Politicians had yet to grasp how devastating climate change would be to society in this century, a leading economist said yesterday.

Wars, famines, floods and hurricanes would wreak havoc unless greenhouse gas emissions were controlled, Professor Nicholas Stern told scientists at the conference.

Another speaker warned that Britain could expect severe droughts and that much of southern Europe would be turned into semi-desert, capable of supporting only a fraction of its current population.

Lord Stern, who wrote the highly influential Stern report, which in 2006 alerted the world to the financial costs of climate change, said that not only was the threat underplayed by politicians but that they did not even understand the extent of the problem.

“Do the politicians understand just how difficult it could be, just how devastating four, five, six degrees centigrade would be? I think, not yet,” he said.

It was beholden on scientists to tell politicians and the public “very clearly and strongly” how severe the risks were. Only by ramming home the message in every country would world leaders appreciate the urgency of the problem. “We have to tell people, you have to tell people, very clearly and strongly just how difficult four, five, six, seven degrees centigrade are because we face very severe risks of going there,” he said.

He predicted that a four or five- degree rise over the next 100 years would result in collapses in crop yields, rivers drying and perhaps billions of people being forced to leave their homes.

“What would be the implication of that?” he asked. “Extended social 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.”

His theme was expanded upon by Rachel Warren, of the University of East Anglia and the Tyndall Centre for Climate Change Research, who predicted that from 2050 to 2100 there would be more than 30 droughts in England and Wales, all lasting more than a year.

Despite the bleak forecasts, Lord Stern remained optimistic that world leaders could reach a deal on reducing emissions of greenhouse gases when they met in Copenhagen in December. “We can get there. I’m more optimistic than I was two years ago. That doesn’t mean it’s going to be easy. We all know it’s going to be tough, but we can get there,” he said.

He was confident that politicians were beginning to head “in the right direction”, despite their failure to appreciate fully the difficulties posed by global warming.

He admitted that the Stern review, which said that the financial cost of climate change could reach 20 per cent of gross domestic product by the end of the century, had underestimated the problem.

“I think that the damages were underestimated by the Stern review and the costs of inaction were even bigger than we argued then,” he said, noting that scientific understanding of greenhouse-gas emission rates and the Earth’s ability to absorb them had advanced.

“The temperatures that could now arise from these greenhouse gases, some of them have higher probabilities than we might have thought.”

He estimated that the costs to society of climate change were now likely to be as much as 50 per cent higher than his 2006 calculation.

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