislodged rocks and soil, destabilizing the peak (GECR, 1998).
Chinn, T. 1996. New Zealand glacier responses to climate change of the past century. New Zealand Journal of Geology and Geophysics 39, 415-428.
Dyurgerov, M. B., and M. Meier. 1997. Mass balance of mountain and sub-polar glaciers: A new global assessment for 1961-1990, Arctic and Alpine Research, 29 (4), 379-391.
Fitzharris, 1996. The cryosphere: Changes and their impacts. In Climate Change 1995: The Science of Climate Change, (Eds J. T. Houghton, L. G. M. Filho, B. A. Callander, N. Harris, A. Kattenberg, and K. Maskell), 241-265, Cambridge University Press, Cambridge, UK.
GECR, 1998. Melting glacier destabilizes Austrian peak. Global Environmental Change Report, vol. X, no. 22, p. 6.
Gregory, J.M. and J. Oerlemans, 1998. Simulated future sea-level rise due to glacier melt based on regionally and seasonally resolved temperature changes. Nature 391, 474-476.
Haeberli, W., and M. Beniston. 1998. Climate change and its impacts on glaciers and permafrost in the Alps. Ambio 27, 258-265
Hastenrath, 1991. Climate Dynamics of the Tropics. Kluwer Academic Publishers, Dordrecht, Netherlands, 488 p.
Hastenrath, S and L. Greischar, 1997. Glacier recession on Kilimanjaro, East Africa, 1912-89. Journal of Glaciology 43 (145), 455-459.
Hodge, S.M., D.C. Trabant, R.M. Krimmel, T.A. Heinrichs, R.S. March, and E.G. Josberger, 1998. Climate variations and changes in mass of three glaciers in western North America. Journal of Climate 11 (9), 2161-2179.
Meier, M., 1998. Land ice on Earth: A beginning of a global synthesis. Unpublished transcript of the 1998 Walter B. Langbein Memorial Lecture, American Geophysical Union Spring Meeting, Boston, MA, 26 May 1998.
Mosley-Thompson, E. 1997. Glaciological evidence of recent environmental changes. Annual Meeting of the Association of American Geography, Fort Worth, Texas.
Oerlemans, J., 1994. Quantifying global warming from the retreat of glaciers. Science 264, 243-245.
Warrick, R. A., C. L. Provost, M. F. Meier, J. Oerlemans, and P. L. Woodworth, 1996. Changes in sea level, in Climate Change 1995: The Science of Climate Change, 359-405
(Eds JT Houghton, LG Meira Filho, BA Calander, N Harris, A Kattenburg, and K Maskell), Cambridge University Press, Cambridge.
Glaciology on the World Wide Web An excellent resource with links to sources of glaciology images and data, publications and research, and glaciology organizations. http://www-nsidc.colorado.edu/glaciology/
World Glacier Inventory Data from over 67,000 glaciers around the world, including glacier location, area, length, orientation, elevation, and classification of type. http://www-nsidc.colorado.edu/NSIDC/CATALOG/ENTRIES/G01130.html
Diaz, H.F., M. Beniston, and R.S. Bradley, 1997. Climatic Change at High Elevation Sites, Kluwer Academic Publishers, Dordrecht, Netherlands, 298 p.