By Richard Black
Environment Correspondent, BBC News website
The area covered by sea ice in the Arctic has shrunk for a fourth consecutive year, according to new data released by US scientists.
They say that this month sees the lowest extent of ice cover for more than a century.
The Arctic climate varies naturally, but the researchers conclude that human-induced global warming is at least partially responsible.
They warn the shrinkage could lead to even faster melting in coming years.
"September 2005 will set a new record minimum in the amount of Arctic sea ice cover," said Mark Serreze, of the National Snow and Ice Data Center (NSIDC), Boulder, Colorado.
"It's the least sea ice we've seen in the satellite record, and continues a pattern of extreme low extents of sea ice which we've now seen for the last four years," he told BBC News.
September is the month when the Arctic ice usually reaches a minimum.
The new data shows that on 19 September, the area covered by ice fell to 5.35 million sq km (2.01 million sq miles), the lowest recorded since 1978, when satellite records became available; it is now 20% less than the 1978-2000 average.
The current rate of shrinkage they calculate at 8% per decade; at this rate there may be no ice at all during the summer of 2060.
ARCTIC SEA ICE EXTENT - SEPTEMBER TREND, 1978-2005
The straight line tracks a more than 8% decline per decade
An NSIDC analysis of historical records also suggests that ice cover is less this year than during the low periods of the 1930s and 40s.
Mark Serreze believes that the findings are evidence of climate change induced by human activities.
"It's still a controversial issue, and there's always going to be some uncertainty because the climate system does have a lot of natural variability, especially in the Arctic," he said.
"But I think the evidence is growing very, very strong that part of what we're seeing now is the increased greenhouse effect. If you asked me, I'd bet the mortgage that that's just what's happening."
One of the limitations of these records is that they measure only the area of ice, rather than the volume.
"One other factor could be movements of sea ice," said Liz Morris, of the British Antarctic Survey, currently working at the Scott Polar Research Institute in Cambridge, UK.
"If it all piles up in one place, you might have the same total amount of ice," she told the BBC News website, "and there is some evidence that ice is piling up along the north Canadian coast, driven by changes in the pattern of winds and perhaps ocean currents."
Most data on sea ice thickness comes from records of military submarines, which regularly explored passages under the Arctic ice cap during the Cold War years.
Submarines can cross the Arctic Ocean along tracks taken decades before, and note differences in the ice thickness above; but that may mean little if the ice itself has moved.
Professor Morris is involved in a new European satellite, Cryosat, which should be able to give definitive measurements of ice thickness as well as extent; its launch is scheduled for 8 October.
But she also believes that the NSIDC data suggests an impact from the human-enhanced greenhouse effect.
"All data goes through cycles, and so you have to be careful," she said, "but it's also true to say that we wouldn't expect to have four years in a row of shrinkage.
"That, combined with rising temperatures in the Arctic, suggests a human impact; and I would also bet my mortgage on it, because if you change the radiation absorption process of the atmosphere (through increased production of greenhouse gases) so there is more heating of the lower atmosphere, sooner or later you are going to melt ice."
Arctic warming fast
Though there are significant variations across the region, on average the Arctic is warming twice as fast as the rest of the planet, according to a major report released last year.
The Arctic Climate Impact Assessment, a four-year study involving hundreds of scientists, projected an additional temperature rise of 4-7C by 2100.
If the current trend can be ascribed in part to human-induced climate change, Mark Serreze sees major reasons for concern.
"What we're seeing is a process in which we start to lose ice cover during the summer," he said, "so areas which formerly had ice are now open water, which is dark.
"These dark areas absorb a lot of the Sun's energy, much more than the ice; and what happens then is that the oceans start to warm up, and it becomes very difficult for ice to form during the following autumn and winter.
"It looks like this is exactly what we're seeing - a positive feedback effect, a 'tipping-point'."
The idea behind tipping-points is that at some stage the rate of global warming would accelerate, as rising temperatures break down natural restraints or trigger environmental changes which release further amounts of greenhouse gases.
Possible tipping-points include
- the disappearance of sea ice leading to greater absorption of solar radiation
- a switch from forests being net absorbers of carbon dioxide to net producers
- melting permafrost, releasing trapped methane
This study is the latest to indicate that such positive feedback mechanisms may be in operation, though definitive proof of their influence on the Earth's climatic future remains elusive.