Arctic Sea Ice Shrinking, Greenland Ice Sheet Melting, According to Study
The total area of surface melt on the Greenland Ice Sheet for 2002 broke all known records for the island and the extent of Arctic sea ice reached the lowest level in the satellite record, according to scientists at the University of Colorado at Boulder.
Researchers from the CU-based Cooperative Institute for Research in Environmental Sciences, or CIRES, say the accelerated melting appears to be linked to shifts in Northern Hemisphere atmospheric circulation patterns.
The 2002 sea-ice record is the most recent evidence of a downward trend in Arctic sea ice in the decades since satellite monitoring began, said Research Associate Mark Serreze, lead author on a 2002 paper on sea ice extent and area in the Arctic. Serreze is a researcher at CIRES' National Snow and Ice Data Center, or NSIDC.
The study also found temperatures during the summer of 2002 were unusually warm over much of the Arctic Ocean. "Since the season also was characterized by very stormy conditions, we believe these two factors contributed to extensive melt and break-up of the icepack," said Serreze.
It is likely that the 2002 minimum sea-ice record in the Arctic is the lowest since the early 1950s and possibly the lowest in several centuries, according to researcher James Maslanik, a co-author of the study and a professor in the College of Engineering and Applied Science.
Satellite monitoring by NSIDC led to the discovery of the record sea-ice retreat, said Research Associate Julienne Stroeve. "We saw an unusually pronounced loss of ice in the Beaufort, Chukchi, East Siberian and Laptev Seas," she said, noting the ice extent in September 2002 was roughly 2 million square miles compared to the long-term average of about 2.4 million square miles.
"We had a hunch it was setting up to be a record year in August," said Ted Scambos of NSIDC, who has been working in Earth's polar regions. "What we saw really surprised us. Not only was sea ice retreating in nearly every sector, but the interior ice was unusually thin and spread out."
Preliminary measurements from the Greenland Ice Sheet show the melt extent of 265,000 square miles, a new record, underscoring the unusual warming there and surpassing the maximum melt extent from the past 24 years by more than 9 percent, said CIRES climatologist Konrad Steffen, a professor in geography and in the Program in Atmospheric and Oceanic Sciences at the University of Colorado.
Steffen's analyses with graduate student Russel Huff show a dramatically higher melting trend since 1979 that appears only to have been interrupted once -- in 1991 -- when the Philippines' Mt. Pinatubo erupted.
Steffen and Huff said the northern and northeastern portion experienced extreme melting reaching as high as 6,560 feet in elevation, where temperatures normally are too cold for melting to occur. The highest point in Greenland rises to nearly 11,000 feet.
Both sea ice and glacier ice cool Earth, reflecting about 80 percent of springtime solar radiation and 40 percent to 50 percent during summer snowmelt. In winter, ice cover slows heat loss from relatively warm ocean water to the cold atmosphere. Without large sea-ice masses at the poles to moderate the global energy balance, warming escalates, said Scambos.
The CU-Boulder findings were reported in a press briefing at the annual fall meeting of the American Geophysical Union, held Dec. 6 to Dec. 10 in San Francisco.
CU scientists estimate that a change in the Greenland climate toward warmer conditions would lead to an increase in the rate of sea-level rise mainly due to the dynamic response of the large ice sheet and not so much to the surface melting.
"For every degree (F) increase in the mean annual temperature near Greenland, the rate of sea level rise increases by about 10 percent," Steffen said. Currently the oceans are rising by a little more than half an inch per decade. In addition, melt water has been shown to directly affect the rate of ice flow off Greenland, penetrating the ice sheet and causing the glaciers to accelerate in speed as they slide over a thin film of melt water.
Excessive melting of sea ice, along with runoff from the Greenland Ice Sheet, also has the potential to "cap" deep water convection in the North Atlantic. This could profoundly impact global ocean circulation and climate, Serreze said. "In other studies, changes in the North Atlantic circulation have been implicated in starting and stopping Northern Hemisphere ice ages."
"The unusual conditions seen in 2002 are part of a larger pattern of recent Arctic change," said Serreze. This includes pronounced warming over sub-Arctic land areas. These changes are associated at least in part with a positive trend in the "Arctic Oscillation," characterized by reductions in atmospheric pressure over the Arctic and higher pressures in the mid-latitudes that are associated with more storms and warmer temperatures in the high Arctic.
"It is likely that sea ice extent will continue to decline over the 21st century as the climate warms," said Serreze. "With these trends, we may see an approximate 20 percent reduction in the annual mean sea ice by 2050, and by then we might be approaching no ice at all during the summer months."
CIRES is a joint institute of CU and the National Oceanic and Atmospheric Administration. Additional information is available via http://nsidc.org/arcticseaicenews/ and http://nsidc.org/greenland-today/.