Ice recap Summer 2011

The good folks at the National Snow and Ice Data Center summarize the season in the Arctic Ocean. Turns out that the weather conditions that helped make 2007 a record for low sea-ice extent didn't recur. And yet, 2011 came within a relative hair's breadth of setting a new record. That means longer-term climate trends are to blame, not seasonal weather variation. The low-down:

Why did ice extent fall to a near record low without the sort of extreme weather conditions seen in 2007? One explanation is that the ice cover is thinner than it used to be; the melt season starts with more first-year ice (ice that formed the previous autumn and winter) and less of the generally thicker multi-year ice (ice that has survived at least one summer season). First- and second-year ice made up 80% of the ice cover in the Arctic Basin in March 2011, compared to 55% on average from 1980 to 2000. Over the past few summers, more first-year ice has survived than in 2007, replenishing the younger multi-year ice categories (2- to 3-year-old ice). This multi-year ice appears to have played a key role in preserving the tongue of ice extending from near the North Pole toward the East Siberian Sea. However, the oldest, thickest ice (five or more years old) has continued to decline, particularly in the Beaufort and Chukchi Seas. Continued loss of the oldest, thickest ice has prevented any significant recovery of the summer minimum extent. In essence, what was once a refuge for older ice has become a graveyard.

Using slightly different algorithms to calculate an estimate of sea ice extent, a University of Bremen team concluded that 2011actually did set a record low. Call it a tie. Whatever the real number, concludes the Bremen group, it's worrying. Unlike the NSIDC, which restricts itself to the geographical data, they dared to mention the ecological implications:

The retreat of the summer sea ice since 1972 amount to 50%. For algae and small animals living at the lower side of the ice, less and less living environment remains since they need a certain time to settle there.

Anyway, when you multiply sea ice extent by ice thickness, you get ice volume, which is just as critical a figure.

That figure is from the the PIOMAS computer model generated by the Polar Science Center of the Applied Physics Laboratory of the University of Washington. It's a model because it's not as easy to directly measure ice thickness across the entire basin, but the model is widely considered trustworthy.

Extrapolating in the future, PIOMAS says we are probably headed for an ice-free summer by 2018 and possibly sooner.

Of course, it's entirely possible something will happen with the weather that allows the ice to hang around a bit longer. But even the most optimistic scenario is dire. Darn albedo.