THE Arctic ice cap is much more vulnerable to global warming than previously thought. A single extra day of summer has a measurable effect on ice thickness the following year, according to the first detailed investigation of year-on-year variations in the thickness of Arctic sea ice.
These findings will stoke fears that the entire Arctic Ocean could be ice-free within a few decades – earlier than climate modellers’ gloomiest predictions.
In the past five years, declassified data from US submarines has shown that Arctic sea ice may have thinned by as much as 40 per cent in the past 50 years. But the data is patchy and open to different interpretations.
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Seymour Laxon, a climate physicist at University College London, has used radar echoes picked up by satellites to plot detailed year-on-year changes. He then compared the changes with the length of the summer. His data covers less than a decade, but reveals two critical facts: the thickness of the ice fluctuates between years by much more than supposed, and the fluctuations correlate almost exactly with the length of the melting season.
“The mass of sea ice can change by up to 16 per cent within one year,” Laxon reports (Nature, vol 425, p 947). He told 91av: “Just a one-day increase in the melting season can cause an extra 5 centimetres of ice to melt.”
The findings confirm the thinning seen in submarine surveys, he says, and they point to global warming as the cause. “Some people have suggested that factors like changes in winds could explain the thinning in some areas. But we have proved pretty certainly that the thickness of the ice depends absolutely on the melting season.” With the length of the summer melting season increasing by an average of five days per decade, that’s really quite dramatic, he adds.
Last week, NASA announced new satellite data showing the Arctic ice sheet is retreating further each summer. “Climate is changing; the Arctic is changing rapidly,” said Mark Serreze of the University of Colorado in Boulder.
Laxon’s findings reinforce growing evidence that the general trend of global warming is much more dramatic and dynamic than expected. “The concept of a slowly dwindling ice pack in response to global warming is just not right,” he says. “The process is very dynamic, and it depends almost entirely on the temperature each summer and not on long-term trends.”
Laxon warns that “existing sea-ice models just don’t explain this data, so we are going to have to build new models before we can give a proper prognosis.” But the future of the Arctic ice cap looks bleak, he says.