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Now to the Arctic Ocean, where researchers say the ice has melted to its second-lowest level on record. The summertime melt coincides with a dramatic warming over the past decade, and it's already affecting the region's wildlife. NPR's Richard Harris has more.

RICHARD HARRIS: Ice floating on the Arctic Ocean comes and goes with the seasons. Typically, about half of the wintertime ice melts away by mid-September, and after that low point, the ice regrows. In 2007, the amount of ice left in September hit a dramatic low. And Mark Serreze, who heads the National Snow and Ice Data Center in Colorado, says this year's low is not far off that 2007 record.

MARK SERREZE: What it's telling us is that the long-term decline in Arctic sea ice is continuing, and even appears to be accelerating at this point.

HARRIS: Serreze says what's notable about this year is how different the weather was, compared with the record-low year in 2007.

SERREZE: In 2007, one of the big reasons we got to a record low was because we essentially had a near-perfect weather pattern that pumped a lot of warm air into the Arctic.

HARRIS: Unusually cloud-free skies hastened the melting, and wind patterns also made the ice prone to disappear. Serreze says this year's melt was nearly as bad, but without the extreme weather.

SERREZE: That's telling us that sea ice is really in trouble. The ice is so thin now, that it just can't take a hit in summer anymore.

HARRIS: And it's setting up a vicious cycle. There's no question that the Arctic Ocean will freeze up again over the fall and winter, but there's less and less really thick ice building back up year to year.

SERREZE: Come next spring, we're just going to have a lot of thin ice that formed over the autumn and winter. That's the stuff that melts out easily the next summer. So, there's a feedback at work here, and that feedback is getting stronger with time.

HARRIS: Serreze says the Arctic could be ice-free in the summer by about 2030, though it is hard to predict, and other scientists say it could be mid-century before we reach that dramatic point. And why does this matter? Well, ice that's floating on the sea surface doesn't raise sea level when it melts. But researchers suspect it will alter the weather that reaches us far to the south, and it's already affecting Arctic wildlife. Thousands of walruses that usually float around on sea ice and dive down to feed on the ocean floor abandoned those floes when the only ice left off the coast of Alaska was over water that was too deep for them.

CHAD JAY: So, after the sea ice ran out, a lot of the walruses came to the shore of northwest Alaska and began hauling out there.

HARRIS: Chad Jay is a research ecologist with the U.S. Geological Survey in Alaska. He says it's harder on the walruses to feed from shore, because they have to swim farther to get back to their food supply. It's also more hazardous for them to be up on the rocks instead of on ice, Jay says.

JAY: When they're hauled out in these large aggregations on the shore, if there's a disturbance or something that causes walruses to flee into the water quickly, then sometimes the calves get run over and die as a result.

HARRIS: Jay is now trying to figure out how the walruses will adapt as the sea ice continues to melt away. Richard Harris, NPR News.

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