A similar, if slightly more sophisticated, case for current policies has been put forward by a distinctly better economist than Stern, Harvard's Professor Martin Weitzman, in what he likes to call his "dismal theorem". After demolishing Stern's cost-benefit analysis, he concludes that Stern is in fact right but for the wrong reasons. According to Weitzman, this is an area where cost-benefit analysis does not apply. Climate science is highly uncertain, and a catastrophic outcome which might even threaten the continuation of human life on this planet, cannot be entirely ruled out however unlikely it may be. It is therefore incumbent on us to do whatever we can, regardless of cost, to prevent this.

This is an extreme case of what is usually termed "the precautionary principle". I have often thought that the most important use of the precautionary principle is against the precautionary principle itself, since it can all too readily lead to absurd policy prescriptions. In this case, a moment's reflection would remind us that there are a number of possible catastrophes, many of them less unlikely than that caused by runaway warming, and all of them capable of occurring considerably sooner than the catastrophe feared by Weitzman; and there is no way we can afford the cost of unlimited spending to reduce the likelihood of all of them.

In particular, there is the risk that the earth may enter a new ice age. This was the fear expressed by the well-known astronomer Sir Fred Hoyle in his book Ice: The Ultimate Human Catastrophe, and there are several climate scientists today, particularly in Russia, concerned about this. It would be difficult, to say the least, to devote unlimited sums to both cooling and warming the planet at the same time.

At the end of the day, this comes down to judgment. Weitzman is clearly entitled to his; but I doubt if it is widely shared; and if the public were aware that it was on this slender basis that the entire case for current policies rested I would be surprised if they would have much support. Rightly so.

But there is another problem. Unlike intelligent adaptation to any warming that might occur, which in any case will mean different things in different regions of the world, and which requires no global agreement, decarbonisation can make no sense whatever in the absence of a global agreement. And there is no chance of any meaningful agreement being concluded. The very limited Kyoto accord of 1997 has come to an end; and although there is the declared intention of concluding a much more ambitious successor, with a UN-sponsored conference in Paris next year at which it is planned that this should happen, nothing of any significance is remotely likely.

And the reason is clear. For the developing world, the overriding priority is economic growth: improving the living standards of the people, which means among other things making full use of the cheapest available source of energy, fossil fuels.

The position of China, the largest of all the developing countries and the world's biggest (and fastest growing) emitter of carbon dioxide, is crucial. For very good reasons, there is no way that China is going to accept a binding limitation on its emissions. China has an overwhelmingly coal-based energy sector — indeed it has been building new coal-fired power stations at the rate of one a week — and although it is now rapidly developing its substantial indigenous shale gas resources (another fossil fuel), its renewable energy industry, both wind and solar, is essentially for export to the developed world.

It is true that China is planning to reduce its so-called "carbon intensity" quite substantially by 2020. But there is a world of difference between the sensible objective of using fossil fuels more efficiently, which is what this means, and the foolish policy of abandoning fossil fuels, which it has no intention of doing. China's total carbon emissions are projected to carry on rising — and rising substantially — as its economy grows.