Sunday, September 8, 2013

With Clouds, Arctic Sea Ice Survives the Summer

We’ve reached the point where the rapidly diminishing Arctic sea ice will set new records every year. This year, though, it is not so easy to find records after last year’s blowout melt-out.

The most reliable measure, NSIDC Arctic sea ice extent, is right along the long-term trend line, about 5.25 million square kilometers at the last reading. That’s about 50 percent above last September’s all-time record low, but it is still lower than any year before 2006, and it reinforces the thought of a declining trend. There are other reasons not to lean too heavily on the extent measure. The extent is so much higher this year partly because the sea ice is more dispersed than we have ever seen before. I am not calling dispersion a record because it can’t really be measured precisely enough, but it probably would be a record if we had a good way to measure it. Dispersion doesn’t mean there is more ice, just that the ice is spread out over a wider area of water, with more open water between pieces of ice.

But there is more ice, for two main reasons, both hopeful signs: outflow and cloud cover. Outflow is simply the flow of ice out of the main body of the Arctic Ocean into the Atlantic Ocean and the Canadian islands. There, ice inevitably melts away within a couple of years. Outflow may historically be as important as melt in removing ice from the Arctic Ocean, but we expect less and less outflow in the future as ice area declines. This summer, outflow came to a stop from May through August. Ice-watchers tell me it wasn’t a complete stop, but it was slow enough that I couldn’t see any outflow on the satellite maps. Here, there are records to watch for: I think the current ice in the North Atlantic Ocean is probably the lowest ever seen by several measures, including extent. The Greenland shoreline too is probably more melted out than we have ever seen before.

Cloud cover is the story of this summer, though. An unprecedented pattern of summer storms kept the Arctic Ocean mostly covered with clouds from June to early August. From what I have heard, it is the highest summer Arctic cloud cover in the satellite record. This too is a hopeful sign. With a warmer ocean, we expected more clouds (even if we were surprised to see this many). It was barely five years ago that scientists recognized the key importance of clouds in protecting sea ice from solar melting, but this year’s experience tells me that cloud cover, especially in June and July, is more important than we previously realized.

It is too soon to celebrate. If cloud cover extends into fall it will delay re-freezing, and the usual pattern of outflow along the east coast of Greenland appears to have resumed in the final days of August. Regardless of weather, in the long run, a warming ocean will melt all the ice. But if the Arctic can have more summers like this one, the melt-out will not occur on September 9, 2016 (as I famously predicted), put possibly as much as a decade later.