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The Arctic zone has moved into a warmer, greener "new normal" phase, which means less habitat for polar bears and more access for development, an international scientific team reported.
Arctic air temperatures were higher - about 1.5degC higher in 2011 than the baseline number for the previous 30 years - and there was a dramatic loss of sea ice and glacier mass, the scientists said in a telephone briefing.
With less bright ice to reflect sunlight, and more dark open water to absorb it, the Arctic's changed characteristics are likely to feed on each other and accelerate, specialists from 14 countries said in an annual assessment called the Arctic Report Card.
"We've got a new normal," said Don Perovich, an expert on sea ice at the U.S. Army Corps of Engineers' Cold Regions Research & Engineering Laboratory in New Hampshire.
"Whether it's a tipping point and it will never recover, who can say? But we have a new normal ... that has implications not just for the ice but other components of the Arctic system."
The turning point for the Arctic came in 2006, when persistent weather patterns pushed sea ice out of the Arctic, setting the stage for 2007, when Arctic ice extent - the area of the ocean covered by ice at summer's end - dropped to its lowest level ever. In 2011, Arctic sea ice reached its second-lowest extent.
Released as UN climate talks proceed in Durban, South Africa, the Arctic report found significant changes in atmospheric, sea ice and ocean conditions, and in land-based ice including glaciers, while marine and terrestrial ecosystems were also changed by the Arctic warming trend.
The Arctic acts as Earth's "air conditioner" and also as a potent global weather-maker. As a result, sweeping changes there influence life across the planet. The report found that even as the Arctic warmed, a shift in weather patterns sent cold Arctic air as far south as the United States and densely populated parts of northern Europe.
With less sea ice to clog potential shipping lanes, development in the Arctic is likely, said Monica Medina of the US National Oceanic and Atmospheric Administration. She suggested that this report and others could help "prepare for increasing demands on Arctic resources" as warming makes these resources more available.
The Arctic "new normal" means oil and gas companies and tourists can begin to expect routine access to the area, according to report co-author Jackie Richter-Menge of the Cold Regions Research and Engineering Laboratory.
The new warmth in the Arctic means more tundra vegetation, with taller shrubs winning out over lower-lying moss and lichens, which could in turn affect caribou and reindeer.
The loss of sea ice cuts into the habitat of polar bears and walruses, which use ice floes as hunting platforms, the scientists said.
Whales were winners, especially those that migrate from temperate areas, because they could stay for longer periods in the Arctic while the water there was open in the summer. Populations of tagged bowhead whales from Alaska and west Greenland were able to mingle in the Northwest Passage, which until this century was blocked by ice.
At the base of the marine food chain, biological productivity soared by 20 percent between 1998 and 2009 as more sunlight penetrates increasingly open Arctic water, the scientists said.
Open Arctic water also absorbs climate-warming carbon dioxide, but that has made the Beaufort and Chukchi seas more acidic, which could erode the shells of some shellfish.