Make clear dire scenarios to prod climate action: Stern

People use a boardwalk to avoid getting wet feet during a period of high water in Venice in this file photo from February 7, 2009.

COPENHAGEN (Reuters) - Scientists must make clear the disastrous effects of climate change so the world takes action now to cut carbon emissions, leading economist Nicholas Stern said on Thursday.

"You have to tell people very clearly and strongly just how difficult (a temperature rise of) four, five, six or seven degrees Celsius is," he said.

"Billions of people would have to move and there would be very severe conflict," said Stern, a professor at the London School of Economics and a former British Treasury economist.

"That's a story that must be told to persuade people it's a very bad idea to go anywhere near five degrees. This is not a black swan, this is a big probability of a devastating outcome," he told a gathering of 2,000 scientists at the Congress on Climate Change in Copenhagen.

"The costs of delay are very big," he said.

Stern, in a 2006 report, warned inaction on greenhouse gas emissions could cause economic pain equal to that of the Great Depression of the 1930s.

Above two degrees warming, hundreds of millions of people would face reduced water supplies, and above three degrees food production worldwide would be very likely to decrease, a U.N. panel of climate scientists said in 2007.

Professor John Schellnhuber of the Potsdam Institute for Climate Impact Research, speaking before Stern, said a warming of five degrees would mean the planet could support less than 1 billion people.

Reaching a new global climate deal in December in Copenhagen, to succeed the Kyoto protocol, depends largely on whether developing countries agree to steps to help solve a problem they say rich countries created.

Stern said rich countries should agree to cut their greenhouse gas emissions by 80 percent by 2050 to bring emissions in the world as a whole down by 50 percent over the same period.

"For 1 or 2 percent of global GDP we can hold concentrations of (atmospheric carbon) below 500 parts per million, then go down from there," Stern said. "This would bring the probability of five degrees warming to only 2 or 3 percent. That sounds like a pretty good deal to me. We can make a very powerful insurance case."

Stern said developing countries were not yet ready to agree to fixed carbon targets, but that rich countries could convince them by sharing green technologies, starting global carbon trading and helping poorer countries to stop deforestation and adapt to the more immediate consequences of climate change.

"China and India are potential deal-breakers. But we can get there. I am more optimistic than I was two years ago, he said."

(Editing by Janet Lawrence)