Leading article: The world cannot wait

At the Meteorological Office's headquarters in an industrial estate on the outskirts of Exeter last week, the world's climate scientists peered into the abyss. As Geoffrey Lean, our environment editor, writes on page 10, most of them will have felt their children or grandchildren standing at their shoulders. Global warming, as the product of human activity, is now established as scientific fact more firmly than ever, and the consequences are worse, more certain and closer than previously thought. Previously remote possibilities, such as the melting of much of the Antarctic ice cap, the collapse of the Amazonian rainforest and deadly acidification of the world's oceans, are becoming ever more likely. This is a challenge to the whole of humankind, and it requires global agreements and institutions to respond. It is, in particular, a vast and historic challenge to political leaders all over the world, and it is to Tony Blair's credit that he has made action on climate change a priority of this year's UK presidencies of the EU and G8. He could have chosen a far easier task.

The Exeter conference went some way to meeting the first of the Prime Minister's objectives: to mobilise and express a clear scientific consensus. But his central problem is to devise a way of tackling global warming that brings both the United States and the rapidly industrialising China and India on board. The two developing countries understandably will not join any successor to the Kyoto protocol unless the US, which is responsible for a quarter of the pollution that causes the climate change, also does so.

President Bush - with much less justification, but a political constituency to satisfy - will not join unless China and India do. In this impasse Mr Blair is naturally inclined to a third way and, in the tradition of most successful diplomacy, is trying to find a baseline on which they can all agree - such as that global warming is indeed an issue and that new technologies should be developed to address it. It is at least possible that the G8 summit at Gleneagles in July will set this process in train, and if President Bush were to agree that the problem is real and that something must be done, that would indeed be a breakthrough.

The trouble is that all the time the clock is ticking. We are running out of time. Unless the world does quickly sign up to new and stricter targets for reducing emissions, it will be too late. Achieving, or even making big progress towards it, will tax Mr Blair's formidable powers of persuasion to the limit. But by common consent he is the leader best placed to pull it off. He should aim at nothing less.