The layer of ice that covers the Arctic Ocean has reached its maximum extent for the year. After several months of expanding over the cold Arctic winter, it has now begun its spring retreat.
According to the National Snow and Ice Data Center (NSIDC), Arctic sea ice reached its seasonal maximum on March 21, after undergoing a brief surge mid-month. It topped out at 14.91 million square kilometers (5.76 million square miles), which is the fifth lowest winter maximum in the satellite record. The lowest maximum yet recorded is 14.63 million square kilometers, or 5.65 million square miles, in 2011.
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The loss of Arctic sea ice is a major concern for scientists because it provides essential habitat for species like polar bears and ringed seals and because its disappearance could alter the entire Arctic marine ecosystem by removing essential algae, which forms the basis of the food web.
Ice also reflects sunlight. As it melts, sea ice is replaced by darker ocean, which absorbs that sunlight, creating further warming. And heat rising from a warmer Arctic Ocean may disrupt atmospheric circulation, prompting frigid winters such as that experienced by much of the northern and eastern United States recently.
This year's figure looked set to be much lower until surface winds helped to spread out the ice pack in the Barents Sea, where ice cover had been anomalously low all winter. Northeasterly winds also helped push the ice pack southward in the Bering Sea, another area where until the ice cover had until then been very low.
Encouragingly, the volume of winter ice increased relative to last year, because of an increase in multiyear ice. One of the reasons scientists speak of Arctic sea ice being in a "death spiral" is that, as the region has warmed and the ice cap has retreated, much of the older, thicker ice that normally survived two or more years has melted, leaving newer, thinner ice that melts more quickly. But the proportion of Arctic sea ice that is multiyear ice increased from 30 percent 12 months ago to 43 percent this year.
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Unfortunately, that doesn't mean less ice will melt this summer. After all, that's still a smaller percentage of multiyear ice than existed at the start of the melt season in 2007, which concluded with a then-record low minimum extent. And a large area of multiyear ice has drifted to the southern Beaufort Sea and East Siberian Sea, where warm conditions are likely to exist later in the year.
In fact, as NSIDC's Julienne Stroeve and colleagues point out in a new paper in the journal Geophysical Research Letters, while the medium-to-long-term declining trend is clear, scientists' ability to make season-to-season, and year-to-year, predictions on sea ice patterns remains low.
But the early signs for the summer aren't promising. Stroeve told Andrea Thompson of Climate Central that, since reaching its maximum, the sea ice extent has been declining rapidly. "It's looking pretty steep," she said.
Photo: A satellite image shows the Arctic sea ice extent on March 21, 2014. Credit: National Snow and Ice Data Center (NSIDC)