The Frozen Poles
How much do you know about the frozen poles of our home planet?
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Ice sheets lose mass when snow and ice at the surface melts and runs off and when ice at the coast enters the neighboring ocean. The three processes of snow accumulation, surface melt and ice loss make up what is known as an ice sheet's "mass budget."
Somewhat counterintuitively, an ice sheet's surface — the area exposed to the air — is its coldest part. This is because snow and ice act as insulation, trapping heat that rises from the bedrock below. Greenland's ice sheet, for example, is melting at its base in several places.
Sea ice is frozen sea water. It forms once the temperature of the water's surface drops below its freezing point. For salty ocean water, that's about 29°F (-2°C). (The salt impurities make the freezing point lower than that of pure water.) In contrast, icebergs, glaciers and ice shelves are made from freshwater, which comes from precipitation on land and other sources.
Ice is bright and helps cool the polar regions. It has an albedo of about 80 percent, so it reflects away most of the incoming sunlight. When sea ice melts, it exposes the dark ocean beneath. The ocean, in contrast, has an albedo of just 10 percent, meaning it absorbs 90 percent of the incoming sunlight. As the Arctic warms in response to global warming, sea ice melts and amplifies global warming by reducing the Arctic albedo further.
The Arctic consists of the Arctic Ocean covered by sea ice, which is surrounded by treeless permafrost (permanently frozen land). That land includes parts of Russia, Canada and the island of Greenland. Antarctica is a vast continent covered in a layer of snow and ice 2 km thick on average, which is surrounded by the Antarctic Ocean.
Sea ice is a key part of our climate. Since 1978, satellites have shown rapid changes in the Arctic. Perennial ice cover (ice that has survived at least one summer) has been shrinking at the rate of about 13 percent per decade. In the Antarctic, the trend is small but opposite, with sea ice cover increasing at about 1-2 percent per decade. Overall, sea ice loss in the Arctic dwarfs any gains in the Antarctic.
When the ice on Greenland and Antarctica melts, the water runs off into the oceans. NASA's Gravity Recovery and Climate Experiment (GRACE) satellites have shown that this huge flow of water changes the distribution of mass on Earth's surface. Meltwater from the polar ice caps entering the oceans measurably affects the axis of Earth's spin.
The salty oceans contain more than 97 percent of all water on Earth, making freshwater relatively scarce. About 70 percent of Earth's freshwater is held in ice caps and glaciers. The rest resides in lakes (27 percent), swamps (3 percent) and rivers (less than 1 percent).
Melting sea ice cannot raise global sea level since the ice is already floating. (Think of an ice cube melting in a glass full of water, which doesn't raise the water level.) However, Arctic sea ice is thinning and the long-term summer average cover has decreased by 34 percent in 1979. Ice from glaciers and ice sheets, which form on land, does add water to Earth's oceans when it melts and does contribute to sea level rise.
Major changes in sea ice cover affect many species, including polar bears living in the Arctic and penguins living in the Antarctic. Polar bears rely on sea ice as a platform for mating, rearing young and for hunting seals. As Arctic ice shrinks, their habitat disappears. Changes in the ice also affect the atmosphere and ocean circulation patterns, as the ice provides strong insulation between those two parts of the climate.