"The longer we go without action (to curb greenhouse gases) the more likely it is that some of the big feedbacks will kick in," Richard Betts, manager of the climate impacts research team at the British Met Office and Hadley Centre.

"We can make a big difference by either choosing a low emissions scenario or a high emissions scenario," said Gunnar Myhre, of the Center for International Climate and Environmental Research in Oslo.

Both were among lead authors of a UN climate report in February, based on the work of 2,500 scientists, that laid out scenarios of temperature rises of 1.1-6.4 Celsius (2 to 11.5 Fahrenheit) by 2100 over 1990 levels.

In the scenarios, the biggest temperature gain comes if the world stays dependent on fossil fuels, with 70 percent of energy in 2100 from sources such as coal and gas, and sharply raises greenhouse gas emissions.

SHIFT TO RENEWABLES

The scenario with the smallest temperature gain, below about 3 Celsius (5.4 F), assumes that carbon emissions will dip by 2100 by when the world will get about half its energy from renewable sources.

The draft report, due for release in Brussels, will build on the first part and lay out the regional impacts of climate change, such as a drying of the Amazon basin or a sharp contraction of vast Himalayan glaciers that feed rivers in Asia.

In the draft, a temperature rise above about 3C could mean a sharp expansion in water shortages, for 1.1 to 3.2 billion people.

At about that level potential crop yields would also start to fall in all parts of the world after briefly benefiting farmers in some regions away from the tropics.

And above about a 4 Celsius (7.2 F) gain, one scenario sees a potential extinction of about 45 percent of Amazonian tree species. "The uncertainties in the emissions scenarios are as large, if not larger, than uncertainties about the response of the climate" to greenhouse gases, Betts said. Both Betts and Myhre declined comment on the regional impacts.