By Deborah Zabarenko, Environment Correspondent

WASHINGTON, Sept 20 (Reuters) - Arctic sea ice melted to its lowest level ever this week, shattering a record set in 2005 and continuing a trend spurred by human-caused global warming, scientists said on Thursday.

"It's the biggest drop from a previous record that we've ever had and it's really quite astounding," said Walt Meier, a research scientist at the U.S. National Snow and Ice Data Center in Colorado.

Sea ice freezes and melts seasonally, but never has it ebbed to this small a patch, the data center said in a statement. Compared to 2005, the previous record-low year for Arctic sea ice, this year saw a decrease of more than 386,100 square miles (1 million square km).

That is about the size of Texas and California combined, or nearly five times the size of the United Kingdom, the center said. It is more than double the drop between 2005 and 2002, the previous record-holding year.

"That's a dramatic change in one year," Meier said of this year's sea ice decrease. "Certainly we've been on a downward trend for the last 30 years or so, but this is really accelerating the trend."

The minimum amount of ice occurred on Sunday and freezing has already begun in some places, according to satellite imagery used by the center.

EARTH'S AIR CONDITIONER

Melting sea ice, unlike the melting glaciers of Greenland and Antarctica, does not contribute to global sea level rise, much as an ice cube in a glass of water does not make the level of liquid rise when it melts.

However, without the bright white of sea ice to reflect the sun's rays, the Earth loses what some climate scientists call its air conditioner. The less ice there is, the more dark water there is to absorb the warming solar radiation.

This year's record was caused by a "perfect storm" of interacting factors, Meier said by telephone.

These included a long-running high pressure system that kept skies cloudless over the Arctic, along with a circulation pattern that pushed ice out of the Arctic towards Greenland, instead of letting it circle around the Beaufort Sea north of Alaska as it usually does.

Also, there was thinner ice to begin with, Meier said.

While this particular year's ice minimum cannot be directly attributed to anthropogenic -- human-caused -- global climate change, the trend that brought it about can, he said.

"This year, the reason why (the ice) was so low was not because there's more anthropogenically generated carbon dioxide dumped in the last year, it's because of this high pressure ... but you can't really explain the overall trend without invoking anthropogenically global warming," Meier said.

He also noted that the decrease in Arctic sea ice was forecast in models used by the U.N. Intergovernmental Panel on Climate Change, which this year said with 90 percent probability that global warming exists and that human activities contribute to it.

However, the sea ice is diminishing much faster than any of the models predicted, Meier said.

More information and images are available at http://nsidc.org/news/press/2007_seaiceminimum/20070810_index.html.

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