Former World Bank chief economist Sir Nicholas Stern said both greenhouse emissions and the effects of climate change were taking place faster than he forecast seven years ago.
His comments are part of an emerging push by the World Bank and the International Monetary Fund to focus on climate change – something IMF managing director Christine Lagarde this week said put global financial stability “clearly at stake”
The IMF last week published a report arguing that fossil fuels are subsidised to the tune of $US1.9 trillion annually by governments around the world and should be more heavily taxed.
The IMF and the World Resources Institute hosted a speech by Stern in which he stated: “We have to go zero carbon more or less where we can” to meet the goal of limiting planetary warming to 2 degrees Celsius over the next 90 years."
"Without changes to emission trends, the planet has roughly a 50% chance that temperatures will soar to 5C above pre-industrial averages in a century,” he said.
"We haven't been above 5C on this planet for about 30 million years. So you can see that this is radical change way outside human experience.
"When we were at 3C three million years ago, the sea levels were about 20-some metres above now. On sea level rise of just 2 metres, probably a couple of hundred million people would have to move."
Stern said 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 fulfil pledges made in 2010 at a UN-led conference in Cancun, Mexico, the world would be on track to warming of 4C,” he added.
Stern's 2006 study, considered a landmark in raising public attention on climate change, predicted that warming would shave at least 5% off GDP per year.
Despite the slow progress in international negotiations, Stern saw signs for hope as a number of countries moved 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.
He said 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 also 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.