Over the past several years, the amount of sea ice in the Arctic Ocean during the summer months took an unexpected plunge, dropping below the levels predicted by many models. As a result, the prospect of an ice-free summer in the Arctic, which would have consequences for everything from the local ecology to global trade, is something the research community is studying intensively. That work is leading to a new understanding of how the age of the ice combines with natural variability to produce sudden changes like the ones we've observed recently.

Two recent reports have focused attention on the state of the Arctic sea ice. On Monday, the National Snow and Ice Data Center announced that the annual summertime decline of Arctic sea ice had started, and issued its annual analysis of the recent dynamics in the Arctic. That analysis followed hot on the heels of a paper that predicts that the Arctic will be effectively ice-free in roughly 30 years. I'll combine those reports with information from a talk given by the Jean-Claude Gascard of the Universit? Pierre et Marie Curie, who spoke at this year's American Association for the Advancement of Science meeting.

First, the current state of the ice. According to the NSIDC, the end of last summer's melt season left Arctic ice cover near the historic lows of the previous year, but the open water froze quickly, leading to a peak level somewhat above that of recent years. The winter's peak ice coverage came earlier than usual, but levels have remained fairly stable so far this spring, leaving ice coverage below historic averages, but well above the amount that's been typical over the last several years.

The expectations for this summer, however, aren't significantly different from those of other recent years. To understand why, you have to consider the age of the ice. In the Arctic, ice that survives a single season puts on weight, growing thicker; that thickness makes it far less likely that the ice will melt over the course of the summer. As shown in the graph below, the amount of multi-year ice has plunged in recent years, having been replaced primarily by one-year old ice that's thinner and more prone to melting. Note that the graph represents the percent of total ice; since the total ice has dropped in recent years, the absolute numbers on this loss are a bit larger.

The sea ice that remains in the Arctic is younger and thinner than it was in the recent past.

From the National Snow and Ice Data Center, courtesy J. Maslanik and C. Fowler, University of Colorado

This past year saw a continuing decline in multi-year ice, but a rebound in ice that's survived two seasons. The reasons for this can be seen when the geographic distribution of the ice is examined. The older ice has largely been pushed against Greenland and the Canadian Arctic where, each year, a fraction is pushed out past the northern tip of Greenland, where it flows into the North Atlantic and melts. The better freezing conditions of the past year have meant that two-year-old ice has largely extended out towards the pole from Canada. These dynamics mean that it will take several years of similar conditions before multi-year ice can start recovering what's lost to the Atlantic.

The remaining multi-year ice is now pinned near Greenland, and flowing into the Atlantic.

From the National Snow and Ice Data Center, courtesy J. Maslanik and C. Fowler, University of Colorado

The emphasis on the thickness of multi-year ice and seasonal variations feeds nicely into the modeling paper, which was published in Geophysical Research Letters. The authors of the paper evaluated a suite of 23 climate models for their ability to roughly match the measured behavior of the Arctic sea ice during the 20 years following 1980. They wound up with six models that seemed to be getting the sea ice right, and noted that half of these "include a multiple sea ice thickness distribution as part of a sophisticated sea ice physics and dynamics package," something that's only present in the most recent generation of climate models.

The authors ran these models forward from the present, using two different IPCC climate projections, and determined how long it took for the summer ice to drop to a million square kilometers, which is the margin they consider effectively ice-free. This point was reached when the remaining ice was anchored to the Canadian Arctic, the current site of the oldest and thickest ice; the rest of the Arctic ocean was open water. Overall, the mean interval for this to happen is 32 years.

The authors note a couple of things about their model runs. For starters, the decline in sea ice doesn't occur in the absence of anthropogenic forcings, which they term a "necessary condition." Indeed, at AAAS, Gascard indicated that, combined, anthropogenic forcings and changes in the albedo caused by the loss of reflective ice account for about 85 percent of the recent temperature change in the Arctic.

But, although anthropogenic forcings are a necessary precondition to cause the loss of ice, natural variability still plays an essential role in controlling the dynamics. The models indicate that it would have taken decades longer to reach the levels of sea ice seen in 2007 in the absence of specific weather conditions, such as a series of consecutive warm years and winds that drove some of the sea ice into the Atlantic. The authors also state that, once sea ice levels reach their current levels, they tended to decline rapidly in all of the models.

These results suggest that a confluence of factors, some stochastic, combined to cause the sudden decline of Arctic sea ice in recent years, so scientists could be forgiven for being surprised by its speed. Moving forward, however, the results suggest that keeping a careful eye on the age of the ice, as well as its extent, may help us stay on top of future developments.

Geophysical Research Letters, 2009. DOI: 10.1029/2009GL037820