Changes in Ice Sheets and Glaciers

An example of the change in a glacier terminus is shown in the next two photographs taken in the Quelccaya Ice Cap in the tropical Andes in Peru. Figure 4 was taken in 1978 and shows large ice masses in the near vicinity of a boulder. The same site photographed in 1995 (Figure 5) shows an ice free rock field surrounding the boulder.

Characteristics of glacier ice on earth are given in Figure 6. The ice mass of the Antarctic continent contains about 90% of glacier ice and most of the remaining ice is on Greenland, with only a small amount in the remaining continental glaciers. Melting the Greenland ice mass would raise sea level by 7 meters and melting the Antarctic glacier would cause a rise of 65 meters. A glacier can lose mass by ablation (evaporation and melting/runoff) and calving (breaking off of icebergs at the outer margin of ice masses extending offshore). If the glacier is in equilibrium (not gaining or losing mass) the accumulation rate must equal the loss rate, defined as the sum of ablation and calving. Note that the mass loss due to ablation is inconsequential on Antarctica but comparable with calving on Greenland. The mass turnover time given in the last line is the total mass of ice in the sheet divided by the accumulation rate. This essentially is an estimate of the length of time it takes to accumulate the total mass of the sheet.

Both accumulation and loss depend on temperature, but in quite different ways as is shown in the next image. Ablation refers to loss of mass by evaporation and melting, and accumulation occurs by snowfall (and possibly some rain or freezing rain on ice fields). Figure 7 gives the dependence of ablation and accumulation (expressed in amount of mass in arbitrary units per year) on temperature. At very low temperatures, ice loss is essentially negligible because melting is non-existent and evaporation is extremely low. Accumulation is proportional to temperature. As the annual temperature rises but remains well below 0 degrees C, accumulation rises slowly with temperature because warmer air permits more water to exist in the vapor phase and hence allow for more snowfall. As annual temperature rises further, evaporation begins to become somewhat more important, and there will be some times during the year at which melting will occur, leading to an increase in ablation. As the annual temperature approaches 0 degrees C, evaporation and, especially, melting increase rapidly. Meanwhile accumulation increased gradually with increased water vapor being available for snowfall. Even when the annual temperature is above freezing, some accumulation will occur at some times of year, but loss will be far larger.

The lower figure (from the image above) shows the combined effects of these individual processes. For very low temperatures, accumulation dominates (curve has values larger than zero). Eventually as temperature rises, the curve peaks and plummets downward passing zero and rapidly becoming negative to a point somewhat less than 0 degrees C where the ice mass is completely gone (ice may be present during winter season, but it no longer persists through all seasons). The ice sheet gains mass for temperatures represented by the dark shading and loses mass for temperatures indicated by the light shading. Also, for temperatures in the range denoted by the letter "b", an increase in temperature causes a loss of ice mass. By contrast, for temperatures in the range indicated by "a", a temperature increase will actually increase the mass of ice.

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