Dec. 12 (Bloomberg) -- Arctic sea-ice retreat is likely to accelerate so rapidly that the Arctic Ocean will be nearly ice- free in summer by 2040, atmospheric scientists said.

Further increases in the atmosphere of so-called greenhouse gases may lead to global warming that causes the already- retreating ice to begin melting four times faster in about 20 years' time, a team led by U.S. National Center for Atmospheric Research scientist Marika Holland says today in research published in the journal Geophysical Research Letters.

``We have already witnessed major losses in sea ice, but our research suggests that the decrease over the next few decades could be far more dramatic than anything that has happened so far,'' Holland said yesterday in a statement posted on the NCAR Web site. ``These changes are surprisingly rapid.''

Using climate-change models, Holland's team forecast that by 2040, ``only a small amount of perennial sea ice'' could be left, according to the statement. The melting can be slowed by cutting emissions of gases such as carbon dioxide and methane, blamed by many scientists as the cause of global warming, it said.

``We don't see this sort of behavior in the absence of increases in greenhouse gas concentrations,'' Holland said in an interview aired today by British Broadcasting Corp. radio's ``Today'' program. The melting ``very definitely is caused in the climate model by increased greenhouse gas levels.''

Scientists posit that the gases remain in the Earth's atmosphere where they trap the sun's energy that is reflected from the Earth's surface. Other scientists say warming is a natural phenomenon that recurs in a cyclical manner over time, or that it is caused by increased activity on the sun.

`Dramatic Implications'

The study builds on previous reports by the Boulder, Colorado-based U.S. National Snow and Ice Data Center, which showed the minimum summer extent of Arctic sea ice retreated last year to a record low. Five of the six lowest values in a series dating back to 1979 have occurred in the past five years, NSIDC scientist Walt Meier said in an Oct. 3 interview.

Meier said at the time the effect was likely to accelerate in a so-called ``feedback loop,'' with the dark patches of exposed ocean absorbing more heat, increasing the ice-melt, and leading to an ice-free Arctic Ocean as early as 2060. The feedback thesis was backed up by NCAR, also based in Boulder.

``As the ice retreats, the ocean transports more heat to the Arctic and the open water absorbs more sunlight, further accelerating the rate of warming and leading to the loss of more ice,'' Holland said in the statement. ``This is a positive feedback loop with dramatic implications for the entire Arctic.''

Winter Ice

Sea-ice extent, the area of ocean covered by at least 15 percent ice, was about 5.7 million square kilometers (2.2 million square miles) according to a five-day average ending September 14, when this year's coverage was at its lowest, according to the NSIDC. The 2006 minimum, the fourth-lowest on record, compared with the low of 5.32 million square kilometers from 2005.

The loss of sea-ice cover poses risks to the way of life of indigenous Arctic people such as the Inuit, who travel between islands across the ice, and to animals such as polar bears who rely on sea-ice as their hunting ground.

This October, temperatures across much of the Canadian Arctic were as much as 9.3 degrees centigrade warmer than the 1951-1980 average for that month, according to graphics on the Web site of NASA's Goddard Institute for Space Studies. Those temperatures are slowing the winter refreezing of the sea.

``What we're starting to see is the winter ice is not recovering anymore,'' NSIDC scientists Mark Serreze said in an interview aired by the BBC. ``2006 is a case in point: we see that at the end of November, we have 2 million square kilometers less ice than we should have in a typical year.''

Perennial Sea Ice

NASA said in September that the Arctic's perennial sea ice -- or the frozen water that usually doesn't melt during the summer -- last winter shrank by about 720,000 square kilometers, an area the size of Texas. Overall ice coverage, which also includes thin seasonal ice, was stable over the winter. The proportion accounted for by perennial ice -- 3 or more meters (10 or more feet) thick, decreased, it said.

Holland's team includes scientists at the University of Washington in Seattle, and McGill University in Montreal, Canada. The research was funded mainly by the National Science Foundation. The National Snow and Ice Data Center is part of the University of Colorado's Cooperative Institute for Research in Environmental Sciences.

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: December 12, 2006 10:38 EST