At the last full-scale United Nations conference on global warming, in Bali, the man in charge broke down and wept. American opposition to mandatory carbon cuts had been implacable. Canada and Japan had joined the US in demanding cuts from the big developing economies before promising any themselves. Then China accused Yvo de Boer, head of the UN Framework Convention on Climate Change, of a minor breach of protocol. After two weeks of trying and failing to save the world, it was the last straw. He had to be escorted from the building.

Next December Mr de Boer will submit himself to a similar ordeal in Copenhagen. Thanks largely to Barack Obama, he stands a better chance than last time of maintaining his composure. But if he and his fellow negotiators are to create realistic global defences against rising carbon levels, temperatures and sea levels, they will need all their stamina and more courage than they have shown so far.

The stakes at Copenhagen could not be much higher. Global surface temperatures have risen by a tolerable three quarters of a degree celsius over the past century, but the rate of increase is accelerating. The Kyoto Protocol has had negligible impact on greenhouse gas emissions, and projections for the mean global temperature rise in the next century range from 1.1 to 6.4 degrees. Whether fast or very fast, the Earth is heating up.

There will be continued argument about the science of climate change over the next 12 months, but not, except on the conspiratorial fringe, about the threat. Climate change is real and worsening, and there is an overwhelming likelihood that much of it is man-made.

Mr Obama accepts this and says that he wants to lead efforts to create a successor agreement to Kyoto at Copenhagen. He admits that it will be harder to break America's addiction to oil at $40 a barrel than at $140, but has refused so far to subordinate environmental plans to his financial rescue package. Europe has responded by re-affirming its goal of a 20 per cent carbon cut compared with 1990 figures by 2020.

Does this mean that the major disputes have been ironed out 12 months before the conference? It does not. Expect three furious arguments between now and then. First, there will be a rearguard action, led by business, to replace a cap-and-trade system with a carbon tax as the main post-Kyoto carbon reduction tool. This deserves to fail. Unlike a tax, cap-and-trade focuses on specific emissions goals that can be tightened each year and which use markets to channel funds to the most cost-efficient ways of achieving them.

Second, soaring unemployment will put pressure on finance ministers, especially in Eastern Europe, to forget about climate change and concentrate on survival. Third, India will argue vehemently for actual carbon cuts and technology transfers from the world's richest nations, not merely promises, before the poor South can be expected to join a new climate change deal.

India's case is compelling. Developed nations have produced most of the man-made greenhouse gases released since the Industrial Revolution, and must lead efforts to deal with their effects. But those effects are global, and the lesson of Bali for Copenhagen is that the response must be global, too. It will be expensive for everyone, but not nearly as expensive as doing nothing.