Arctic Ice Masses, Recent Climate Change, and Implications for
Global Sea Level
Julian A. Dowdeswell, Centre for Glaciology, University of Wales
The Arctic appears to be an area of the globe which is particularly sensitive to climate change. Several General Circulation Model (GCM) simulations of future climatic response to increasing proportions of "Greenhouse gases" in the atmosphere have predicted that Arctic regions will experience enhanced warming relative to lower latitudes. Ice-core records from Greenland have also indicated very abrupt past environmental responses to shifts in the linked ocean atmosphere system. The winter inputs and summer losses of mass from the glaciers, ice caps and ice sheets covering over 2 million km2 of the Polar North vary in response to climate changes, and affect global sea level as increments of water are decanted to the oceans or stored in solid form. This area includes the 1.7 million km2 Greenland Ice Sheet, together with the glaciers and ice caps which extend over 250,000 km2 of the Arctic, on both the heavily glacierized archipelagos of the Canadian and Eurasian High arctic and the glaciers north of about 60oN within continental North America, Europe and Iceland.
High Arctic climate change over the last few hundred years includes the relatively cool Little Ice Age (LIA), followed by warming over the last hundred years or so, according to the oxygen isotopic and melt layer signals from ice cores and from the few long time series of direct meteorological observations available from the circum-polar North. Meteorological data from the Eurasian High Arctic (Svalbard, Franz Josef Land, Severnaya Zemlya) and Canadian High Arctic islands are scarce before the mid-Twentieth Century, but longer records from Svalbard and Greenland show warming from about 1910-20 (Dowdeswell, 1995). Other evidence of recent trends in High Arctic temperatures and precipitation is derived from ice cores, which show cooler temperatures (by 2-3oC) for several hundred years before 1900, with high interdecadal variability. The proportion of melt layers in ice cores has also risen over the last 70-130 years, indicating warming.
Two forms of observations have been made on recent trends in the mass balance of Arctic ice caps and the Greenland Ice Sheet: field and satellite measurements. There is widespread geological evidence of glacier retreat in the Arctic since about the turn of the century linked to the end of the LIA. An exception is the rapid advance of some surge-type ice masses. Field measurements of mass balance on ice caps in Arctic Canada, Svalbard and Severnaya Zemlya since 1950 show either negative or near-zero net balances, suggesting glacier thinning in response to recent climate warming (Dowdeswell, 1995). The total net input of melt from small glaciers and ice caps throughout the world is estimated at 0.4±0.2 mm yr-1 of sea-level rise, averaged over the past 60-80 years (Meier, 1993). On the Greenland Ice Sheet, satellite radar altimetry is being used to measure whether the ice is thickening or thinning. The 18 km diameter footprint of the ERS-1 radar altimeter makes it unsuitable for making similar measurements of small ice caps (<10,000 km2), but laser ranging systems of high accuracy will be used in future to measure changing surface elevation on these ice caps. Repeat radar altimeter measurements for the southern 40% of the Greenland Ice Sheet (constrained by satellite orbital parameters) shows a thickening by 230±70 mm yr-1, equal to between 0.2 and 0.4 mm yr-1 of sea-level fall (Zwally and others, 1989; Wingham, 1995).
Predictions of ice-mass response to climate change have been modeled using two approaches: simple energy balance and more complex GCM experiments. A "greenhouse-induced" warming of 1oC in the High Arctic is predicted to produce a global sea-level rise of 0.063 mm yr-1 from melting of Arctic ice caps (excluding Greenland) based on simple energy balance calculations. By contrast, recent GCM results for the Greenland Ice Sheet using a high resolution grid (T106) to improve modeling of altitudinal effects, suggests an increase of ablation by about 200 mm yr-1 (water equivalent) for a doubling of atmospheric CO2. (Ohmura, 1995). This is equivalent to a sea-level rise of 1.1 mm yr-1. However, similar high-resolution modeling of the Antarctic Ice Sheet shows an increase in mass balance of 23 mm yr-1, or a sea level fall of 0.9 mm yr-1. According to these GCM experiments, the differing effects of doubling CO2 on the mass balance of the two great ice sheet may cancel each other out.
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Dowdeswell, J.A., 1995, Glaciers in the High Arctic and recent environmental change: Philosophical Transactions of the Royal Society, Series A., v. 352, p. 321-334.
Ohmura, A. 1995, Mass balance of polar ice sheets: Unpublished Manuscript.
Meier, M.F., 1993, Ice, climate, and sea level; do we know what is happening?, in Ice in the Climate System: Peltier, W.R.,ed., NATO ARW Series C - Mathematical and Physical Sciences - Global Environmental Changes, Berlin and Heidelberg, Germany, Springer-Verlag, p. 141-160.
Wingham, D.J., 1995, Elevation change of the Greenland Ice Sheet and its measurement with satellite radar altimetry: Philosophical Transactions of the Royal Society, Series A, v. 352, p. 335-346.
Zwally, I.I.J., and others, 1989, Growth of Greenland Ice Sheet - measurement: Science, v. 246, p. 1587-1589.