Dramatic thinning and shrinking of polar ice

The reduction in ice may be part of a natural cycle, researchers say, but a comprehensive new study points to a human contribution that could lead to the complete melting of the permanent portion of the ice cap in a few short decades.

While scientists are cautious about possible human contributions (through burning of fossil fuels and other activities that add to climate-warming greenhouse gases) there appears to be little doubt that ice is melting at an incredible pace. The biggest question surrounding the findings is whether it is all no more than a short-term blip in a long, natural cycle of advance and retreat.

In Friday's issue of the journal Science, Norwegian scientist Ola Johannessen, of the Nansen Environmental and Remote Sensing Center, presents satellite measurements of microwave emissions that show declines in permanent Arctic ice of 7 percent over each of the past two decades. Permanent ice, or "multiyear" ice as researchers call it, is the portion that remains even after seasonal melting.

Johannessen relied on satellite data stretching back to 1978, obtained by the Nimbus-7 Scanning Multichannel Microwave Radiometer and later by Defense Meteorological Satellite Program spacecraft. Ice emits microwave radiation that varies depending on whether it is fresh or has thickened over the years.

In a separate study appearing in the Dec. 1 issue of Geophysical Research Letters, Andrew Rothrock of the University of Washington, along with colleagues, reported a surprising decrease in the thickness of Arctic sea ice. The average thickness declined by 4.3 feet, or 40 percent, between 1958 and 1997, the scientists said. Using data collected by nuclear submarines, the researchers found the thinning had occurred at each of more than two dozen sites observed.

But other sites that might have been measured are difficult to reach, and it's possible that the observed thinning was caused by wind and ocean currents pushing ice to different locations, said John Walsh, a polar researcher at the University of Illinois.

Walsh told explorezone.com that "there is pretty good agreement that ice in many areas has been retreating in summer," but he added that "we should reserve the possibility that ice is becoming thicker in areas that haven't been measured."

Possible climate effects

If Arctic ice continues to melt at its present rate, in a few decades it could be nonexistent during summer months, though no one is saying this is inevitable.

The effect on Earth's climate would be huge. Researchers say the ice reflects roughly 80 percent of solar energy that reaches it back into space. Remove the ice, and the Arctic becomes a giant heat collector, absorbing 80 percent of the sunlight that reaches the far north.

Major ocean currents, slow-moving "rivers" within the seas that exchange energy from the tropics to the poles, would be altered. World weather patterns would shift significantly. Storms would take different paths and precipitation patterns would be altered globally.

Are we to blame?

If the thinning and shrinking of Arctic ice is part of a natural cycle, then it should reverse itself at some point. But another study in Friday's issue of Science bolsters the belief by many scientists that humans are contributing to the warming of the planet, which in turn is melting Arctic ice at an unnatural pace.

University of Maryland's Konstantin Vinnikov, working with several top climate experts from NASA, NOAA and other organizations, shows that melting ice reported in five separate studies could not have occurred in the natural course of things. Vinnikov and his colleagues compared satellite and surface observations of sea ice with predictions from two existing climate computer models, which simulated the effects of increased carbon dioxide and known greenhouse particles produced by humans.

The results showed that the chances of the ice shrinkage since 1953 being due to natural cycles are less than 0.1 percent, implying that humans are to blame.

"These results offer strong support for the theory of global warming, which will have consequences far beyond the direct effect of disappearing sea ice," said Alan Robock of the Department of Environmental Sciences at Rutgers University.

Part of a natural cycle?

"We may be seeing a trend in one direction that could be associated with anthropogenic impacts," said David Robinson, a Rutgers geography professor who is an expert in the variability of snow cover in the Northern Hemisphere. "But it could be associated with a decadal or multidecadal cycle."

Robinson, who was not involved in the Science report but is familiar with it, is typically cautious about inferring long-term trends from relatively short-term data sets. Computer models, he notes, tend to show less variation than what nature provides when applied to time spans of a few months or years, meaning that they are often less than accurate.

"That's the difficulty when you're working with only four or five decades," Robinson said in a telephone interview. "You could be seeing something associated with a longer pulse. I'm not saying that's the case. I'm saying it's something that has to be considered."

Robinson said it might be, however, that the models are more accurate when applied to periods of 10 years or more, a possibility that would lend credence to the Vinnikov study. He explained that the computer models on which the study is based are used to represent an estimate of "natural variability" in the ice quantity, against which actual data are compared.

"The big fly in the ointment is that natural variability is what the computer runs," Robinson said. "So you have to make this leap that the computer model's variability is right. That's a bit of a sticking point."

Still, Robinson said the work is solid and may well prove accurate.

As a reflection of the standards set in the Vinnikov study, snow cover measurements that Robinson collects and analyzes were not used as part of the analysis, because they were not perceived to be as well pinned down as the ice data. But Robinson said the snow data shows "the same trend of exceeding the natural variability."

Robinson is still collecting snow data. If it becomes a more reliable measure, and is still found to show a trend similar to the ice data, then the study by Vinnikov and his colleagues would be bolstered.

Watch and see

Walsh, of the University of Illinois, provided one of the ice data sets that went into the Vinnikov study. Walsh said that while the humans probably are contributing to global warming and ice melting, the recent trend is likely also part of a natural cycle.

"My own view is that both forces are at work," Walsh said. "I'd be surprised if the next few decades have such an alarming rate of melting."

Walsh said the data is solid, but only time will tell if the models have made an accurate prediction.

"It's the best data we have," he said. "I think we should certainly pay close attention to what goes on over the next 10 years in the Arctic."