The sea ice of the Arctic shrank to its lowest-ever level this week, shattering the previous record, set two years ago, by an enormous amount, American scientists have confirmed.
In what will be widely seen as one of the most alarming signs yet of accelerating global warming, the summer melt-back exceeded the September 2005 low point by 22 per cent – an area of 1.2 million square kilometres – more than 385,000 square miles. This represents an area five times the size of the UK.
The colossal shrinkage is immediately and dramatically visible on satellite images of the two low points. Furthermore, the difference between 2005 and this year is more than double the difference between 2005 and 2002, the previous lowest year.
"It's the biggest drop from a previous record that we've ever had and it's really quite astounding," said Walt Meier, a research scientist at the US National Snow and Ice Data Center (NSIDC) in Colorado. "That's a dramatic change in one year. Certainly we've been on a downward trend for the last 30 years or so, but this is really accelerating the trend."
The ice cover of the Arctic Ocean shrinks in the summer and regrows in the autumn and winter, in a regular cycle. By Sunday last week, it had shrunk to 1.6 million sq miles, the NSIDC said. This compares with the 2005 low point of 2.07 million sq miles. The contrast is even greater with the long-term average over the past 20 years or so. Between 1979 (when regular satellite monitoring had just started) and 2000, the long-term average minimum was 2.6 million sq miles.
The remarkable increase in the rate of melting in the past two or three years has led to a revision of estimates of when the Arctic might be wholly ice-free in summer. Early predictions by the UN's Intergovernmental Panel on Climate Change (IPCC), based on computer models of global warming, suggested that as climate change advances, this might happen by 2080. But now scientists are increasingly thinking the models have seriously underestimated the rate, and it may happen much earlier.
According to Mark Serreze, another senior NSIDC researcher, it might take only 25 years or less. "If we were talking even two or three years ago, I'd have said the transition to an ice-free Arctic summer might be between 2070 and 2100," he said. "But we're starting to see that that is rather optimistic, and an educated guess right now would be 2030. It's something that could be within our lifetime."
He added: "We're on strong spiral of decline; some would say a death spiral. I wouldn't go that far but we're certainly on a fast track. We know there is a natural variability, but the magnitude of change is too great to be caused by natural variability alone."
The melting of the Arctic ice will not add to global sea level rise because, following Archimedes' principle, it is already displacing its own volume of water when floating in the sea. (When the ice-cube melts in the gin and tonic it does not raise the level of liquid in the glass). However, the melting of giant land-based ice sheets, such as those covering Greenland and Antarctica, is likely to add substantially to sea levels.
What the melting of the Arctic ice will do is spell doom for much of the wildlife of the region, led by polar bears, which need the ice to hunt seals. Polar bears were officially notified as threatened species last year by being included in the Red List of the World Conservation Union. Some conservationists think they could be gone by mid-century.