After two years of work, and 12 years after their last attempt, 190 nations gather in Copenhagen from Monday to try to avert dramatic climate change -- what one minister called the most difficult talks ever embarked upon by humanity.

Already the sheer size of the measures needed, and splits between rich and poor about who should pay, mean that a historic U.N. pact to fight global warming and ease dependence on fossil fuels may be put off in favor of a less binding declaration.

The conference runs from December 7-18 and will draw 15,000 officials, campaigners and journalists, making it the biggest climate summit yet.

U.N. scientists predict ever more heatwaves, floods, desertification, storms and rising sea levels this century.

But recession has sapped willingness to invest in a green future, and many opinion polls suggest that public concern about global warming is declining.

These are the most difficult talks ever embarked upon by humanity, said Norwegian Environment Minister Erik Solheim.

The effects will be felt by the rice farmer in Sichuan in China, by Google headquarters in Seattle, or by the oil worker in Norway, he said. It's much more difficult than disarmament, global trade or previous environmental agreements.

PLEDGES FALL SHORT

Experts say pledges made so far are not enough to reach the benchmarks that have been set for averting the worst of climate change, such as ensuring that global emissions fall after 2020.

And rich nations have not yet come up with cash to help developing nations kickstart a deal.

It's unlikely that we'll achieve what's necessary so that emissions will peak before 2020, said Hans Joachim Schellnhuber, head of the Potsdam Institute for Climate Impact Research in Germany.

He hoped for a magic moment of concessions when more than 100 leaders come to the summit for a final push on the last two days, but added: It's possible that Copenhagen could end up as a fiasco.

After an offer by India on Thursday to slow the rise of its greenhouse gas emissions, mainly from burning coal and oil, all the top emitters, led by China and the United States, have pledged curbs.

We have a full house in terms of targets from industrialized countries and indications from major developing countries of what they intend to do, said Yvo de Boer, head of the U.N. Climate Change Secretariat.

But he said there was still a huge challenge to work out a deal that produces action fast enough to slow climate change.

POLITICAL AGREEMENT?

At best, most experts say the talks will reach a political agreement including targets for cuts in greenhouse gases by rich nations by 2020, and new funds for the poor.

Agreement on a legally binding treaty text to succeed the 1997 Kyoto Protocol will be put off until 2010.

Most say a full treaty is out of reach at least partly because the United States has not yet joined other industrialized nations in passing carbon-capping laws.

The U.S. Senate is still debating a bill, although U.S. President Barack Obama will come to Copenhagen on December 9, on his way to collect the Nobel Peace Prize in Oslo.

What we need is a two-step process with some real momentum and a political agreement coming out of Copenhagen, said Eileen Claussen, head of the Washington-based Pew Center on Global Climate Change. A second step would be a legal treaty.

There are also deep rifts between rich and poor nations about how to share the burden of fighting global warming.

China, India, Brazil and South Africa have outlined domestic goals on carbon emissions but rejected some core demands by rich nations, including a goal of halving world emissions by 2050.

They say the rich -- who have benefited from decades of the industrialization that has boosted carbon emissions -- first have to set deeper cuts in their own output by 2020.

So far, cuts on offer by the rich total about 14 to 18 percent by 2020 below a U.N. benchmark year of 1990. Obama will offer 3 percent, or a 17 percent cut judged from 2005 levels.

(Additional reporting by Erik Kirschbaum in Berlin and David Fogarty in Singapore; Editing by Kevin Liffey)

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