Sea ice in the Arctic Ocean underwent a sharp recovery this year from the record-low levels of 2012, with 50 percent more ice surviving the summer melt season, scientists said Friday. It is the largest one-year increase in Arctic ice since satellite tracking began in 1978.
The experts added, however, that much of the ice remains thin and slushy, a far cry from the thick Arctic pack ice of the past. Because thin ice is subject to rapid future melting, the scientists said this year's recovery was unlikely to portend any change in the relentless long-term decline of Arctic sea ice.
"I'm not at all surprised there was a jump upward — we've never set two record lows in a row," said Walt Meier, a NASA scientist who has monitored sea ice for years. "I would say I'm a little surprised the jump is as big as it is."
Last year's ice extent was so low that this year's recovery looks larger by comparison, Dr. Meier said. The main reason for this year's growth, Dr. Meier added, was that the region was colder and cloudier through the spring and summer than in the recent past.
"We had cool conditions, cooler than the long-term average, and yet it is still going to be the sixth-lowest ice minimum on record," Dr. Meier said.
Playing out over a generation, the decline of the Arctic ice cap has been one of the most striking effects of global warming, a change in the planetary aspect so large it would have been visible to an observer on the moon. "We could be looking at summers with essentially no sea ice on the Arctic Ocean only a few decades from now," said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo., in a statement.
Contrary to popular impression, the melting of sea ice does not cause a rise of sea level, since the ice is already floating and displacing its weight in water. But the replacement of white ice with dark water does mean the surface of the Arctic Ocean can absorb far more heat in the summer, which could contribute to the melting of nearby land ice in Greenland, raising sea level.
During the winter, the Arctic plunges into near-darkness 24 hours a day, and a skin of ice grows across most of the ocean surface. But during the summer, the sun shines continuously. The key to the status of the ice cap is how much of it survives the summer melt season to become thick, resilient ice.
When satellite monitoring began in the late 1970s, about half the surface of the Arctic Ocean would be covered by ice at the end of the melt season, which usually occurs in September. By last year, that figure had fallen to 24 percent of the ocean surface, rising this year to 36 percent.
The National Snow and Ice Data Center announced Friday that the low point in sea ice this year had occurred on Sept. 13. The timing varies from year to year, and the agency always waits several days before making an announcement to be certain the ice pack has begun to regrow.
Lately, a new low in summer sea ice has been set every few years, followed by a few years of recovery, followed by yet another low that typically exceeds the previous one by a substantial margin.
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