The new report by the Intergovernmental Panel for Climate Change (IPCC) "shows the situation has clearly got worse," a European diplomat told AFP.

"The question now is to translate this goal into an international regime able to tackle carbon emissions," he said.

"The developed countries must take the lead, but the question is whether the United States is going to get serious about returning to international negotiations. The effort also implies emissions reductions by big developing countries, especially China."

Despite its flaws, complexities and critics, the only game in town for tackling carbon emissions is the UN's Kyoto Protocol.

Yet in its present form, the treaty will barely make headway against the carbon gases that are piling up invisibly in the atmosphere and dangerously trapping solar heat.

One reason is that both the United States and China, for different reasons, lie outside Kyoto's constraints.

The United States walked away from Kyoto in 2001, saying it would not ratify the deal as it was not in US interests to do so, given the cost of converting to cleaner energy use.

It also said the pact was unfair because big developing countries are not required to make targeted pollution curbs. These curbs only apply to industrialised countries, whose burning of oil, gas and coal largely caused today's climate damage.

Kyoto runs out in 2012, and talks are underway for its post-2012 commitments -- and these pledges must establish machinery that will deliver massive cuts.

By some estimates, today's emissions must be cut by half globally by 2050 to peg the temperature rise by 2100 to 2 C (3.6 F) compared with pre-industrial levels -- a rise that many scientists caution may not give any guarantee of safety anyway.

How can this machinery be built?

In the failure of voluntary or regional initiatives to cope with what is a global problem, the task will be to encourage the United States to return to the Kyoto fold or at least build some emissions-cutting link with Kyoto parties.

It would also have to wean China, but also probably Brazil and India, away from dangerous burning of oil, gas and coal.

But at present those countries are hanging tough, saying the industrial countries bear most responsibility for today's problems and so they have to make the big sacrifices. As developing economies, they oppose commitments that would drag their rise to prosperity.

Yvo de Boer, the executive secretary of the UN Framework Convention on Climate Change (UNFCCC), has been leading appeals for new UN Secretary General Ban Ki-Moon to call a global summit. He has been supported by the UN Environment Programme.

In Paris Friday, de Boer said the IPCC report "is a call for immediate action." He highlighted the linchpin roles of the United States and the big developing countries, but skirted making recommendations as to what they should do.

There is now "a general recognition that a global response means that you also need to engage major developing countries like China, India and Brazil," said de Boer.

"(...) In any case, it is absolutely essential to have continued US engagement on the question of climate change and to listen carefully to concerns to the US to make sure that what we put in place is for everbody."

Among Kyoto's defenders, hopes were barely disguised Friday that the report, helped by the newly-elected Democratic majority Congress, might pressure US President George W. Bush to return to the international fold in some form.

South Africa said the IPCC was "a wake-up call to the world's largest emitter, the United States," which had a moral obligation to join the global effort.

In Brussels, the European Union's executive Commission said "it is now more urgent than ever" that the international community gets down to "serious negotiations."

Last month, the European Commission took the lead in the negotiation poker unfolding among the main Kyoto parties.

It called on developed countries to slash emissions to 30 percent below 1990 by 2010. If that initiative failed, the 27-nation EU would unilaterally cut its own emissions by "at least 20 percent."

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