The Mystery at the Heart of This Year's Record-Setting Arctic Ice Melt

You have probably heard that the Arctic has less sea ice right now than humans have ever recorded. The new record, set yesterday, beat the previous low, which was measured in September 2007. 

"By itself it's just a number, and occasionally records are going to get set," said National Snow and Ice Data Center scientist Walt Meier in an official statement. "But in the context of what's happened in the last several years and throughout the satellite record, it's an indication that the Arctic sea ice cover is fundamentally changing."

There are two odd things about this sad record of global change. 

First, it's only late August, several weeks before the traditional time when the sea ice melting stopped. That could mean that the melt is stopping earlier and could begin to recover earlier. Or we may have several weeks to go of melting, in which case, this year's low could not just break but shatter 2007's record. 

Second, if the melt continues for days or weeks more, the melt will end up catastrophically lower than anyone anticipated.

After 2007's low -- which scared many Arctic scientists into statements like, "The Arctic is screaming" -- the sea ice up north recovered, though not to pre-2007 levels. Counting this year, the six years with the least sea ice on record all occurred in the last six years. 

In that sense, it is not a monumental surprise that 2012 did not see an overabundance of sea ice or return to the norms of earlier this century. On the other hand, the catastrophic drop off of sea ice in the last few weeks was not something that was easy to model or predict. 

What happened?

***

Each year, a program called the Sea Ice Outlook gathers predictions about sea ice from different teams of scientists. The groups use different techniques to predict what the next summer's ice melt season might look like. Some do straight statistics, others build models, and other groups use heuristics or a combination of methods. Teams can submit in June, July, and August. In all cases, they are all trying to guess how much sea ice will be left in the Arctic come mid-September, which (as noted above) had traditionally been the low point. The measure that scientists use to describe the sea ice's extent is the millions of square kilometers of ice that our satellites can see from orbit.

In May of this year, it looked as if the Arctic was going to have a year much like the past four, if a little worse. A lot of ice would melt, more than in any of the years before or since 2007's record-setting low, but none predicted a catastrophic year. The median guess was 4.4 million square kilometers of sea ice would be left, and the band was pretty tight around that number, with only a single group predicting a sea ice extent of 4.1 million square kilometers or lower. A few numbers for comparison: The average low from 1979 to 2000 was 6.7 million square kilometers. In 2010 and 2011, 4.9 and 4.6 million square kilometers of sea ice remained in September. The record low was 4.17 million square kilometers of sea ice in 2007.

And it is this last number that the Arctic crashed through this week with a measurement of 4.1 million square kilometers of sea ice left. 

Now, the modelers may not turn out to have been wrong if, as we mentioned above, the sea ice melt season ends early. "We'll see how September turns out," Robert Grumbine, an ice modeler with the National Oceanic and Atmospheric Administration who worked on three of the predictions, told me. "It isn't unheard of for the annual minimum to be about now, rather than in September."

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