Arctic losing ice much faster than expected

New satellite images show polar ice coverage is dwindling in its extent and thickness

By Robin McKie  /  The Guardian, LONDON

Wed, Aug 15, 2012 - Page 9

Sea ice in the Arctic is disappearing at a far greater rate than previously expected, according to data from the first purpose-built satellite launched to study the thickness of Earth’s polar caps.

Preliminary results from the European Space Agency CryoSat-2 probe indicate that 900km3 of summer sea ice has disappeared from the Arctic ocean over the past year.

That rate of loss is 50 percent higher than most scenarios outlined by polar scientists and it suggests that global warming, triggered by rising greenhouse gas emissions, is beginning to have a major impact on the region. In a few years, the Arctic ocean could be free of ice in summer, triggering a rush to exploit its fisheries, oil, minerals and sea routes.

Using instruments on earlier satellites, scientists could see that the area covered by summer sea ice in the Arctic has been dwindling rapidly, but the new data indicate this ice has been thinning dramatically at the same time. For example, in regions north of Canada and Greenland, where ice thickness regularly stayed at about 5m to 6m in summer a decade ago, levels have dropped to between 1m and 3m.

“Preliminary analysis of our data indicates that the rate of loss of sea ice volume in summer in the Arctic may be far larger than we had previously suspected,” said Seymour Laxon, of the Centre for Polar Observation and Modelling at University College London (UCL) where CyroSat-2 data are being analyzed. “Very soon we may experience the iconic moment when, one day in the summer, we look at satellite images and see no sea ice coverage in the Arctic, just open water.”

The consequences of losing the Arctic’s ice coverage, even for only part of the year, could certainly be profound. Without the cap’s white brilliance to reflect sunlight back into space, the region would heat up even more than it is doing at present. As a result, ocean temperatures would rise and methane deposits on the ocean floor could melt, evaporate and bubble into the atmosphere.

Scientists have recently reported evidence that methane plumes are now appearing in many areas. Methane is a particularly powerful greenhouse gas and rising levels of it in the atmosphere are only like to accelerate global warming, and with the disappearance of sea ice around the shores of Greenland, its glaciers could melt faster and raise sea levels even more rapidly than they are at present.

“With the temperature gradient between the Arctic and equator dropping, as is happening now, it is also possible that the jet stream in the upper atmosphere could become more unstable,” Chris Rapley of UCL said. “That could mean increasing volatility in weather in lower latitudes, similar to that experienced this year.”

CryoSat-2 is the world’s first satellite to be built specifically to study sea-ice thickness and it was launched, on a Dniepr rocket from the Baikonur cosmodrome in Kazakhstan, on April 8, 2010. Previous Earth monitoring satellites had mapped the extent of sea-ice coverage in the Arctic, but the thickness of that ice proved more difficult to measure.

The US probe ICESat made some important measurements of ice thickness, but it operated intermittently in only a few regions before it stopped working completely in 2009.

CryoSat was designed specifically to tackle the issue of ice thickness, both in the Arctic and the Antarctic. It was fitted with radar that can see through clouds. (ICESat’s lasers could not penetrate clouds.) CryoSat’s orbit was also designed to give better coverage of the Arctic Ocean.

“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.”