Wednesday, September 17, 2008

Less Arctic Ice Than Ever

The Arctic ice coverage seems to have leveled off as the sun disappears from the Arctic and the ocean starts to freeze over for the winter. The ice coverage area is more than last year’s shocking record low, but not by much, and the total amount of sea ice on the Arctic Ocean is less than ever.

The most accurate daily measure of Arctic ice we have is the ice extent from the National Snow and Ice Data Center (NSIDC). It’s the right thing to look at to measure ice trends, but it is not a reliable reflection of the total amount of ice on the Arctic Ocean. That’s because the NSIDC measures the ocean area covered at least 15 percent by sea ice. By this measure, the same cluster of ice that covers 10 square kilometers one day could cover 50 square kilometers the next if the waves toss it around — or vice versa, if winds pile up ice against a shoreline.

This effect, dispersion, is just one of the reasons why ice coverage is not a measure of the total amount of ice. Ice is also thicker at some times and thinner at others. The average ice thickness measured this year was less than previously recorded. In addition, thinner ice, being lighter, is more easily dispersed. Based on this, the total mass of Arctic sea ice is probably 15 to 20 percent less than at the minimum level a year ago.

And next year? A lot depends on the weather in the next few weeks. A sharp cold snap early this fall could lead to near-normal winter ice formation this winter, in spite of the low ice extent and warmer water temperatures. If that happens, next summer could look a lot like this summer. If it does not, the Arctic will start next summer with less thick multiyear ice than ever and will surely melt away in a way we have never seen. That is the more likely scenario, and shipping companies are already making plans for freight runs across the Arctic Ocean next August and September.