Arctic Melting: Polar Ice Cap
Arctic Melting: Polar Ice Cap
The North and South Poles of Earth are both covered with ice because the light of the sun strikes them at a glancing angle or not at all, depending on the time of year. The South Pole is occupied by a large continent, Antarctica, which bears a thick ice sheet containing about 90% of all the ice on Earth. The North Pole is covered by ocean that is skinned over by frozen seawater. This patch of floating ice expands and contracts with the seasons. The Arctic has been more strongly affected by climate change than any other region except the western peninsula of Antarctica, and in recent years has shown rapid thinning of its ice and reduction of its ice area, especially in summer. Within a few decades the Arctic may be completely ice-free in summer.
Historical Background and Scientific Foundations
Although indigenous peoples such as the Inuit (Eskimos) have inhabited the region around the Arctic for thousands of years, they had no reason or ability to reach its more isolated areas, such as the vicinity of the North Pole or the interior of the Greenland ice cap. The Arctic interior was first explored by Europeans in the nineteenth century, when a number of expeditions tried to reach the North Pole itself. This was finally achieved in 1909 by American engineer and explorer Robert E. Peary (1856–1920). The North Pole has since been visited by many expeditions, including nuclear-powered submarines, aircraft, adventurers on foot, and icebreakers.
For decades, scientific knowledge of the Arctic depended on aircraft and ground stations. However, simultaneous observing of the whole region is practical only from orbit. Satellite data was first obtained from the Landsat satellites in the late 1970s. Infrared radiation emitted by all objects reveals the object's temperature, so infrared observations from space have allowed tracking of temperatures all over the Arctic since 1979.
The Arctic sea-ice cap is an important part of the global climate system. It reflects solar energy back into space like a mirror, thus helping control circulation of water in the oceans via the North Atlantic branch of the Great Conveyor Belt or thermohaline overturning circulation by which all the oceans of the world remain in constant cyclic motion. When seawater freezes in the winter, it rejects salt into the water, increasing the ocean's local density. This denser, saltier water then sinks, helping to force the global ocean conveyor. Rainfall patterns in the Northern Hemisphere and the livelihoods of humans and animals living in the Arctic are also strongly influenced by the Arctic ice.
The Arctic has been particularly vulnerable to global warming. Warming is started by increased atmospheric concentrations of greenhouse gases, such as carbon dioxide and methane. Aerosol pollutants, tiny particles from incomplete burning of fossil fuels, forests, and other combustibles, also cause Arctic warming. Arctic haze has been observed by aircraft pilots since the 1950s, but it was not until the 1970s that scientists realized that the haze was air pollution, namely aerosols. Aerosol particles, which are dark, absorb energy from sunlight, warming the air. They also can settle on ice and snow darkening them and so making melting more likely. Over the last 50 years, temperatures in Alaska, western Canada, and eastern Russia have increased by 4–7°F (2–4°C), twice the average global warming. As of 2007, scientists were forecasting that during the twenty-first century temperatures in these areas would rise by another 7–13°F (4–7°C).
Snow and ice have a high albedo (brightness) and reflect about 90% of incoming solar energy to space, but open sea and ground have low albedo, absorbing much more energy than they reflect. Positive feedback occurs, accelerating warming, when melting of snow cover or sea ice exposes dark ground or ocean. This greatly increases the amount of solar energy absorbed and so speeds up the warming that started the melting initially.
The vulnerability of the Arctic to warming has been seen in increased atmospheric temperatures, melting of permafrost, and—most dramatically—the rapid shrinkage of the north polar ice cap.
The size of the polar sea-ice cap varies greatly, between a maximum in winter and a minimum in summer. Therefore, to see changes in sea-ice area from year to year, each month's ice area must be compared to the ice area of the same month in the previous year. Satellite data show that while Antarctic sea ice has been growing, Arctic ice has been declining in every month since at least 1979, when reliable data began to be collected. Although there had been ups and downs, the trend from 1979 to 2004 was a decrease in September (annual minimum) ice area by 7.7% per decade.
WORDS TO KNOW
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
GREAT CONVEYOR BELT: The overturning circulation of the world's seas, driven by temperature and salinity differences between the poles and tropics; also called the thermoha-line circulation or meridional overturning circulation. Because the great conveyor belt transports thermal energy from the tropics toward the poles, it is a central component of Earth's climate machine.
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.
NORTH POLE: Point at which Earth's axis passes through its surface in the Arctic. The north pole is near, but not identical to, the northern magnetic pole, the point at which the field lines of Earth's magnetic field pass vertically through its surface.
PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F (0°C) for several years.
SOUTH POLE: The geographically southernmost place on Earth.
THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.
Moreover, the pace of shrinkage has been rapidly accelerating in the early 2000s. Polar cap area reached historic minimum lows in 2002, 2005, and 2007. The ice-area low of September 2007 was particularly shocking to scientists, about 25% less area than the previous record low in 2005. Arctic specialist William L. Chapman of the University of Illinois at Urbana-Champaign characterized the extreme shrinkage as “simply incredible.” The Northwest Passage—the sea route around the northern perimeter of North America from the Pacific Ocean to the Atlantic Ocean—was free of ice and open to navigation by ships for the first time in recorded history.
The polar ice has also been getting thinner since the late 1970s. Changing winds have moved older, thicker ice out of the Arctic bit by bit, driving it into the North Atlantic (where it melts) and replacing it with younger, thinner ice. As of 2007, the Arctic ice pack had thinned to about half its average thickness of 50 years earlier.
Impacts and Issues
One of the most common scientific criticisms of the authoritative 2007 report on climate change from the Intergovernmental Panel on Climate Change (IPCC),Climate Change 2007, is that its predictions are too conservative, particularly because they disregard new knowledge about accelerated melting in the Arctic and the western peninsula of Antarctica. The IPCC barred the use of scientific papers published after December 2005 in its 2007 report, so recent data about accelerated Arctic melting were not used. Scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, announced in April 2007 that Arctic sea-ice melting was happening more rapidly than predicted by all 18 of the computer climate models used by the IPCC in preparing its 2007 report. In 2006, NCAR scientists announced that if greenhouse gases continue to build up in the atmosphere at their current rate, the Arctic could be completely free of ice in summer as early as 2040.
Such drastic changes in sea-ice patterns may slow the ocean conveyor and accelerate melting of Greenland's ice cap by making the polar ocean a much more effective absorber of solar energy. Polar bear populations would almost certainly be decimated, since polar bears depend on sea ice for access to seals, one of their main sources of food. In 2007, the U.S. Geological Survey announced that the world population of polar bears could be reduced by two-thirds by 2050 even under conservative (less drastic) assumptions about global warming.
The livelihoods of indigenous peoples in the Arctic are also threatened by Arctic warming. Radical changes in Arctic ice may trigger changes in rainfall patterns at the temperate latitudes, including reduced rainfall in the American West and increased rainfall in parts of Europe.
Primary Source Connection
One of the most debated and least predictable effects of global climate change is the issue of rising sea levels. Scientists expect sea levels to rise over the next several centuries as the ice sheets of Antarctica and Greenland melt. However, predicting the extent of rise in sea levels has been difficult for scientists because of the complex relationship between the ice sheets and air temperature and sea temperature. This article details some of the difficulties that scientists face in trying to construct computer models that accurately estimate the rise in sea level.
Juliet Eilperin is a staff writer for the Washington Post.
CLUES TO RISING SEAS ARE HIDDEN IN POLAR ICE
Few consequences of global warming pose as severe a threat to human society as sea-level rise. But scientists have yet to figure out how to predict it.
And not knowing what to expect, policymakers and others are hamstrung in considering how to try to prevent it or prepare for it.
IN CONTEXT: LOSSES FROM ICE SHEETS
“New data since the TAR [IPCC Third Assessment Report, 2001] now show that losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003. Flow speed has increased for some Greenland and Antarctic outlet glaciers, which drain ice from the interior of the ice sheets. The corresponding increased ice sheet mass loss has often followed thinning, reduction or loss of ice shelves or loss of floating glacier tongues. Such dynamical ice loss is sufficient to explain most of the Antarctic net mass loss and approximately half of the Greenland net mass loss. The remainder of the ice loss from Greenland has occurred because losses due to melting have exceeded accumulation due to snowfall.”
Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.
SOURCE:Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
To calculate sea-level rise, the key thing researchers need to understand is the behavior of the major ice sheets that cover Greenland and Antarctica. The disintegration of one would dramatically raise the ocean. But while computer models now yield an increasingly sophisticated understanding of how a warming atmosphere would behave, such models have yet to fully encapsulate the complex processes that regulate ice sheet behavior.
“The question is: Can we predict sea level? And the answer is no,” said David Holland, who directs New York University's Center for Atmosphere Ocean Science. Holland, an oceanographer, added that this may mean researchers will just have to watch the oceans to see what happens: “We may observe the change much more than we ever predict it.”
In its executive summary report for policymakers in February, the Intergovernmental Panel on Climate Change, composed of hundreds of leading climate scientists, barely hazarded a guess on sea level, predicting that it would rise between 7.8 inches and two feet by the end of the century. However, the United Nations-sponsored panel—which operated under the assumption that, by 2100, the Greenland ice sheet would lose some mass but that the Antarctic ice sheet would gain some— did not venture a best estimate or an upper limit for possible sea-level rise.
The panel could agree to say only there is a 50–50 chance that a global temperature increase of between
1.8 and 7.2 degrees Fahrenheit would lead to a partial melting of the ice sheets over a period of several hundred to several thousand years.
Because so much is at stake—a three-foot increase in sea level could turn at least 60 million people into refugees, the World Bank estimates—ice sheet modelers are working furiously to try to unravel the mystery of how these sheets accumulate and lose mass.
Michael Oppenheimer, a Princeton University professor of geosciences and international affairs, does make a prediction: He figures that if the Greenland ice sheet disintegrates, sea level would rise about 23 feet. If the West Antarctic sheet melts, as well, it would add an additional 17 feet or so.
“If either of these ice sheets were to disintegrate, it would destroy coastal civilization as we know it,” Oppenheimer said.
One of the biggest challenges facing researchers is that ice sheets are under “attack from the edges,” in the words of Richard B. Alley, a Pennsylvania State University geosciences professor. Each sheet amounts to a pile of snow compressed over time into a two-mile thick, continent-spanning sheet of ice, which spreads out under its own weight, Alley said.
Near the coast, the pile develops quick-moving “ice streams,” which flow between slower-moving sections of ice and float out onto the ocean in an “ice shelf.” While recent satellite data have indicated that these ice streams are flowing faster and delivering more water to the oceans, many uncertainties remain.
David Vaughan, a glaciologist with the British Antarctic Survey in Cambridge, said the terrain beneath the ice streams helps determine how they move, but the contours of the land are largely unknown because it is buried so far under the ice. The streams may run aground on elevated bedrock, slow down as they move past rocky fjord walls or speed up as they move over mud.
“There's a continent of topography sitting under Antarctica,” Vaughn said. “Everything there has an impact on how the ice sheet flows, and very little of that has been mapped.”
Researchers are also trying to measure the layer of water that lies under the ice sheets, as that also helps regulate ice stream flows.
“They're essentially afloat on their own sub-glacial water, even if there's not much water there,” said Garry Clarke, a glaciology professor at the University of British Columbia. “We don't know very much about how water flows underneath ice sheets.”
Another uncertainty is how much the oceans surrounding the ice sheets are warming, something that is difficult to measure because the areas are remote. Vaughan and his colleagues suspect that warmer waters around Antarctica have contributed to melting the Western Antarctic ice sheet, but there is little good data because few ships venture there.
Researchers are now going to extraordinary lengths to collect the data they need. Holland at NYU recently returned from a trip to Greenland, where he was collecting information about the Ilulissat glacier, which has doubled its speed over the past decade as it flows toward the ocean and melts. To test the temperature and salinity of the water surrounding the glacier, Holland and other researchers had to hover in a helicopter and lower their instruments into an opening in the ice.
“It's kind of beautiful, and scary and fun,” he said.
Even with better data, scientists find it difficult to enter the information into computer models. Most models do not attempt to calculate what could happen to ice sheets at their edges.
Adding to the challenge, Oppenheimer said, is that models “are only good at explaining things that happen at a large scale. Ice sheets are very complex beasts, and the water moves at a very small scale.”
Ice streams move along narrow channels, and plugging such detail into a computer model takes a long time. But without that level of detail, the results are incomplete.
Researchers have made some progress in ice sheet science over the past decade by using satellites to measure the sheets' changing mass.
Last month, for example, a team of NASA and university scientists used readings from NASA's QuikScat satellite to measure snow accumulation and melt in Antarctica from July 1999 through July 2005. They discovered that broad areas of snow had melted in west Antarctica in January 2005 in response to warmer temperatures. The finding was surprising because Antarctica had shown relatively little warming in the recent past.
Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder, who led the study, said increases in snowmelt “definitely could have an impact on larger-scale melting of Antarctica's ice sheets if they were severe or sustained over time.”
Because ice sheet modeling has not ranked as a high priority for government laboratories and has not been integrated into large-scale climate models, scientists from around the world are now collaborating to develop more sophisticated models to inform policymakers about potential sea-level rise. The researchers have convened two major meetings this year, one at the NOAA Geophysical Fluid Dynamics Laboratory at Princeton University and one at the University of Texas at Austin, in an effort to generate a new generation of ice sheet models.
Vaughan, who attended both conferences, said he is hopeful that he and others will solve the question of ice sheet modeling by the time he ends his career: “It will be 15 years before I retire, and I want it nailed by then.”
But other researchers are less optimistic. Holland, who like Vaughan is in his mid-40s, doubts that scientists will master the problem before greenhouse gas emissions trigger significant melting of the ice sheets that he studies.
“We will get there eventually, but it won't be for a long time. It won't be in my lifetime,” Holland said. “There's no plan; there's no program. There's no one responsible for sea-level rise.”
eilperin, juliet. “clues to rising seas are hidden in polar ice,” washington post (july16 2007):a06.
See Also Albedo; Antarctica: Melting; Arctic Melting: Greenland Ice Cap; Arctic People: Climate Change Impacts; Great Conveyor Belt; Greenland: Global Implications of Accelerated Melting; Polar Bears; Soot.
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