LOS ANGELES, Aug. 24 (Xinhuanet) -- The Arctic glaciers and ice sheets are melting more quickly because of global warming, which may lead to ice-free summer in a century, a team of scientists reported on Wednesday.

    The researchers said there is no known natural processes that might slow the de-icing of the Arctic. Such substantial additionalmelting of Arctic glaciers and ice sheets will raise sea level worldwide, flooding the coastal areas where many of the world's people live.

    Melting sea ice has already resulted in dramatic impacts for the indigenous people and animals in the Arctic, which includes parts of Alaska, Canada, Russia, Siberia, Scandinavia and Greenland, said the researchers.

    "What really makes the Arctic different from the rest of the non-polar world is the permanent ice in the ground, in the ocean and on land," said Jonathan Overpeck, geoscientist at the University of Arizona.

    "We see all of that ice melting already, and we envision that it will melt back much more dramatically in the future as we move towards this more permanent ice-free state."

    The report by Overpeck and his colleagues is published in the Eos, the weekly newspaper of the American Geophysical Union. The researchers said a seasonally ice-free state hasn't been seen in the Arctic region for more than 1 million years.

    The past climates in the Arctic include glacial periods, where sea ice coverage expanded and ice sheets extended into Northern America and Europe, and warmer interglacial periods during which the ice retreats, as it has during the past 10,000 years.

    By studying natural data such as ice cores and marine sediments, scientists have a good idea what the "natural envelope"for Arctic climate variations has been for the past million years,Overpeck said.

    The team of scientists synthesized what is currently known about the Arctic and defined key components that make up the current system. They identified how the components interact, including feedback loops that involve multiple parts of the system.

    "In the past, researchers have tended to look at individual components of the Arctic," said Overpeck. "What we did for the first time is really look at how all of those components work together."

    The team concluded that there were two major amplifying feedbacks in the Arctic system involving the interplay between seaand land ice, ocean circulation in the North Atlantic, and the amounts of precipitation and evaporation in the system.

    Such feedback loops accelerate changes in the system. For example, the white surface of sea ice reflects radiation from the sun. However, as sea ice melts, more solar radiation is absorbed by the dark ocean, which heats up and results in yet more sea ice melting.

    While the researchers identified one feedback loop that could slow the changes, they did not see any natural mechanism that could stop the dramatic loss of ice.

    In addition, Overpeck warned that permafrost, the permanently frozen layer of soil that underlies much of the Arctic, will melt and eventually disappear in some areas. Such thawing could releaseadditional greenhouse gases stored in the permafrost for thousandsof years, which would amplify human-induced climate change.

    Overpeck said humans could step on the brakes by reducing carbon dioxide emissions.

    "The trouble is we don't really know where the threshold is beyond which these changes are inevitable and dangerous," he said."Therefore it is really important that we try hard, and as soon aswe can, to dramatically reduce such emissions." Enditem