- 13:12 16 September 2008
- NewScientist.com news service
- Catherine Brahic
As the Arctic sea-ice reaches its summer minimum extent, it is clear that it has yet again shrunk to one of the smallest areas in recent decades, 10% above the record minimum set last year.
New Scientist takes a look at the reasons behind the dramatic melt. How much is to do with global warming – and how much can be blamed on the weather?
In 2007, temperatures were unusually warm, and the sky was very clear at the beginning of the summer when solar radiation is strongest. What's more, winds pushed ice away from the Siberian coast and helped it move out into the Atlantic. These factors led to a record ice minimum and the opening of the North-West Passage.
"Last year was a 'perfect storm' of conditions leading to the extreme low anomaly," says Walt Meier of the US National Snow and Ice Data Center. In other words, the meltdown could be explained by natural variations in the weather, not long-term climate trends.
Yet global warming undoubtedly worsened the situation. "The ice responded much more dramatically to the 'extreme weather' than it would have when the ice cover was thicker and more extensive back in the 1980s and earlier," says Meier.
So what is driving the long-term decline in sea-ice?
1. Increasing temperatures
Arctic temperatures have warmed more than anywhere else on Earth. Computer models show this is linked to the greenhouse effect, which increases the likelihood of unusually warm years.
Year-on-year, the warming has weakened the ice. Satellite data clearly shows the summer ice cap has been shrinking since the late 1970s. As a result, more and more winter ice is growing on open stretches of water, making it thinner and more vulnerable to melting during its first summer.
2. Thinning ice
This thinning has happened in spite of the Arctic Oscillation, a natural variation that sees a decade during which ice tends to be pushed out into the Atlantic followed by a decade when ice is retained and can build up. If the AO is working as we expect, ice should eventually build back up. "However, with warming temperatures, the ice may not be able to recover," says Meier.
In 2008, 72% of the winter ice was thin first-year ice – usually it would be about 30% – opening the possibility that the North Pole would be ice-free for the first time. But cloudier, cooler weather staved off the event.
3. Albedo effect
Another positive feedback of global warming is the albedo effect: less white summer ice means more dark open water, which absorbs more heat from the sun. Thinner ice also breaks up more easily, and the thinner the ice, the faster it seems to be exported from the Arctic to the Atlantic, says Rune Graversen of the Royal Netherlands Meteorological Institute.
4. The Arctic greenhouse
As the climate warms, warmer temperatures and more open water will mean more water vapour entering the atmosphere – itself a powerful greenhouse gas. This will help drive temperatures even higher. "Global warming may change the water vapour content of the atmosphere that will alter the greenhouse warming in the Arctic," says Graversen.
5. Tropical energy
As well warming direct from the sun, recent evidence shows that increasing amounts of heat energy are being transported from the tropics to the Arctic (Nature DOI: 10.1038/nature06502). What is not clear is how much this extra warming has to do with anthropogenic climate change.
"If the energy-transport increase is confirmed from other data sets, and it continues to increase over the next years, it will be more and more likely that it is coupled to global warming," says Graversen.
When, not if
"We are now well outside the range of natural variability," says Meier. "It is clear from how low the ice extent has been recently, the significant long-term trend, and the way the ice-cover is responding to atmospheric conditions and ocean circulation, that we've entered an entirely new regime of the Arctic sea ice".
"I think most glaciologists would be very surprised if the Arctic went back to normal," agrees Graversen.
Computer models previously forecast that the Arctic would be free of ice in the summer by 2050. "This now looks to be too conservative. Most scientists are thinking it will happen around 2030, some even much earlier," says Meier.
"Natural variability will have something to say about how fast we get there, but we seem inexorably sliding to such a state sooner or later."
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