Fishermen unload their catch today in Bali, Indonesia. Fish stocks, already under pressure from overfishing are further imperilled by the prospect of global warming. Photo / AP

Fishermen unload their catch today in Bali, Indonesia. Fish stocks, already under pressure from overfishing are further imperilled by the prospect of global warming. Photo / AP

BALI - Top scientists jumped into the political battle over global warming at a climate conference, urging mankind to make deep cuts in carbon dioxide and other heat-trapping gases, but the United States refused to back away from its opposition to mandatory cuts.

More than 200 experts issued a declaration at the international gathering on Bali island, which entered its fifth day on Friday, calling for a 50 per cent reduction in such emissions by 2050 - a rare policy prescription by scientists who usually limit themselves to presenting evidence and leaving the politicians to choose which remedies to take.

The scientists aimed to spur talks here over launching negotiations, to extend over the next two years or so, on an emissions-cutting agreement to succeed the Kyoto Protocol, which expires in 2012. The current pact requires 36 industrial nations to reduce the gases by a modest 5 per cent below 1990 levels. The United States is the only industrial nation to reject it.

"What this declaration is about is delivering a clear message. It's got the weight of the scientific community behind it," said Australian climatologist Matthew England, a group spokesman. "It means we have to have a radical change to the way we power this planet. We have to start reducing greenhouse gas emissions as soon as we possibly can."

Delegates from nearly 190 nations are attending the two-week conference.

It has broken into a tense standoff between two camps, with a majority supporting mandatory emissions cuts on one side, and opponents such as the United States on the other, delegates said. Failure to act, experts say, would allow rising temperatures to trigger devastating droughts, flooding and other environmental damage.

The United States, the world's largest producer of climate-warming gases, resisted calls for strict limits on emissions, even as the US Senate Environment and Public Works Committee passed a bill Wednesday to cut pollutants by 70 per cent by 2050 from electric power plants, manufacturing and transportation. The bill now goes to the full Senate.

US officials said neither the Senate action nor the decision earlier this week by Australia to sign the Kyoto Protocol - after years of standing with the US in opposition - would have an impact on Washington's stance at the Bali conference.

Top American negotiator Harlan Watson shrugged off the latest scientific declaration, suggesting it lacked the weight of the Intergovernmental Panel on Climate Change, or IPCC, the scientific group that shared this year's Nobel Peace Prize with former Vice President Al Gore.