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?

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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."

Some of the more recent (July and August) predictions for the Sea Ice Outlook have revised their estimates downwards on the basis of the early summer. Nonetheless, it still stands: going into this summer, we were not expecting a record low for sea ice.

And now here's the most recent data.

What's befuddling about 2012, relative to 2007, is that the Arctic has not seen the kind of ice-melting weather that 2007 did. "I'm at a loss at this loss," wrote sea ice blogger (yes they exist!) Neven Acropolis. "The 2007 record that stunned everyone, gets shattered without 2007 weather conditions."