Lord Stern, the economist who produced the single most influential political document on climate change, says he underestimated the risks of global warming and the damage that could result from it.

The situation was worse than he had thought when he completed his review two-and-a-half years ago, he told a conference yesterday, but politicians do not yet grasp the scale of the dangers now becoming apparent.

"Do politicians understand just how difficult it could be, just how devastating rises of 4C, 5C or 6C could be? I think, not yet," Lord Stern posed to the meeting of scientists in Copenhagen.

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"A rise of 5C would be a temperature the world has not seen for 30 to 50 million years. We've been around only 100,000 years as human beings. We don't know what that's like.

"We haven't seen 3C for a few million years, and we don't know what that looks like either."

Lord Stern said new research done in the past two or three years had made it clear there were "severe risks" if global temperature rose by the predicted 4C to 7C by 2100. Agriculture would be destroyed and life would be impossible over much of the planet, the former World Bank chief economist said.

Now a professor at the London School of Economics, Lord Stern referred to the higher temperature rises several times at the conference. The scientists are meeting in the Danish capital as a precursor to the major UN meeting in December which aims to find a successor to the Kyoto Protocol climate treaty.

Commissioned by Gordon Brown when Chancellor, and issued in October 2006, the 700-page Stern Review on the Economics of Climate Change is the single most influential political document on global warming yet published. It has been closely studied by the governments of every major country.

The report said the costs of acting to counter climate change, by stabilising emissions of carbon dioxide in the atmosphere, might be about 1 per cent of annual global GDP by 2050. But the cost of doing nothing was found to be far greater – risking up to 20 per cent of the world's wealth.

Yesterday, Lord Stern revised this prediction, saying the cost of inaction would be "50 per cent or more higher" than his previous highest estimate – meaning it could cost a third of the world's wealth.

The conference has been hearing detail on research done following the Stern review, including claims sea levels are likely to rise twice as fast as predicted in the last UN climate change report in 2007.

Lord Stern said the world's population needed to be aware of the implications of climate change, with many areas devastated by hurricanes and others drying out.

"Much of southern Europe would look like the Sahara. Many of the major rivers of the world, serving billions of people, would dry up in the dry seasons or re-route."

Billions of people would have to relocate as a result, he said.

"What would be the implication of that? Extended conflict, social disruption, war essentially, over much of the world, for many decades.

"This is the kind of implication that follows from temperature increases of that magnitude. I think it's vital that people understand the magnitude of the risks, but also that they understand that [by cutting emissions] we can reduce the probability of going there very dramatically," Lord Stern said.

Then and now: How Stern's view changed

Temperatures

A central assumption of the 2006 Stern Report was global temperatures would rise by between 2C and 3C over the current century if nothing was done to counter global warming.

Stern also mentioned the possibility of a 4C rise.

Yesterday, Stern said 4C, 5C, 6C and even 7C degree rises were a real possibility by the end of the 21st century, taking the world into new territory - agriculture would be destroyed and life impossible in many areas.

Costs

Stern created a sliding scale in the 2006 report which measured the costs of doing nothing on climate change. At the upper limit was the chance the damage would amount to 20 per cent of global Gross Domestic Product – a fifth of the world's wealth.

Yesterday, Stern revised his estimate saying the cost would be 50 per cent higher "or more" than the previous highest guess – risking a third of the world's wealth or a 30 per cent plus reduction in consumption per head.