A route that has opened up by global warming can cut travel times between the Atlantic and Pacific Oceans. Picture: REUTERS/US Coast Guard/Patrick Kelley/Handout

London - A few years ago, just after the Arctic sea ice had retreated to its (then) all-time record in September 2007, I found myself in the company of a distinguished scientist and polar explorer who had taken a rather sceptical line on global warming. In short, he thought it was all a hoax.

Rising to the bait, I asked him what he thought about one of the most obvious manifestations of a warmer world, the continuing retreat of the Arctic sea ice in the summer months. How do you explain that? I asked him.

Simple, he said. The sea ice varies naturally and what we are seeing is well within the bounds of natural variability. What people forget, he added, is that sea ice is not fixed to the ground, it is easily blown about by the winds, making it seem as if it has disappeared when in fact it is just piled up on top of itself.

Although winds do affect the floating sea ice, there is little dispute about the satellite observations showing the extent of its retreat in recent decades. The US military satellites used by the US National Snow and Ice Data Centre in Boulder, Colorado, use passive microwaves to measure sea ice cover and they show a clear decline in surface area of more than 30 percent since records began in 1979. But could such a significant decline be the consequence of natural variability?

Computer models of global warming suggest that sea ice would be one of the most obvious consequences of a warmer world, but these models did not predict that the observed decline would be quite so rapid.

Earlier this year, scientists from the Max Planck Institute for Meteorology in Hamburg tried to address this question of natural variability - looking at the impacts of seasonal variations in the weather and conditions such as the direction and strength of the Arctic winds.

Instead of using computer models, the scientists crunched the numbers on statistical associations to determine whether a particular phenomenon could be one of the causes behind the observed decline in sea ice.

While variable winds and weather can have dramatic effects on sea ice, they on their own could not explain the intensity of the observed decline, the scientists said. They also discounted other variables, such as solar radiation, cosmic rays, volcanic eruptions and oceanic heat transport. In the end, the only thing they were left with was the rising levels of CO2 in the atmosphere caused by human activities.

Their conclusion: the disappearing sea ice was indeed the manifestation of man-made global warming. - The Independent