Oliver Letwin: Climate change is the big test for Blair and G8

On Africa, even if Gordon Brown's International Finance Facility proposal has been reduced in scale, there has been real progress. But on climate change, the signs so far are not encouraging. If, despite frantic, last-minute negotiations, the G8 climate change discussion ends with a whimper rather than a bang, we won't merely have failed to move forward - we will have moved backwards.

The risk is that, if the Prime Minister's hype isn't followed by visible advance, the world's media will conclude that it is all too difficult; the diplomats will become disenchanted; and the politicians will find it even more difficult to raise the necessary enthusiasm the next time round.

Some sceptics will, of course, be delighted if Mr Blair's attempt to put climate change centre-stage fails. They argue that we should not commit resources to the uncertain challenge of climate change when we could be addressing problems whose existence is certainly not in doubt - above all, global poverty. There are mischievous moments when one could wonder whether Mr Brown is covertly in this category.

But the sceptics are taking a reckless gamble. They are betting that, by avoiding action now, we can save some eco- nomic costs in the short term, without facing an immense social and economic penalty in the long term. If they are wrong, and the mainstream scientists are right, the long-term price of saving some money now will be terrible.

This is an unequal bet. It is not worth risking catastrophic climate change in the future for the sake of avoiding a mild reduction in the rate of global economic growth today - especially when many of the measures required to reduce greenhouse gas emissions also have wider environmental benefits.

The key to progress is the UN Convention on Climate Change which the last Conservative Government persuaded the US to sign. Ironically, this convention explicitly signs up to the scientific concerns that Mr Blair is now finding it difficult to persuade Mr Bush to accept.

It begins with the words: "acknowledging that change in the Earth's climate and its adverse effects are a common concern of humankind, concerned that human activities have been substantially increasing the atmospheric concentrations of greenhouse gases, that these increases enhance the natural greenhouse effect, and that this will result on average in an additional warming of the Earth's surface and atmosphere and may adversely affect natural ecosystems and mankind ..." One could hardly be clearer than that.

Given the existence of the convention, and the clarity of its intent, there is little (if any) point in the G8 having long, metaphysical debates. The Prime Minister's task, as the master-negotiator, is to find a practical way of maintaining momentum. This must mean using the framework provided by the convention - and the meeting of the convention countries in Montreal later this year - to coax the United States on one side and the emerging giants of India and China on the other into simultaneous movement. Simultaneity is the crux, because it is the fear of competitive disadvantage, rather than the fear of the absolute economic costs, that is really driving the reluctance of the three great, dynamic economies of the US, India and China to take serious action.

What is needed is a system of mutually assured action, in which each party eventually agrees to a package of targets and delivery mechanisms, secure in the knowledge that all other parties are headed in the same direction.

It would be wholly unrealistic to expect an agreement of mutually assured action at Gleneagles. But what could still be achieved is the establishment of a critical path towards such an agreement. This would provide a new momentum and prevent the over-hyped Gleneagles meeting turning into a serious reverse.

The right analogy for the climate change negotiations is not global poverty. It is nuclear weapons and strategic arms reduction. We need to move from a position in which we are threatened with mutual destruction to one in which, through a series of strategic carbon reduction treaties, we achieve progressive economic and environmental détente. Perhaps what we are missing is Henry Kissinger?

The writer is the shadow Environment Secretary