At least 400 scientists and experts from about 120 countries are attending the week-long third session of the Intergovernmental Panel on Climate Change (IPCC), the UN's leading authority on global warming.

Two reports issued earlier this year by the panel warned that the Earth was already warming and predicted severe consequences including drought, flooding, violent storms and increased hunger and disease.

The third report expected to be released on Friday will lay out ways to reduce greenhouse gas emissions and prevent a climate catastrophe without seriously impacting on the global economy.

"The time to act is now," Chartree Chueyprasit, one of Thailand's top environment officials, told the opening of the meeting.

"Global warming is increasingly becoming a hot agenda that requires harmonised cooperation from all nations."

But achieving that harmony between so many diverse nations and agreeing on exactly what action to take is expected to be the subject of fierce debate, with some delegates predicting that the meeting will be hijacked by politics.

"It's very difficult at these negotiations to try to find that level of compromise and to try to find sustainable solutions that are equitable," said Peter Lukey, of South Africa's Department of Environmental Affairs.

"This is a highly politicised event. You take these political stances. It's highly frustrating to see."

The European Union -- which has pledged to reduce its carbon dioxide emissions by 20 percent by 2020 -- may face off with the United States and China, the world's biggest carbon polluters.

"I hope this gathering can produce balanced views, not just the views of the developed countries, but also the views of the developing countries," Chinese delegate Sun Guoshun told AFP.

Rajendra Pachauri, chair of the IPCC, played down tensions between the major players and said they were hopeful of hammering out their differences and reaching a conclusion by Friday.

"I think the nature of the subject is such that there will be a lot of intensive debate but I'm sure we will be able to resolve any outstanding issues without any major disruption in the proceedings," he told reporters.

"Ultimately it's a balanced assessment of the science which prevails."

An early draft of the panel's 24-page summary, seen by AFP, says that world leaders have little time to waste, but that the tools for reducing greenhouse gas emissions -- at a modest cost -- already exist.

Renewable energy, nuclear power, bio-fuels and reforestation are all in the mix, but a spokesman for the UN Environment Programme said the draft report was likely to be "completely rewritten" by the end of the week.

Sticking points at the Bangkok meeting could include taxes and caps on carbon dioxide (CO2) emissions and any references to the Kyoto Protocol, which US President George W. Bush abandoned after he took office.

Any mention of nuclear energy in the final report would incur the wrath of many environmental groups.

One of the key issues set to be hotly debated is a so-called carbon price -- finding a way to make consumers and businesses pay for the pollution they create.

The findings of the report, which stops short of making recommendations, will be used by governments and international organisations to map out their own plans for preventing worst-case climate scenarios.

The report will also play a key role in Kyoto negotiations, which will take place in December on the Indonesian island of Bali.

"The IPCC doesn't have any muscle, it has grey matter," Pachauri said. "The muscle will have to come from somewhere else."

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