Summing up what many scientists, environmentalists and politicians now think about the threat of climate change is simple: the world is drinking in the last chance saloon.

Time is still available to tackle the warming of the atmosphere, which every government (including that of George Bush) today accepts is real, and being caused by human actions. But the window of opportunity is rapidly closing, and the last chance for the world to act in concert to bring the process under control is clearly visible: it is the UN Climate Conference in Copenhagen scheduled for December 2009.

The international community will meet to try to agree a new global deal to cut rising emissions of greenhouse gases – the waste gases from industry and transport, principally carbon dioxide, which are retaining the sun's heat in the atmosphere and causing temperatures to rise.

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But, if such a deal is not achieved in Denmark, many observers feel it will be too late subsequently to stop climate change causing devastating problems for the world in general and human society in particular, from the widespread failure of agriculture to the swamping of low-lying countries by sea-level rise.

This week, in the Polish industrial city of Poznan, a dress rehearsal for Copenhagen is taking place. Diplomats and officials from every member state of the United Nations have come together to shape the outline of next December's agreement – or at least, to try. For it would be difficult enough to get 150-plus delegations, each with its own domestic pressures and national interests, to agree on the colour of an orange, never mind what, if it is successfully negotiated, will probably be the most complex treaty the world has seen.

Who is to do what? That's the simple, but fantastically difficult question at the heart of everything. Most of the excess CO2 (above normal historical levels) currently in the atmosphere was put there by the rich developed countries such as the US, Britain, Germany and Japan, in the two centuries since industrialisation began, and particularly in the years since the end of the Second World War when economic growth shot forward so dramatically. But developing countries such as China, India, Indonesia and Brazil are rapidly catching up, and it is probable that China, for example, with its exploding economy growing at 10 per cent a year, has overtaken the US to become the largest CO2 emitter in the world.

So who is more to blame? How are we to balance what is known in the jargon as the burden-sharing? How are we to divide up the pain? (For cutting carbon emissions out of your economy is costly and may restrict your economic growth.) Can China cut less CO2 than the US, even if it is emitting more? Will the US even be willing to make cuts under those circumstances?

The first world climate treaty, the Kyoto protocol, signed in 1997, recognised that nations had "common but differentiated" responsibilities and so it committed the industrialised countries, including Britain and the rest of the European Union, the US and Japan, to making fairly modest reductions in their CO2 by 2010, while the developing countries, led by China, began to monitor their emissions, but were not obliged to cut them. But Kyoto ran aground. Never mind that few nations (Britain being a rare exception) will meet their modest Kyoto targets. George Bush withdrew the US in 2001 and robbed it of its significance; without the world's biggest polluter involved, Kyoto ceased to mean anything.

What is to be negotiated at Copenhagen is a successor treaty to Kyoto, and it is clear what that must involve, if it is to succeed. Firstly, all the major industrialised countries, including America, must agree a programme of deep cuts in their emissions. They have to lead the way.

Secondly, the developing countries, led by China, have to do something in terms of reducing their emissions; they cannot simply continue with business as usual.

Thirdly, there will have to be new flows of funds from the rich nations to the developing countries, to help them take action, both to cut their emissions, and to protect themselves againstthe effects of climate change which will be unavoidable, such as sea-level rise.

And fourthly, there may have to be a special agreement to protect the world's forests, which lock up a vast amount of the Earth's carbon.

If such a deal is eventually done, it will be close to the wire; it will be behind closed doors in Denmark, brokered at the last minute between pragmatic and powerful people. The original Kyoto agreement was reached in this way, in the early hours of the morning. An important player was Al Gore, who was then the US Vice-President (and who has since gone on to become, with his film An Inconvenient Truth, the world's single most influential voice on climate change). Another was Britain's John Prescott, who then combined the jobs of Deputy Prime Minister and Environment Secretary – Kyoto was probably his finest hour. Perhaps the key player was the Argentine Raul Estrada, chairman of the central negotiating committee, who rescued the talks when they seemed on the brink of collapse.

Such figures will be necessary in Copenhagen. One of them will almost certainly have to be Chinese. But, towering above them all is one new figure: Barack Obama. The President-elect has already made it clear he sees tackling global warming as a priority, and he will reverse George Bush's obstructionism and fully re-engage with the talks. The election of Mr Obama was like an electric shock: it changed the dynamic and meant a deal was possible; if the US were to take a leading role, it would mean it was very possible. Britain's new Energy and Climate Change Secretary, Ed Miliband, who is taking part in the Poznan talks this week, wants a leading role for Britain, although the UK position, of course, is part of the position of the EU.

Therein lies a difficulty, as this week the EU is trying to finalise its own energy and climate package – with some difficulty. This aims ambitiously at member states providing 20 per cent of their energy requirements from renewable sources by 2020, and cutting 20 per cent of EU carbon emissions by the same date, if no global climate deal is achieved at Copenhagen – and 30 per cent if a global deal is achieved. Some member countries, principally Poland, Italy and the Czech Republic, are unhappy with this, and it is possible that European heads of government (including Gordon Brown), who will try to complete the deal tomorrow and on Friday, may have to secure their adhesion by watering it down.

This possibility worries environmentalists and other observers in Poznan, who were hoping that if the EU set a forceful example, a very positive statement about the way forward to Copenhagen might be agreed by the end of the week. Another worry is that the Obama effect is not yet being felt: remember that it remains the Bush administration until the new president is inaugurated on 20 January. It is possible that the Poznan declaration may not be as positive as many would like.

But what cannot be disregarded is the pressure of the science, which gets more ominous. "The world desperately needs an effective equitable global deal to be secured in Copenhagen next year, and Poznan fires the starting gun on that process," said Robin Oakley, head of climate and energy for Greenpeace UK. "At the moment, we are heading for something like four degrees of temperature rise, which would be catastrophic," said Ed Matthew, head of climate change for Friends of the Earth. "We are running out of time."