Mr Cameron’s green concerns undoubtedly upped the ante. Greenpeace says that the Treasury was lukewarm and the office of the Deputy Prime Minister closed to them before Mr Cameron’s election. But the Chancellor’s interest in climate predates the Tory shift. He has been concerned that global warming will wreak most havoc on the poorest countries about which he cares so much.

Two years ago he was involved in convincing Russia to ratify the Kyoto treaty. He listens to Michael Jacobs, the environmental economist who used to head the Fabian Society, and now we understand to Al Gore, whose Administration made strenuous efforts to sabotage the Kyoto Protocol in the 1990s but who is now free to speak from the heart.

Mr Brown posed a vital question: would it cost more to try to prevent climate change, or adapt to it? Climate scientists had struggled to provide any robust economic analysis. One of the few economists to have addressed this is Paul Ekins, who calculated in 2004 that meeting the Kyoto Protocol targets would cost the US no more than 1 per cent of GDP. Sir Nicholas’s emphatic answer — that unchecked warming could devastate the world economy on the scale of the world wars and Great Depression — has strengthened the hands of politicians who want to act. By unleashing this report, the Chancellor is building a momentum that he knows may prove unstoppable.

Sir Nicholas’s emphasis on international action plays to Mr Brown’s strengths. The flabby EU Emissions Trading Scheme, for example, on which so many hopes now rest, comes to an end in 2012. The Government is pressing member states to tighten it up in 2009. Mr Brown’s experience in navigating EU, IMF and World Bank meetings will stand him in good stead in the negotiations that he hopes Britain will lead. This must include bringing the US, China and India more clearly into the equation, and making best use of facilities such as the UN Clean Development Mechanism and the World Bank’s Clean Energy Investment Framework.

The science debate is effectively over. The Stern review means that the economic debate is all but over. Only the political debate is left: and Mr Brown and David Miliband, Environment Secretary, will not encounter much opposition if they decide to make fundamental changes.

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