It's now official: The 2012 Arctic summer sea ice melt blows past the previous record low.
According to the National Snow and Ice Data Center, "Arctic sea ice cover likely melted to its minimum extent for the year on September 16." The total coverage of the ice was about 1.32 million square miles, the lowest ever recorded since we began monitoring the Arctic with satellites in 1979.
Check out this NASA video to get a visual sense of this year’s dramatic loss of sea ice.
The Historical Context
The previous record low in 2007 was a doozie, measuring only about 1.6 million square miles -- a shocking 20 percent plus below 2005′s previous record low of about 2.1 million square miles. The 2007 record was so low, in fact, that some scientists began to worry about an imminent ice-free Arctic.
But then the sea ice recovered somewhat in 2008 and 2009. There were those who used this reversal as evidence that global warming was over and predicted that sea ice would continue to grow in the coming years as global temperatures began to fall. (See here and rebuttal here.)
Of course one lesson in all this is that when it comes to climate, you shouldn't get too carried away by any one year's extreme -- on the high side or the low side. What really counts is the long-term trend. And from that point of view, it’s pretty clear that Arctic sea ice is retreating pretty rapidly -- it's taking a jagged path with dips and blips but the overall trend is downward. We are now about 50 percent below the 1979-2000 average minimum. Here's a comparison of the 2007-2012 ice melt seasons with the 1979-2000 average.
But while we should not read too much into this year’s new observed low in terms of the long-term trend, there is one aspect of this year's event that is troubling. We don't completely understand why the sea ice extent was so low or why the sea ice is declining so much more rapidly than climate models have predicted.
Weather-wise an Uneventful Summer
Can we lay the cause of the exceptional melt on weather? Not really. With the exception of one summer storm, during which melt rates doubled for a few days, the season was meteorologically speaking fairly uneventful.
Some scientists have speculated that we are seeing the result of long-term warming on the ice. With each year's melt season, the Arctic begins the next year with less and less multi-year ice, ice that tends to be thicker. As the old ice is lost, the Arctic becomes ever-more vulnerable to dramatic declines. (See image.)
Most of the current ice cover is young and weak and as such melts more readily.
If correct, this suggests that we are beginning to see the effects of a positive feedback system that will continuously eat away at the Arctic's summer ice until there isn't any. Melting ice in the summer means less multi-year ice the next spring, which leads to more summer melting and even less multi-year ice.
Climate Models Underestimating Melt
And that brings us to the issue of the climate models. Right now climate models offer up a wide range of predictions that as a group do a poor job of reproducing observed melt rates -- more specifically they tend to underestimate observed melt rates. This generally still holds true in spite of the fact that a newer generation of model provides more consistent results. Factors that may contribute to such shortcomings include deficits in how the models handle the system's inherent natural variability, ocean currents, weather, ice thickness, and natural feedbacks (like ponding on the ice or the impact of ocean warming from below).
One worrisome aspect of this situation is that if greenhouse gases continue to rise, all models predict that the planet is headed to a future with an Arctic that is ice-free in the summer. The predictions of when this will occur vary by model, from as soon as a decade or two from now to closer to the end of the century. But if the models underestimate the decadal rate of melting now, is it not reasonable to suspect that they also overestimate how long it will be till the Arctic ice melts entirely during the summer?
What Might This New Minimum Portend?
So who cares if Arctic sea ice disappears? We probably all should. Ice provides critical habitat for seals and bears, and there is growing evidence "that the recent decline of Arctic sea ice has played a critical role in recent cold and snowy winters" (more here) including in parts of Europe and the United States (see here and here). And research suggests that warming in the Arctic is altering phytoplankton blooms, which drive the oceanic carbon cycle and fuel the bottom of the marine food chain.
So, what's ahead for the Arctic and our fast-approaching winter? Do the new Arctic ice lows mean more of the unusual weather patterns that dumped excessive amounts of snow and cold on us and our European friends? In my book predictions are tough, especially these days. A warming world is a changing world and when it comes to meteorology, there are few safe bets. The best I can advise is to hold on to your hats.