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Environment

As a New Movement in Earth's Seasonal Symphony Begins, Climate Change in the Arctic Keeps up its Brisk Tempo

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A mosaic of images from NASA's Terra satellite shows the entire Arctic region, centered on the North Pole (where the dark spot indicates missing data). The images were acquired on Sept. 17 — when Arctic sea ice likely reached its minimum extent for 2014. (Source: NASA) In 2014, Arctic sea ice has continued in its long-term and dramatic decline, a process that is likely helping to accelerate the pace of overall climate change in the north. That's the news from the National Snow and Ice Data Center, announced on Monday — just as world leaders were preparing to meet at the United Nations in New York to grapple with climate change. According to the NSIDC, Arctic sea ice — which floats atop the ocean — likely reached its minimum extent for the year on Sept. 17th:

This is now the sixth lowest extent in the satellite record and reinforces the long-term downward trend in Arctic ice extent. Sea ice extent will now begin its seasonal increase through autumn and winter.

Meanwhile, at the opposite pole, Antarctic sea ice has:

. . . surpassed the previous record maximum extent set in 2013 and is now more than 20 million square kilometers (7.72 million square miles) for the first time in the past thirty-five years. It is too soon to determine if Antarctic sea ice has reached its annual maximum.

Many seasonal rhythms help pace the great pulsating symphony of natural systems on our planet. And like an overeager conductor, we humans are pushing the orchestra to depart in significant ways from the original score. Nowhere is that more evident than in the Arctic, which is warming faster than any other region on Earth, thanks to our emissions of heat-trapping greenhouse gases. And while the increase in Antarctic sea ice may seem contradictory, it's important to keep this one thing in mind: The symphonic score of planetary systems is more like the complex and unpredictable work of a composer like György Ligeti than the more obviously well-ordered work of Johannes Sebastian Bach. Up in the Arctic, warming of the atmosphere has caused Arctic sea ice to shrink more and more in summertime, resulting in increasing reaches of open water. This is important because bright ice reflects solar energy back into space, whereas dark ocean water tends to absorb it. As a result, the warming in the Arctic has been amplified as the seas there have absorbed energy. And this, in turn, has helped drive further decreases in sea ice extent. Since 1979, in fact, the decrease in average sea ice extent in the Arctic equals an area equivalent to about a third of the United States. Research published earlier this year in the Proceedings of the National Academy of Sciences suggested that this so-called Arctic amplification effect, hypothesized nearly 50 years ago, is considerably larger than climate models have predicted. (I wrote about the details here.)

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The width of the diagram above shows how the geographic extent of Arctic sea ice in September shrank between 1979 and 2913. The height shows the decrease in volume. (Source: NASA) But it's not just the extent of sea ice that has been changing. As this graphic shows, the volume has also been decreasing. That means Arctic sea ice is thinner, on average, than it was in 1979, when satellite monitoring began. And thinner ice is more prone to break up and melt — an example of another positive feedback effect. So why has this not been happening in the Antarctic? It's important to recognize that the environment there is vastly different than in the Arctic. The Antarctic continent is surrounded by ocean, whereas the Arctic region consists of an ocean that is almost completely landlocked. As a result, the two regions tend to respond differently in some respects to changes in climate. Figuring out exactly what's happening in the Antarctic is an area of ongoing, cutting edge research. So scientists cannot yet offer an unambiguous explanation. Some of the factors hypothesized to play a role are changes in winds, precipitation, and the saltiness of the upper layers of the ocean — all possibility related to climate change. And, in fact, some computer models do predict short-term increases in the extent of sea ice around Antarctica, even as the average temperature of the globe rises, according to NASA.

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