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Shrinking Arctic Tells Many Stories
Greenbelt - Dec 01, 2003
In 2002, a series of scientific studies pointed to dramatic changes in Arctic sea ice. Sea ice that survives the summer and remains year round—called perennial sea ice—is melting at the alarming rate of 9 percent per decade, according to a study by NASA Goddard Space Flight Center senior researcher Josefino Comiso. The extent of Arctic sea ice at summer's end reached a record low in 2002, reported NASA-funded researchers at the University of Colorado's National Snow and Ice Data Center, in Boulder. Early findings suggest that summertime melting of Arctic sea ice in 2003 is on pace to rival last year's low.
In support of this evidence of a changing Arctic climate, Comiso shows in a new paper that most of the Arctic warmed significantly in the 1990s compared to the 1980s. The study also finds that the seasons when sea ice melts, between early spring and late fall, have gotten longer and warmer each decade, and that Arctic regions within North America have warmed more per decade than other Arctic areas. The study, which appears in the November 1 issue of the American Meteorological Society's Journal of Climate, uses surface temperature data taken from satellites between 1981 and 2001.
It is tempting to take solace in the idea that these striking changes are happening somewhere far away. But in reality, such shifts in the Arctic are likely early indications of a global climate in a state of flux. "People talk about global warming, and the Arctic really is the best place to detect global warming because the effects are amplified there," Comiso says.
The reasons why climate changes get amplified in the Arctic are many and complex, but one of the main features concerns the ice itself. Ice reflects the Sun's rays up into the atmosphere and out to space, which keeps solar radiation from warming the Arctic lands and ocean. Both sea ice, which floats on water, and glaciers and ice on land cool the Earth in this manner. Without large ice masses at the poles the Earth would absorb more heat and warming would escalate. When ocean temperature rises sea ice becomes thinner, exposing more water, thus reinforcing the warming trend and creating a positive feedback loop.
Researchers suspect that loss of Arctic sea ice may be caused partly by global warming and partly by changing atmospheric pressure and wind patterns over the Arctic that move sea ice around, which also help to warm Arctic temperatures. Changes in air pressure and wind patterns may likewise be a result of greenhouse gas buildup in the atmosphere.
"The warming we see is another indication that climate is now changing, and in ways that may not have been experienced in several million years," says David Rind, a senior researcher at the NASA Goddard Institute for Space Studies in New York.
Ice at a Threshold?
"It appears that ice extent for September 2003 is the second lowest in the satellite record—2002 barely beat it," Serreze says. " In other words, we have not seen a recovery." This year's sea ice extent appears to be reinforcing a general downward trend, Serreze adds.
Researchers at the University of Washington have shown that the decreasing sea ice trend is linked in part to a cyclical atmospheric pressure system, called the Arctic Oscillation. This oscillation refers to a pattern of low- and high-pressure systems between the Arctic and the mid-latitudes. When the oscillation is in its positive phase, as it has been generally over the last 20 years, air pressure tends to be low over the Arctic Ocean. This pattern pulls in warmer air from the surrounding area in a low-level counter-clockwise spiral and helps to break up the sea ice and blow it out of the Arctic.
Serreze believes that the September 2002 low-ice mark was reached due to unusually warm temperatures and frequent storms that worked in tandem to break up and melt the ice. The Arctic oscillation was in a positive phase the previous winter and appears to have played a role. But the Arctic Oscillation doesn't explain everything, and there are signs that it may be moving back to a more neutral phase. Whether this will be just a temporary shift is not known. Yet the ice continues to retreat.
"The more recent years have shown indications of a recovery in the Arctic Oscillation towards more neutral conditions, but we've still seen decay in sea ice," Serreze says. He wonders if the ice has thinned to a point where it has reached a threshold; a situation where thin ice and warming waters reinforce each other, regardless of pressure patterns like the Arctic Oscillation.
Evidence of Arctic Warming
The summer warming and longer sea ice melt season appear to be affecting the volume and extent of perennial sea ice, the study suggests. Also, a longer melt season could in turn cause thinner sea ice at the end of winter, making it even more susceptible to early thawing. Winter cooling, on the other hand, caused sea ice to actually advance in the Bering Sea, Greenland Sea, and Baffin Bay.
Researchers observed varied temperatures across different regions within and near the Arctic Circle. Average temperature trends increased by one-third of 1 degree C per decade over sea ice, and they also rose half a degree C per decade over the lands of Eurasia. Temperatures over North America experienced the highest regional warming, increasing by 1.06 degrees C per decade. Greenland cooled by less than one-tenth of a degree C per decade. The cooling found over Greenland was mainly at high elevations, while warming trends were observed around its periphery. These results are consistent with a National Snow and Ice Data Center study that found record loss of sea ice around Greenland's periphery in 2002.
The surface temperature records from 1981 to 2001 were based on thermal infrared data from the Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA satellites. The AVHRR sensor detects radiation in the visible and thermal infrared range of the electromagnetic spectrum. These satellite temperature data were compared with accurate ground-based data taken during the one-year-long Arctic ice station project called the Surface Heat Budget in the Arctic (SHEBA) from 1997 to 1998. Comiso found that the two datasets are extremely consistent with each other.
"This study is unique in that previously, similar studies made use of data from very few points scattered in various parts of the Arctic region," says Comiso. "These results show the large regional and seasonal differences in the trends that only satellite data can provide."
For starters, as sea ice melts, Arctic waters warm, greatly altering ocean processes, which in turn have an effect on Arctic and global climate, says Michael Steele, senior oceanographer at the University of Washington, Seattle. As the oceans warm and ice thins, more solar energy gets absorbed by the water, creating a positive feedback that leads to further melting and warming.
Such mechanisms can change the temperature of ocean layers and impact ocean circulation and salinity, Steele says. For example, the Arctic Ocean during winter is usually very cold and produces lots of sea ice, which creates cold, salty water that sinks to deep levels and drives ocean circulation. But if surface waters warm and ice does not form as well in winter, these processes involving salinity and circulation could be reduced or eliminated. "Then the Arctic Ocean and the North Atlantic Ocean will look very different," Steele says.
Also, if expansive areas now covered by year-round sea ice were to melt and the water were to open up, it could create shipping lanes where none are possible today. While that may be good for some industries, loss of sea ice would also alter wildlife habitats. Polar bears, walrus, and seals, for example, are all adapted to life on sea ice. Indigenous communities based around the Arctic may face enormous changes in terms of both culture and how they meet their people's basic needs of food, shelter, and clothing.
Moreover, Arctic warming and sea ice loss are likely to influence the loss of land ice, says Rind. Temperatures may rise to levels where land ice melts, and feedbacks created by sea ice loss reinforce regional Arctic warming, which in turn could cause more land ice to melt. While sea ice already floats and does not contribute to sea levels, melting land ice adds new water to the oceans. By 2100, sea levels are projected to rise by almost 1 meter (0.9 meters), according to the 2001 Intergovernmental Panel on Climate Change report. Loss of land glaciers, which is already evident, could cause sea levels to rise even further.
"Sea levels will continue to rise long after greenhouse gases are stabilized, further endangering coasts and island communities," says Rind. The loss of sea ice by itself initiates ocean and high-latitude warming regardless of greenhouse effects.
Additionally, some scientists say Arctic warming could change our atmosphere. If Arctic areas continue to warm, scientists speculate that thawing Arctic soils may release significant amounts of carbon dioxide and methane currently trapped in permafrost. Slightly warmer ocean water may also release frozen natural gases in the sea floor, all of which act as greenhouse gases in the atmosphere, says Rind. However, the extent to which Arctic warming will add greenhouse gases to our atmosphere is a matter of debate.
To test the future climate, Rind used the Goddard Institute for Space Studies climate model to run a few simple experiments on how a warming Arctic might impact, say, the state of Kansas. The model was run to test how Kansas might be affected by sea ice loss by itself—that is, without assuming any additional greenhouse warming and without any temperature increases to the oceans. The air over sea ice is generally cold, and when you remove that ice, the air masses above warm considerably. Those Arctic air masses consistently blow south over North America during winter. As a result, the model predicts that sea ice loss will result in warming of over 2 degrees Celsius (4 degrees Fahrenheit) in Kansas in the winter, with a loss of 40 percent of the typical snow cover.
"While to some this may sound positive, the vernalization of winter wheat requires subfreezing temperatures; warmer winters would likely require a change in the type of wheat that would be grown which would affect inputs and markets. Worldwide agricultural impacts are likely," Rind says.
In summer, the model tests found that loss of sea ice by itself will raise the temperature a few degrees Fahrenheit and reduce soil moisture in Kansas by up to 10 percent. "Loss of snow cover and associated spring runoff from this reservoir will affect wide regions," Rind adds.
Rind's simple model simulation clearly illustrates that changes to Arctic ice alone alter climates the world over. And that's just the start. As the Arctic warms and sea ice declines the ramifications will play out in social, political, and scientific realms. "The implications of Arctic warming can be enormous," Comiso says.
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Recent Warming Of Arctic May Affect Worldwide Climate
Greenbelt - Oct 24, 2003
Recently observed change in Arctic temperatures and sea ice cover may be a harbinger of global climate changes to come, according to a recent NASA study. Satellite data -- the unique view from space -- are allowing researchers to more clearly see Arctic changes and develop an improved understanding of the possible effect on climate worldwide.