Published online 17 September 2008 | Nature | doi:10.1038/news.2008.1115

News

Arctic sea ice reaches annual low

Floating pack fails to break record shrinkage of last year.

This summer's Arctic sea-ice cover has apparently bottomed out at 4.52 million square kilometres, which does not crack 2007's record low. The US National Snow and Ice Data Center (NSIDC), at the University of Colorado in Boulder, announced the likely 2008 minimum on Tuesday after tracking five consecutive days of rising ice levels, from 10 to 14 September.

At this point, the ice extends over an area 9.4 per cent greater than it did in 2007. That year, unusually warm weather melted much of the ice, and persistent winds drove what was left together into a 4.13-million-square-kilometre area, well below the 1979–2000 average of nearly 7 million square kilometres. It was the least icy year seen in the Arctic during the satellite era.

Sea-ice cover on 12 September reached its seasonal low.NSIDC

This year, however, there were fewer warm days, and the winds came from various directions rather than pushing the ice in one direction. "In some ways, this year is more remarkable than last year," says Walt Meier, a research scientist at NSIDC.

Thin and getting thinner

Because of last year's massive melt, 2008's ice had little hope for a major rebound; the ice was thinner and particularly vulnerable to melting. In March, NASA scientists reported that thick, multi-year ice covered less than a third of the Arctic last winter, in comparison to the 50–60 per cent it used to coat. Since 1987, the average Arctic ice thickness has declined from 3.7 metres to 2.6 metres, according to a recent paper by Ron Lindsay and colleagues at the Polar Science Center at the University of Washington in Seattle1.

Although this year's minimum is a bit more than last year's, ice experts warn that 2008 does not indicate an upward trend. "People might want to call it a recovery, but I don't think recovery is a good term," Meier says. It would take several cold seasons — which aren't likely, he says — to bring the ice back to 1980s-era levels.

Individual seasons will vary, but one summer there will likely be no Arctic ice at all. Meier estimates that point could come around 2030, although others predict that the first ice-free summer could be as far off as 2050, or as close as 2012. Meier calls the latter estimate extreme, but acknowledges it's not impossible. "Five years ago, saying that would have gotten you laughed out of the room," he says. "Now, no one's laughing."

Complicating feedbacks

Part of the reason ice cover is tricky to predict is that as ice area decreases, the remaining ice melts faster. While white ice reflects sunlight, dark water absorbs it and transmits the extra heat to melt any ice nearby. The reflectivity, or albedo, controls how much solar energy is absorbed. "The albedo of the ice is very complicated, in part because of melt ponds that form on the surface of the ice in the summer," Lindsay says.

Bill Chapman, who tracks ice levels independently from NSIDC, says there's an outside chance that more melting could occur in the days to come. "I still wouldn't call it the minimum for another week or two," says Chapman, a senior research programmer in atmospheric sciences at the University of Illinois, Urbana-Champaign.

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NSIDC acknowledges that possibility, but, Meier says, "generally once it starts going up, it continues." 

  • References

    1. Lindsay, R. W., Zhang, J., Schweiger, A., Steele, M. & Stern, H. J. Climate published online, doi:10.1175/2008JCLI2521.1 (2 July 2008).
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