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The Arctic's 30-year warming trend persisted in 2014, with air temperature increases at twice the rate of the rest of the world, the U.S. government reported Wednesday in its eighth annual Arctic Report Card. "The global rate of temperature increase has slowed in the last decade, but Arctic temperatures continued to increase," the report found.

A special secton on polar bears found a mixed story: A population decline in Canada's western Hudson Bay was attributed to a shorter sea ice season. In Alaska's southern Beaufort Sea, however, numbers stabilized after about a 40 percent decline over the previous decade.

While there were no big climate surprises, the persistent warmth is itself worrisome, according to experts who prepared the report. "We can't expect records every year. It need not be spectacular for the Arctic to be changing," Martin Jeffries, the report's chief editor and an Arctic adviser for the Navy's Office of Naval Research, told reporters. "We're seeing the power of persistence," added Jacqueline Richter-Menge, a scientist with the federal Cold Regions Research and Engineering Lab.

While the report card didn't actually assign grades, Richter-Menge told NBCNews.com that she'd have to give a "grade of incomplete due largely to the fact that it is an evolving story." Craig McLean, a senior National Oceanic and Atmospheric Administration official, noted that eight nations share the Arctic. "This is a class project among many nations working together," he said.

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