Effects of Global Warming: Ice Sheets and Glaciers

Arctic Ice Cover in 1979 and 2005

Surface temperatures in the Arctic have risen almost twice as much as the global average. Satellites have made continual observations of Arctic sea ice since 1978. The extent of Arctic sea ice has declined by about 8.5 percent per decade from its size in 1979 (top image). Since 2002 satellite records have revealed unusually early onsets of springtime melting in the areas north of Alaska and Siberia, as well as a smaller recovery of sea ice during the winter season. In 2005 (bottom image) satellites showed the lowest extent of Arctic sea ice on record.

     Warming temperatures are already causing significant changes to mountain glaciers around the world, ice sheets in Greenland and the Antarctic, and polar sea ice in the Arctic. From Europe to Africa to Asia to North America, mountain glaciers have receded over the 20th century, and melting is becoming more rapid. The large-scale melting of ice may accelerate the pace of global warming in what is known as a feedback process. Because ice reflects sunlight back out to space, it has a cooling effect. Water and land, which are darker than ice, absorb and retain more heat.

    Glaciers on Kilimanjaro, the highest mountain in Africa, have lost 82 percent of their ice since 1912 and are estimated to be gone completely by 2020. Glaciers in the lofty Himalayas of Asia are melting at a rate of 9 to 15 m (30 to 50 ft) per year. Annual runoff from these glaciers feeds major rivers such as the Ganges, Yangtze, and Mekong. Glacier National Park in Montana is projected to have no glaciers left by 2030, and the number of glaciers has already dropped from an estimated 150 in 1850 to 26 in 2007.

Shrinking Greenland Ice Sheet

The Greenland ice sheet underwent extensive surface melting from 1992 to 2002, according to the Arctic Climate Impact Assessment report released in 2004. Tinted areas in these satellite images show the extent of surface melting in 1992 and in 2002. The report warned that the melting of Greenland’s ice sheet and the melting of glaciers in Alaska and Canada are increasingly contributing to a rise in the world’s sea level.

      In the Arctic annual average temperature has increased at almost twice the global rate over the past few decades. The area covered by sea ice during summer has declined by 15 to 20 percent in the last 30 years, and is projected to disappear almost completely late in the 21st century. Many species, including polar bears, seals, and walrus, depend on sea ice for their survival. The rapid loss of Alaskan glaciers represents almost half of the total loss of ice in glaciers worldwide, and makes a significant contribution to observed sea level rise. Melting of the Greenland ice sheet, which could raise sea level by 7 m (23 ft) if it melted completely, is also accelerating. The area that is experiencing at least some melting increased by 16 percent from 1979 to 2002, and scientists estimate that warming of more than a few degrees Celsius could cause widespread and possibly unstoppable melting, leading to significant sea level rise.

    Fresh water flowing from melting Arctic ice into the North Atlantic Ocean could disrupt ocean circulation patterns, which have a significant influence on the global climate. According to scientific projections, a collapse of ocean circulation patterns is unlikely to occur by 2100. However, scientists do expect there to be a weakening and slowing of the thermohaline circulation, also known as the ocean conveyor belt. In addition, a disruption of surface circulation patterns in the North Atlantic, known collectively as the Gulf Stream, could lead to cooling in Europe.

    In Antarctica the situation is somewhat different than in the Arctic. The Antarctic Peninsula, the “tail” of land reaching toward South America, has experienced dramatic warming at a rate several times the global average over the past 50 years. However, other parts of Antarctica have not shown similar trends, with some areas warming and some cooling. Overall, Antarctica is estimated to be warming at about the global average rate. Unlike the Arctic, there has been no clear general trend in sea ice. In the Antarctic Peninsula, however, ten floating ice shelves have lost more than 14,000 sq km (5,400 sq mi) of ice, and probably have not been at such a low level in the past 10,000 years. As in Greenland, scientists estimate that warming of more than a few degrees Celsius could lead to widespread melting of the West Antarctica ice sheet. This melting alone would raise sea level by as much as 5 m (16 ft).