Melting is the change that occurs when a solid becomes a liquid, as, for example, when ice melts to become liquid water. This kind of change is called a change of state.
Melting may take place in the cryosphere, the regions of Earth's surface where water exists in the frozen state. The cryosphere usually is comprised of snow or ice, including sea ice, freshwater ice, snow, glaciers, and permafrost, any ground that is perennially frozen.
Global warming impacts the cryosphere, and changes in the cryosphere affect global warming. The melting of sea ice is accelerating, resulting in larger expanses of open water that absorb solar radiation rather than reflect it as sea ice does. This increase in the absorption of solar radiation further contributes to global warming. Large land areas near the North Pole have been frozen since the last Ice Age, but as the permafrost thaws, it releases greenhouse gases into the atmosphere, adding to the gases that are already contributing to global warming.
In addition to the impact of melting on global warming, the melting of large glaciers also causes sea levels to rise. Although the sea level is only rising by a few millimeters each year now, global sea level would rise
by meters if the great ice sheets covering Greenland and Antarctica melt. Heavily populated coastal and island dwellers would be forced to find new homes. Ocean temperatures would change in polar regions, and this change, in turn, would impact polar plant and animal life and alter the climate as ocean circulation changes around the globe.
Historical Background and Scientific Foundations
The Arctic and Antarctic are located around the opposite poles of Earth. Because the sun's radiation reaches these areas at the greatest angle, these regions are extremely cold. However, the geography of the two regions is completely different. Antarctica is a landmass surrounded by ocean waters, and the Arctic region is a semi-enclosed ocean that is almost surrounded by land.
Sea ice that builds up in the Arctic stays largely confined in the Arctic Ocean. Since it does not completely melt in the summer, this sea ice accumulates and piles up into thick ridges. About 5.8 million square mi (15 million square km) of sea ice builds up in the Arctic in the winter. In the summer, some of it melts, leaving around 2.7 million square mi (7 million square km) of ice still floating, although these expanses of sea ice are diminishing with global warming. In September 2006, Arctic sea ice only covered 2.3 million square mi (5.9 million square km).
Antarctica is covered by an ice sheet, an accumulation of snow and glacial ice that extends beyond the edge of the landmass into large ice shelves—thick layers of ice that float on the ocean around Antarctica. Scientists estimate that the ice covering Antarctica is at least 40 million years old. A similar ice sheet covers most of Greenland. About 6.9 million square mi (18 million square km) of sea ice forms on the surface of the ocean waters surrounding Antarctica in the winter, but melting reduces this ice expanse to about 1.1 million square mi (3 million square km) of sea ice in Antarctica's summer.
Global warming is causing surface melting on the ice shelves. This melt water trickles into fractures in the ice shelves, causing them to break away to become giant icebergs. In 2002, a 1,040-square-mi (2,700-square km) section of ice shelf on the eastern side of Antarctica broke loose to form a large floating ice mass. Similar processes affecting the ice shelves around Greenland spawn the icebergs of the North Atlantic Ocean.
Particularly in the Arctic, the melting of sea ice is related to an ice-albedo feedback mechanism. Albedo is the amount of solar radiation that is reflected from a surface. Sea ice and snow have a high albedo, which means they reflect back into space a very large portion of the solar energy that is received. Ocean water absorbs more of the solar radiation received and becomes warmer, leading to more melting.
WORDS TO KNOW
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
CRYOSPHERE: One of the interrelated components of Earth's system, the cryosphere is frozen water in the form of snow, permanently frozen ground (permafrost), floating ice, and glaciers. Fluctuations in the volume of the cryosphere cause changes in ocean sea-level, which directly impact the atmosphere and biosphere.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
ICE AGE: Period of glacial advance.
ICE SHEET: Glacial ice that covers at least 19,500 square mi (50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.
ICE SHELF: Section of an ice sheet that extends into the sea a considerable distance and that may be partially afloat.
PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F(0°C) for several years.
SEA ICE: Ice that forms from the freezing of ocean water. As the salt water freezes, it ejects salt, so sea ice is fresh, not salty. Sea ice forms in relatively thin layers, usually no more than 3–7 ft (1–2 m) thick, but it can cover thousands of square miles of ocean in the polar regions.
SINK HOLE: Depression in the surface of the ground caused by removal of material beneath the surface. Sinkholes are often caused by depletion of groundwater or by the dissolving of minerals by groundwater. Hundreds of sinkholes from groundwater depletion riddle Mexico City; some have forced evacuations of neighborhoods.
As more ice melts as a result of atmospheric warming, the heating of the ocean and, therefore, the atmosphere above it intensifies as well. Ice-albedo feedback occurs every summer in the Arctic. The Arctic Ocean sea ice expanse varies from year to year, but seasonal contraction of sea ice is increasing with global warming. The Arctic summer is getting longer and more sea ice is melting each year. In 2006, the sea ice in the Arctic Ocean was reduced by about 15–20% over the level of sea ice that existed in the 1980s and 1990s.
Impacts and Issues
A study conducted in 2007 by scientists associated with the U.S. National Oceanic and Atmospheric Administration (NOAA) indicates that the summer sea ice coverage of the Arctic Ocean will be reduced more than 40% by 2050 compared to coverage during 1979 to 1999. The impact of the shrinking sea ice will fall heavily on the polar bear, a species being considered for listing as an endangered species under the U.S. Endangered Species Act. Polar bears essentially have no food supplies on land, but hunt seals and other marine mammals from ice sheets. According to a U.S. Geological Survey study, released in 2007 that discussed the impact of global warming on polar bears, about two-thirds of the world's polar bears will disappear by 2050. Steve Amstrup, lead biologist for the study team states, “As the sea ice goes, so goes the polar bear.”
The changes that are occurring in Antarctica also affect the species of penguins, whales, and seals that live in the region because these animals depend on krill, a small crustacean that lives under sea ice. Disappearing ice is threatening the supply of this keystone of the Antarctic marine ecosystem. A July 2007 press release from the Center for Biological Diversity states that “the emperor penguin colony at Pointe Geologie, which was featured in the film March of the Penguins, has declined by more than 50% due to global warming. Krill …has declined by as much as 80% since the 1970s over large areas of the Southern Ocean.”
Sea level rise is another potential threat associated with the melting of the cryosphere. Over the past 100 years, the sea level has risen an average of 0.04–0.08 in (1–2 mm) each year, but the accelerated melting of the ice covering land areas in Greenland and Antarctica is now causing the sea level to rise faster. If all the ice sheets were to melt, global sea level would rise about 224 ft (70 m). Although scientists do not expect all the ice sheets to melt, 2007 data show that the vulnerable Greenland ice sheet is moving toward the sea at an accelerated rate of close to 2.5 times the rate of the previous two years. If the entire Greenland ice sheet melts, the average global sea level would rise 21 ft (6.5 m).
In addition to the effect on sea level, when glaciers melt into the sea some of Earth's freshwater supply is lost. About 75% of Earth's freshwater occurs in the form of ice. Melting of the Greenland and Antarctic glaciers changes the salinity of adjacent ocean water. Mountain glaciers that have been the source of freshwater for populations living near them are also melting.
Melting of the permafrost (frozen ground) in countries that border the north polar region is also an issue. The top 10 ft (3 m) of perennially frozen ground may melt by the year 2100 according to simulation studies carried out at the U.S. National Center for Atmospheric Research (NCAR). Melting permafrost in parts of Canada, Alaska, and Siberia produces sink holes where ice chunks have melted. The thawing ground is not as level when it melts as it was when frozen, so roads and buildings are being damaged and Arctic taiga forest trees are tipping as the ground shifts.
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