"We have a closing window of opportunity in terms of putting a post-2012 approach in place," de Boer, head of the UN Climate Change Secretariat, told Reuters during May 7-18 talks among 166 nations in Bonn about how to curb climate change.

"Bali represents an opportunity to launch such a process. Whether that will happen and exactly what form the launch will take is difficult to predict," de Boer said.

Many delegates in Bonn say they have become gloomier about the chances of a start of formal negotiations in Bali, likely to last two years. Many had expressed confidence of a launch at Bali at the last ministerial talks in Nairobi in November.

Officials in Bonn are seeking ways to widen and extend the UN's Kyoto Protocol on limiting greenhouse gases, released mainly by burning fossil fuels, to include outsiders led by the United States, China and India.

Kyoto binds 35 industrial nations to cut greenhouse gases by 5 percent below 1990 levels by 2008-12 but Kyoto backers only account for about a third of world greenhouse gas emissions.

Time is running short because diplomats reckon it will take two years to negotiate a successor to Kyoto, and then another two years for national governments to ratify. Businesses want to know new rules quickly to help plan investments.

BUSH

Some delegates say new talks might have to wait until after US President George W. Bush leaves office in 2009. Bush opposes Kyoto-style caps on emissions on the grounds they would cost jobs and wrongly exclude poor nations.

Still, governments are under pressure to act after the UN climate panel this year squarely blamed human activities for stoking global warming and said it could bring more hunger in Africa, water shortages for billions and rising ocean levels.

De Boer said the findings of those reports should be presented to ministers at the start of the Bali meeting as a reminder of the scientific findings, including that the costs of coping with change would scarcely brake world growth.

"I think it should be presented to the ministers, yes absolutely," he said. Some delegations -- including China, the United States and Saudi Arabia -- have raised questions about whether ministers need to take time with a presentation.

One European diplomat said a presentation at the start of the meeting, including projections of more hunger in Africa or water shortages in Asia, could help shame governments into action.