Arctic clear for summer sailing by 2040 - Nature
|Published online: 11 December 2006; | doi:10.1038/news061211-1|
Models predict rapid decline of sea ice.
Some projections from climate centres worldwide have suggested previously that Arctic sea ice could vanish in September, at the end of the summer melt, by as early as 2050. But the most recent calculations, performed on the Community Climate System Model at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, predict the most rapid and imminent decline yet.
Models differ in their assumptions of how sensitive Arctic ice is to warming air and water temperatures at different times of year. "We're in the more sensitive bracket among models, but we're not an outlier by any means," says Cecilia Bitz, an atmospheric scientists at the University of Washington and a co-author of the study.
The research team, led by Marika Holland, based at NCAR, modeled fluctuations in the sea ice since 1870 using seven distinct simulations. Their work, published this week in Geophysical Research Letters, indicates that in one scenario, September ice-cover could be reduced from 6 million to 2 million square kilometres in just one decade. About 20% of present-day ice cover would remain. But most of this ice would hug the coasts of Canada and Greenland, leaving the Arctic Ocean nearly free of ice at the end of summer.
Across all the seven simulations, which closely match scientists' observations of actual sea ice disappearance up to the present, the latest that the summer ice is expected to disappear is by 2055.
Sea ice waxes and wanes seasonally. The ice pack melts throughout the warm, summer months, usually reaching its minimum thickness and extent in the first two weeks of September. In autumn and winter, as the Arctic deep freeze sets in, the ice is replenished to an annual average of some 15 million square kilometers, blanketing the Arctic Ocean. The thickness and extent of sea ice at summer's end play an important role in the formation of ice the following winter.
For five straight years now, the ice cover has dipped well below average during the summer months, with 2005 setting a record minimum for September ice: Arctic sea ice dropped 20%, or 1.3 million square kilometres, below the 1979-2000 average. This loss equals roughly twice the size of the state of Texas. (September 2006 was expected to set another record in sea ice loss, but an unseasonably cool August stopped it from doing so.) The trend over the past five years translates to an 8% loss in sea ice each decade.
One of the reasons for this potentially dramatic decline in sea ice cover is what climate scientists refer to as a feedback in the climate system known as the 'albedo effect': ice reflects more sunlight than dark, open water and serves as an insulator to the warmer ocean below it. As the ice pack shrinks and thins in the summertime, the insulating effect is weakened and the dark waters below store more heat, further accelerating the ice melt.
Losses at both ends
The increased summer thaw is having an impact on the winter ice cover too. Over the past two winters, the area of sea ice has fallen by 6% per year, compared with an average of 1.5% per decade since 1979, according to NASA.
"The recovery in autumn is no longer what it once was," says Mark Serreze, a sea ice expert with the National Snow and Ice Data Center in Boulder. "What we're starting to see now is that we're losing ice on both ends of the seasons."
All this should open passage for ships and submarines wishing to traverse the Arctic Ocean, at least in summer. It also means a loss of habitat for polar bears, as the winter ice becomes too thin to support their weight as they travel long distances. And plankton that thrive in icy conditions and provide the base of the food chain may not be suited to ice-free conditions.
Serreze notes that while climate model predictions vary on timescales for the disappearance of summer sea ice, "these climate models are in near universal agreement that as the climate warms in response to greenhouse gas loading, we're going to lose the Arctic sea ice cover."