E. Linacre and Bart Geerts
Glaciers and climate
Glaciers act as slow-response weather instruments, growing when temperatures fall and/or more snow falls, shrinking when temperatures increase and/or less snow falls. They respond only to environmental trends that last a few decades, therefore their trends are useful for climate studies. Some glaciers occasionally surge, largely due to internal ice/water dynamics, and glacier flow in general depends on the details of the flow profile, the ice volume, the ice temperature, and the roughness of the stream bed. Therefore the short-term (a few years) movement of the glacier terminus should not be interpreted in terms of climate change.
Mountain glaciers and small ice caps outside the Antarctic and Greenland ice sheets make up only about 6 percent of the world's total ice mass. A survey of 160,000 mountain glaciers and ice caps in all glaciated regions of the world (1) shows that the volume of the world's glaciers isdeclining, and the rate of ice loss continues to accelerate. There has been a significant decrease in the area and volume of mountain glaciers, especially at mid- and low-latitudes.
Winter snow deposition, when globally averaged, has increased slightly during this century, especially at higher altitudes. However, this is outweighed by a greater rate of melting, particularly at lower altitudes, pointing to warmer summers. The result is a net loss.
(a) The smaller, low-latitude glaciers seem to be affected most. The largest glacier on Africa's Mount Kenya lost 92 percent of its mass in the last century whilst Mount Kilimanjaro glaciers shrank by 73 percent. The edge of the Qori Kalis glacier from the Quelccaya ice cap in the Peruvian Andes retreated at 4 m/year between 1963-78, but by 1995 was retreating at 30 m/a (2). Glaciers are almost extinct on the island of Papua New Guinea/ Iryan Jaya.
(b) Mid-latitude glaciers also are showing significant losses. The number of glaciers in Spain fell from 27 in 1980 to only 13 now. The ice volume loss in the European Alps, has been reduced by more than 50 percent since 1850, the end of the Little Ice Age (p. 338). The volume of glacier ice in the Caucasus Mountains of Russia has decreased by about 50 percent in a century, and 22 percent of the ice volume has disappeared in the past 40 years from the thousands of glaciers in the Tien Shan Mountain Range bordering China and Russia. Ice cores from the Dunde ice cap in eastern Tibet have shown that the last 50 years have been the warmest yet (2). At the current rate of glacier shrinkage in Montana's Glacier National Park, there will be no glaciers left by 2050. Perhaps it will then be renamed "Glacier National Historic Monument".
There appears to have been little change in the length of New Zealand glaciers from 1600 to the later part of the 19th century, but then a substantial loss of ice during the past century (3). The total volume has shrunk from about 100 km3 to 64 km3. Records of precipitation point to no notable reduction of moisture input, so the recent loss is due to higher temperature increasing the rate of ablation, reckoned as between 0.7 - 1.3 degrees since the late 1800ís. On average, the 127 glaciers in the Southern Alps of New Zealand have become 25 percent smaller in area and 38 percent shorter in length since a century ago, receding 13.3 metres annually on average (4). The retreat of the Fox glacier (1977 image) is quite evident. New Zealand glacier termini now lie about 94 m higher than in 1900, implying a warming by 0.6 K, if the vertical temperature gradient is the customary 6 K/km, approximately. Such reduction is in agreement with the observed global warming.
(c) Until recently, the volume of the high-latitude (especially Arctic) glaciers has shown little change. However these need to be monitored intensely because climate change is expected to have the largest magnitude at high latitudes, and because in terms of their water content these glaciers are much more important than the lower-latitude glaciers.
Effect on sea level
The observed ice wastage is in sync with the observed global mean rise of the sea level, but its volume contributes only about 20% of the observed rise (1). This is because there are other water reservoirs whose volume is changing, such as atmospheric water vapour, manmade dams, and groundwater. Water from high-latitude glaciers is recycled more quickly and contributes more to sea level rise than do low-latitude glaciers. The International Panel on Climate Change (5) projected in 1996 that the world's oceans will rise by more than 45 cm by the year 2100, with a third of that contributed by glaciers and ice caps.
On the other hand, there seems to have been little loss of sea ice area from Antarctica and the Arctic, at least between 1978-1996, despite temperature rises there being above the global average. However it is not known whether the average thickness of sea ice (and hence its volume) has changed.
Case study: the Bering Glacier
The Bering Glacier is North America's largest glacier (6). Originating some 25 km east of the Alaskan border with Canada, the glacier flows 191 km to its end in south central Alaska. The glacier covers an area of about 5170 km2 and in some places it is over 800 m thick. But its size has varied during the last 10,000 years in response to climate changes, and even during this century it is affected by occasional surging and an overall reduction.
Glacial surging is defined as periodic, brief and rapid movements of large quantities of ice within the glacier, often accompanied by a significant advance of the snout. The Bering Glacier surged around 1900 and 1920 and in 1938-40, 1957-60, 1965-7 and 1993-4. It is unclear whether climatic factors influence glacial surging.
Overall, the glacier's area has declined 130 km2 since the beginning of the century. Moreover, the rate of glacial retreat has been increasing over recent decades, with maximum rates occurring after 1990. Also, the glacier has thinned dramatically since 1900. Parts of the glacier have thinned by as much as 180 m in the last 50 years and in some places the glacier has lost between 20-25% of its thickness.