“Before CryoSat, we could see summer ice coverage was dropping markedly in the Arctic, but we only had glimpses of what was happening to ice thickness,” Rapley said. “Obviously, if it was dropping as well, the loss of summer ice was even more significant. We needed to know what was happening — and now CryoSat has given us the answer. It has shown that the Arctic sea cap is not only shrinking in area, but is also thinning dramatically.”
Sea-ice cover in the Arctic varies considerably throughout the year, reaching a maximum in March and a minimum in September. By combining earlier results from ICESat and data from other studies, including measurements made by submarines that have traveled under the polar ice cap, Laxon said preliminary analysis now gives a clear indication of Arctic sea-ice loss over the past eight years, both in winter and in summer.
In winter 2004, the volume of sea ice in the central Arctic was approximately 17,000km3. This winter it was 14,000km3, according to CryoSat.
However, the summer figures provide the real shock.
In 2004, there were approximately 13,000km3 of sea ice in the Arctic. This year, the value is 7,000km3, almost half the figure eight years ago.
If the current annual loss of about 900km3 continues, summer ice coverage could disappear in about a decade in the Arctic.
However, Laxon urged caution.
“First, our figures are based on preliminary studies of CryoSat figures, so we should take care before rushing to conclusions. In addition, the current rate of ice volume decline could change,” he said.
Nevertheless, experts say computer models indicate rates of ice volume decline are only likely to increase over the next decade.
As to the accuracy of the measurements made by CryoSat, they have been calibrated by comparing them to measurements made on the ice surface by scientists, including Laxon; by planes flying beneath the satellite’s orbit; and by data supplied by underwater sonar stations that have analyzed ice thickness at selected places in the Arctic.
“We can now say with confidence that CryoSat’s maps of ice thickness are correct to within 10cm,” Lazon added.
Laxon also pointed out that the rate of ice loss in winter was much slower than that in summer.
“That suggests that, as winter starts, ice is growing more rapidly than it did in the past and that this effect is compensating, partially, for the loss of summer ice,” he said.
Overall, the trend for ice coverage in the Arctic is definitely downwards, particularly in summer — a point recently backed by Peter Wadham, who this year used aircraft and submarine surveys of ice sheets to make estimates of ice volume loss. These also suggest major reductions in the volume of summer sea ice, about 70 percent over the past 30 years.
“The Arctic is particularly vulnerable to the impact of global warming,” Rapley said. “Temperatures there are rising far faster than they are at the equator, hence the shrinking of sea-ice coverage we have observed. It is telling us that something highly significant is happening to Earth. The weather systems of the planet are interconnected, so what happens in the high latitudes affects us all.”