If someone had told me, just six months ago, that the UK government would be sponsoring a major report which - in Tony Blair's words - "demolishes the last remaining argument for inaction on climate change", I would have refused to believe it. Yet, so it came to pass with the publication of the Stern report. In accepting the report's findings, the government achieved several things at once. First, it regained ground lost to the Tories and countered David Cameron's dog-sled photo opportunity in the Arctic. Second, it suggested that Gordon Brown - as the man who commissioned Lord Stern and who introduced several proposals at the report's launch - would be a prime minister who "gets it" on climate.
Last, and most usefully, the government has helped environmentalists tear down the last bastion of climate-change denial: economics. For several years the Danish statistician Bjørn Lomborg has generated unwarranted press attention with his gatherings of famous economists who have declared that climate change is too expensive to tackle and that we would be better off spending the money elsewhere.
Now, Nicholas Stern has ridden out in full armour and slain the Lomborgian dragon. His 700-page report entirely demolishes that last ditch of climate-change denial - that the world "cannot afford" to cut fossil-fuel emissions. Stern points out that the potential cost comes in at roughly 1 per cent of global GDP, a tiny price to pay for averting the greatest crisis ever to face human civilisation.
This in itself is cause for celebration: after years of deliberation and the submission of thousands of pages of evidence, Stern has come to essentially the same conclusion as Friends of the Earth and Greenpeace: that "we can grow and be green". And this draws the sting from objections made by renegade rich countries such as Australia and the United States, whose governments persist in claiming, absurdly, that even making tiny Kyoto-style cuts would bankrupt them. I have some objections to a purely economic approach to climate change. How do you assign a monetary value to the disappearance of the polar bear, for example? How do you put a price on people losing their livelihoods, and their lives, to droughts, floods and food shortages? But the forthright approach of the Stern report makes it, in many ways, more radical than numerous papers put out recently by environmental groups.
Stern also implicitly throws down a challenge to the British government with regard to its own policies, a challenge that the government must respond to urgently if it is to avoid charges of hypocrisy. First, it should make the bold stroke of cancelling the entire £12bn roads programme: the last thing we need now is to be spending billions on catering for motorists when pitiful amounts of cash are being spent on helping households use less energy and generate their own power. Next, as I argued two weeks ago in these pages, David Mili band must embrace carbon rationing with enthusiasm and announce a strategy for national implementation.
Crucially, the government must commit itself to a clearer framework for the next stage of international negotiations: as Stern points out, the UK's carbon emissions are only 2 per cent of the global total and an equitable agreement based on contraction and convergence is the only way to bring in China, India and other developing nations.
Nicholas Stern's report could be a step change in British politics. As always, there is a catch, and it is a grave one: Stern's 1 per cent price tag would not actually save us from the worst effects of global warming. Specifically, his figure refers to the estimated cost of stabilising atmospheric greenhouse-gas levels at 500-550 parts per million (ppm). We are currently at 430ppm. Stabilising in the 500-550ppm range would require global emissions to peak in the next ten to 20 years, and then fall by between 1 and 3 per cent a year.
However, the European Union and many environmental groups believe that global warming must never exceed 2°C. That danger threshold, according to the latest science, is the line beyond which global warming could run rapidly out of control because of "positive feedbacks" - the collapse of the Amazon rainforest or the release of methane from melting permafrost in Siberia. But, according to Stern's own figures, his 550ppm stabilisation target gives us only a 10 per cent chance of keeping temperature increases below 2°, and a 50-50 chance of passing 3°. This is rather like playing Russian roulette with four out of the five chambers loaded - the odds are not just silly, but suicidal. Stern dismisses the target of 450ppm, which is much more likely to keep us within the magic 2° threshold, as "almost out of reach, given that we are likely to reach this level within ten years".
Yet what choice do we have but to grasp at this straw? It might cost much more than 1 per cent of GDP, but the earth does not strike bargains: we have to reach the targets that are set by the planet, or else go out of business. Two degrees is that target, and yes, we have less than ten years to act.