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Monday, 6 December, 1999, 08:18 GMT
Humanity blamed for ice loss
Arctic Many studies have pointed to rapid thinning

A new study says there is now very little doubt that human-induced warming is behind the rapid thinning of Arctic sea ice seen in recent years.

An area equivalent to the size of Texas has been lost over the past 20 years.

Polar researchers combined five different data sets and two computer models of global warming trends and found that the probability of such a loss occurring purely as a result of natural climate forces was less than two percent.

Although many studies have shown how the ice cover and thickness in the northern polar region is in sharp decline, not all scientists have been in agreement as to the cause.

However, the researchers led by Konstantin Vinnikov from the University of Maryland are uncompromising in blaming human influence.

Computer models

"The probability is very low that the observed and modelled trends are due exclusively to random variations, assuming that the models' natural variability is similar to that found in nature," the team report in the journal Science.

"This strongly suggests that the observed decrease in Northern Hemisphere sea-ice extent is related to anthropogenic (man-made) global warming."

The scientists used a computer model to show how much ice there would be if humans had not pumped carbon dioxide into the atmosphere. A global climate model simulating natural changes over 5,000 years was used to put the findings in a longer-term context.

It revealed that the probability of a negative trend over 19.4 years as large as that seen from the satellite data was less than two percent. Another computer simulation was then run which included increases of warming greenhouse gases and cooling aerosols.

The results incorporating these human-induced changes matched the observed sea ice losses much more closely. They suggested that the sea ice decreases were largely the result of greenhouse gases accumulating in the atmosphere during the second half of the 20th Century.

Arctic oscillation

The loss of a large ice mass over the northern polar region would have a dramatic impact on the global climate system. Snow and ice help to cool the Earth by reflecting energy from the Sun - up to 80% - straight back out into space. The sinking, cold, dense, salty water which is created as ice forms also helps to drive ocean currents which redistribute heat around the planet.

But not all experts are yet prepared to blame the loss of Arctic sea ice solely on human activity. They point to the influence of a natural climate phenomenon known as the Arctic Oscillation (AO). This is an erratic see-saw that alternately raises and lowers atmospheric pressure over the North Pole while lowering and raising it in a ring around the polar region.

For the last 10 years, this has led to distinct wind patterns and warm-air movement that can also result in sea-ice retreat.

Polar researcher Dr Andrew Rothrock, who has used US Navy data to show how Arctic sea ice has thinned, believes it may take five to 10 years for the true culprit behind the thinning to emerge.

"I'm going to wait and see," he tells the latest edition of Science. "I would lean toward the view that this is a fairly extreme state [of the AO], and it will likely come back toward more normal conditions."

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See also:
16 Nov 99 |  Sci/Tech
Arctic sea ice gets thinner
 |  Sci/Tech
Global warming could starve polar bears
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