Arctic ice more vulnerable to sunny weather, new study shows

The shrinking expanse of Arctic sea ice is increasingly vulnerable to summer sunshine, new research concludes. The study, by scientists at the National Center for Atmospheric Research (NCAR) and Colorado State University (CSU), finds that unusually sunny weather contributed to last summer's record loss of Arctic ice, while similar weather conditions in past summers do not appear to have had comparable impacts.

The study, which draws on observations from instruments on a new group of NASA satellites known as the "A-Train," will be published tomorrow in Geophysical Research Letters. It was funded by NASA and the National Science Foundation, which is NCAR's principal sponsor.

"In a warmer world, the thinner sea ice is becoming increasingly sensitive to year-to-year variations in weather and cloud patterns," says NCAR's Jennifer Kay, the lead author. "A single unusually clear summer can now have a dramatic impact."

The findings indicate that summer sunshine in the Arctic produces more pronounced melting than in the past, largely because there is now less ice to reflect solar radiation back into space. As a result, the presence or absence of clouds now has greater implications for sea ice loss.

Satellite data offer clues to record-shattering 2007 melt

Last summer's loss of Arctic sea ice set a modern-day record, with the ice extent shrinking to a minimum of about 1.6 million square miles (4.1 million square kilometers) in September. That was 43 percent less ice coverage than in 1979, when accurate satellite observations began.

Looking at the first two years of data from radar and lidar on the A-Train satellites, Kay and her colleagues found that total summertime cloud cover in the Western Arctic was 16 percent less in 2007 than the year before. A strong high-pressure system centered north of Alaska kept skies relatively clear. Over a three-month period in the summer, the increased sunshine was strong enough to melt about a foot of surface ice. Over open water, it was sufficient to increase sea-surface temperatures by 4.3 degrees Fahrenheit (2.4 degrees Celsius). Warmer ocean waters can contribute to sea ice loss by melting the ice from the bottom, thereby thinning it and making it more susceptible to future melt.

"Satellite radar and lidar measurements allow us to observe Arctic clouds in a new way," says CSU scientist Tristan L'Ecuyer, a co-author of the study. "These new instruments not only provide a very precise view of where clouds exist but also tell us their height and thickness, which are key properties that determine the amount of sunlight clouds reflect back to space."

The research team also examined longer-term records of Arctic cloud and weather patterns, including a 62-year-long record of cloudiness from surface observations at Barrow, Alaska. They found that the 2007 weather and cloud pattern was unusual but not unprecedented. At Barrow, five other years--1968, 1971, 1976, 1977, and 1991--had less summertime cloud cover than 2007, but without the same impact on sea ice.

A summer feedback cycle

The research suggests that warmth from the Sun will increasingly affect Arctic sea ice loss in the summer. As the ice shrinks, incoming sunshine triggers a spiraling effect: the newly exposed dark ocean waters, much darker than the ice, absorb the Sun's radiation instead of reflecting it. This warms the water and melts more ice, which in turn leads to more absorption of radiation and still more warming.

"Our research indicates that the relative importance of solar radiation in the summer is changing," Kay says. "The sunshine reaching the Arctic is increasingly influential, as there is less ice to reflect it back into space. Dry, sunny conditions in a single summer can now act as a potent force to melt sea ice."

The authors note that, in addition to solar radiation, other factors such as changes in wind patterns and possibly shifts in ocean circulation patterns also influence sea ice loss. In particular, strong winds along regions of sea ice retreat were important to last year's loss of ice. The relative importance of these factors, and the precise extent to which global climate change is driving them, are not yet known.

The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under primary sponsorship by the National Science Foundation. Opinions, findings, conclusions, or recommendations expressed in this release are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Source: National Center for Atmospheric Research

Related stories:

Global sea-rise levels by 2100 my be lower than some predict, says new study
(PhysOrg.com) -- Despite projections by some scientists of global seas rising by 20 feet or more by the end of this century as a result of warming, a new University of Colorado at Boulder study concludes that global sea rise of much more than 6 feet is a near physical impossibility.
Canada's Arctic ice shelves break apart, drift away
Two ice shelves in Canada's far north have lost massive sections since August while a third ice shelf now is adrift in the Arctic Ocean, said researchers Wednesday who blamed climate change.
Unexplored Arctic region to be mapped
A scientific expedition this fall will map the unexplored Arctic seafloor where the U.S. and Canada may have sovereign rights over natural resources such as oil and gas and control over activities such as mining.
Bad sign for global warming: Thawing permafrost holds vast carbon pool
Permafrost blanketing the northern hemisphere contains more than twice the amount of carbon in the atmosphere, making it a potentially mammoth contributor to global climate change depending on how quickly it thaws.
Arctic ice on the verge of another all-time low
Following last summer's record minimum ice cover in the Arctic, current observations from ESA's Envisat satellite suggest that the extent of polar sea-ice may again shrink to a level very close to that of last year.
Arctic sea ice drops to 2nd lowest level on record
(AP) -- Arctic Ocean sea ice has melted to the second lowest minimum since satellite observations began, according to scientists at the National Snow and Ice Data Center. Sea ice melt recorded on Monday exceeded the low recorded in 2005, which had held second place.
Canada to map Arctic's vast resources
Canada's Prime Minister Stephen Harper announced Tuesday an ambitious plan to discover and tap mineral, oil and gas riches believed to be hidden beneath the cool Arctic.
RV Polarstern on its way to East Siberian Sea
Bremerhaven, August 19th 2008. German research vessel Polarstern, operated by the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, transits the Northwest Passage for the first time. Polarstern left the port of Reykjavik on August 12th, sailed around Greenland on a southern course and is located right in the Northwest Passage. Its destination is the East Siberian Sea where geoscientific measurements at the junction between the Mendeleev Ridge and the East Siberian Shelf are at the focus of the participants of this expedition. The measurements striven for in the framework of the International Polar Year shall help to understand how the undersea ridges and basins were built. This expedition takes the researchers in 68 days around the North Pole because the return voyage is to lead via the Northeast Passage.

News discussion:

Space & Earth science news