To general surprise, the world's largest country has revealed itself as also one of the nimblest. Speaking to the United Nations, Hu Jintao, China's president, said his country would reduce its carbon intensity – the amount of CO2 it emits for each dollar of economic output. After months of rich and poor countries digging themselves deeper into their respective trenches, Mr Hu may just be leading the world out of the standoff.

With the Kyoto agreement ticking towards expiry, talks on a new treaty have stumbled on seemingly incompatible demands. Greenhouse gas emissions increasingly come from poor countries – China just surpassed the US as the greatest CO2 emitter. The developed world insists on their participation in a global climate deal (none more stubbornly than the US, where China's non-participation became an excuse for not ratifying Kyoto).

For their part, developing countries – India most stridently – have resisted binding pledges on emissions, pointing out that rich countries put most of the CO2 into the atmosphere – a good point, but one that undermines any global deal if taken to mean that now-industrialised countries must shoulder the entire burden of emissions control.

Poor nations may have fairness on their side, but were losing the war of words. In contrast to India's pouty reaction to being cast as impeding progress, however, China has turned impressions around remarkably swiftly. Mr Hu's offer marks its first contribution as a great power constructively engaging with the world's big problems.

More importantly, it is an excellent idea. Reducing carbon intensity will limit how much emissions can foreseeably rise, without stunting China's growth. This is not only fair, but astute politics: if made legally binding, it could meet some of rich countries' concerns.

An intensity target does not cap emissions, but it makes it easier to do so in the future. It gives China and others following its lead an incentive to join global emissions trading at a later time (even based on absolute caps) by putting them in a good position to profit from it.

Mr Hu did not quantify the intensity target, nor was this a binding pledge. China must now declare its willingness to negotiate a tough and binding intensity target at the Copenhagen summit in exchange for absolute cuts by rich countries. It must also use its heft to bring other poor nations behind it. India, already softening its aversion to binding targets, should align its position with China's.

This is a chance at a good compromise. The world must seize it.