The team studied core samples taken from the bottom of the ocean, which allow C02 levels to be tracked millions of years ago. They show that when the world began to glaciate at the start of the Ice age about 35m years ago, the concentration of CO2 in the atmosphere stood at about 450ppm.
“If you leave us at 450ppm for long enough it will probably melt all the ice – that’s a sea rise of 75 metres. What we have found is that the target we have all been aiming for is a disaster – a guaranteed disaster,” Hansen told the Guardian.
At levels as high as 550ppm, the world would warm by 6C, the paper finds. Previous estimates had suggested warming would be just 3C at that point.
Hansen has long been a prominent figure in climate change science. He was one of the first to bring the crisis to the world’s attention in testimony to Congress in the 1980s.
But his relationship with the Bush administration has been frosty. In 2005 he accused the White House and Nasa of trying to censor him. He has steadily revised his analysis of the scale of the global warming and was himself one of the architects of a 450ppm target. But he told the Guardian: “I realise that was too high.”
The fundamental reason for his reassessment was what he calls “slow feedback” mechanisms which are only now becoming fully understood. They amplify the rise in temperature caused by increasing the concentration of greenhouse gases. Ice and snow reflect sunlight but when they melt, they leave exposed ground which absorbs more heat.
As ice sheets recede, the warming effect is compounded. Satellite technology available over the past three years has shown that the ice sheets are melting much faster than expected, with Greenland and west Antarctica both losing mass.
Hansen said that he now regards as “implausible” the view of many climate scientists that the shrinking of the ice sheets would take thousands of years. “If we follow business as usual I can’t see how west Antarctica could survive a century. We are talking about a sea-level rise of at least a couple of metres this century.”
The revised target is likely to prompt criticism that he is setting the bar unrealistically high. With the US administration still acting as a drag on international efforts, climate campaigners are struggling even to get a 450ppm target to stick.
Hansen said his findings were not a recipe for despair. The good news, he said, is that reserves of fossil fuels have been exaggerated, so an alternative source of energy will have to be rapidly put in place in any case. Other measure could include a moratorium on coal power stations which would bring the C02 levels to below 400ppm.
Hansen’s revised position will pile yet further pressure on Britain over plans to build a new generation of coal power stations. Last year he wrote to Gordon Brown urging him to block the first such power station; the Royal Society has made similar suggestions to the government.