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An Arctic alert on global warming

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Global warming is heating the Arctic at a rapid pace - with impacts that could range from the disappearance of polar bears' summer habitat by the century's end to a damaging rise in sea levels worldwide.

That assessment, released Monday by a group of international climate experts, amounts to one of the most urgent warnings on climate change to date, and could put new pressure on the US and other nations to curb fossil-fuel emissions.

This comes at a time of growing concern about the effects of global warming, which scientists generally agree is increasingly driven by rising carbon-dioxide levels in the atmosphere from human industrial activity and changing land-use patterns.

Monday's report called for "strong near-term action" to reduce output of gases that, when they rise into the atmosphere, trap heat in what is called the greenhouse effect.

The trends cited in the Arctic Climate Impact Assessment are stark:

• Rapid melting of Arctic glaciers, including the vast sheet of ice that covers Greenland. The sheet locks up enough fresh water to raise sea levels by as much as 27 feet over the course of several centuries. The group calculates that during this century, Greenland temperatures are likely to exceed the threshold for triggering the long-term meltdown of the island's ice sheet.

• Arctic temperatures rising up to twice as fast as the global average. Over the past 50 years, average winter temperatures in Alaska, western Canada, and eastern Russia have risen as much as 7 degrees F. Over the next century, temperatures are projected to rise by up to 13 degrees F.

• A dramatic reduction in the extent of the summer ice pack in the Arctic Ocean. Late-summer ice coverage already has declined by as much as 20 percent over the past three decades. The summer ice pack is projected to shrink by another 10 to 50 percent by the end of the century. Some climate models show the summer ice vanishing by 2040.

Either change could accelerate warming by allowing the ocean to absorb solar heat. The change could threaten species such as polar bears and some seals with extinction. Researchers also worry that an influx of fresh water into the North Atlantic could disrupt large-scale ocean currents worldwide, altering weather patterns and the locations where nutrients rise from the depths to support regional fisheries.

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