"The push for greater mitigation will come through catastrophes and other extreme events," he said.

Next week, the IPCC is scheduled to release the third and final part of its global warming report in Bangkok, which will look at measures to reduce emissions and ways the world can adapt to stave off the impacts of climate change.

"What we want is confirmation," Munasinghe told Reuters. "The report should come out saying there are mitigation options available that can be done at relatively low cost. Technologies are available."

The IPCC groups 2,500 scientists, is considered the world authority on the issue, and has said all regions of the planet will suffer from sharp global warming.

Its findings are approved by consensus of more than 100 governments and will guide policy on issues such as extending the Kyoto Protocol, the main UN plan for capping greenhouse emissions, beyond 2012.

"There is a very clear trade-off between the pain -- how much economic cost it might be -- and how much of climate change we want to avoid," Munasinghe said.

MORE STORMS

To keep the temperature rise at 2 degrees Celsius or less by 2100, measures must be adopted to check CO2 levels in the air at 450 parts per million, which will shave 2-3 percent off global gross domestic product by the year 2030, he said.

Taking steps to limit CO2 at 550 parts per million, allowing for a temperature rise of 3 degrees C, will cost less than 1 percent of GDP. The costs of letting CO2 go to 650 parts per million with a 4 degree rise in global temperatures -- widely viewed as dangerous -- would be "hardly noticeable", he said.

"I think it's unlikely that people will try to aim for 450," said Munasinghe, who is also chairman of the Colombo-based Munasinghe Institute for Development.

"My guess ... is that 550 is the kind of comfort level now." Most scientists say climate change will cause seas to rise, glaciers to melt and storms to intensify, potentially leading to more natural disasters around the world.

Hurricane Katrina, which struck the US Gulf coast and inundated New Orleans in 2005, "immediately highlighted the fact that extreme events like storms and hurricanes will be much more frequent as the climate worsens", he said.

Disasters will also enliven the debate about how to balance between efforts to mitigate climate change and efforts to adapt to it and guard against it, he said.

Rising costs are another factor that will help motivate people to improve emissions. "Price is a great motivator," he said, noting how the 1973 oil crisis essentially gave birth to a range of cars that were much more fuel efficient.

Increased awareness of the problem, which "has exploded" in the past two years, can push policymakers to make the necessary changes to cut emissions, he said.