The author of an influential 2006 study on climate change warned Tuesday that the world could be headed toward warming even more catastrophic than expected but he voiced hope for political action.
Nicholas Stern, the British former chief economist for the World Bank, said that both emissions of greenhouse gas and the effects of climate change were taking place faster than he forecast seven years ago.
Without changes to emission trends, the planet has roughly a 50 percent chance that temperatures will soar to five degrees Celsius (nine degrees Fahrenheit) above pre-industrial averages in a century, he said.
"We haven't been above five degrees Centigrade on this planet for about 30 million years. So you can see that this is radical change way outside human experience," Stern said in an address at the International Monetary Fund.
"When we were at three degrees Centigrade three million years ago, the sea levels were about 20 some meters (65 feet) above now. On sea level rise of just two meters, probably a couple of hundred million people would have to move," he said.
Stern said that other effects would come more quickly including the expansion of deserts and the melting of Himalayan snows that supply rivers on which up to two billion people depend.
Even if nations fulfill pledges made in 2010 at a UN-led conference in Cancun, Mexico, the world would be on track to warming of four degrees (7.2 Fahrenheit), he said.
Stern's 2006 study, considered a landmark in raising public attention on climate change, predicted that warming would shave at least five percent of gross domestic product per year.
Despite the slow progress in international negotiations, Stern saw signs for hope as a number of countries move to put a price on greenhouse gases.
"My own view is that 2013 is the best possible year to try to work and redouble our efforts to create the political will that hitherto has been much too weak," Stern said.
Stern said that French President Francois Hollande was keen for nations to meet their goal of sealing an accord in 2015 in Paris.
Stern also voiced hope that German Chancellor Angela Merkel, long a prominent voice on climate change, would become more active after this year's elections.
US President Barack Obama has vowed action on climate change after an earlier bid was thwarted by lawmakers of the rival Republican Party, many of whom reject the science behind climate change.
Emissions have risen sharply in recent years from emerging economies, particularly China.
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