NICHOLAS STERN , Can he SAVE the world

Middle-aged, soberly suited, grey and compact, he speaks softly in the fastidious register of academia, comprised of paragraphs constructed almost entirely out of words such as "policy framework", "costs and benefits", "transparency of governance" and so on. Yet, when he speaks, the whole world now listens.

Since publishing the Stern Review in 2006, the professor has become the global authority on climate change. Commissioned by Gordon Brown, his study of the economics of climate change shifted the debate away from polar bears and unseasonal summers, and reframed it in the cold hard language of the balance sheet. Unless we invested 1% of global GDP per annum in measures to prevent climate change, the review warned, it would cost us 20% of global GDP.

Suddenly, the CBI and the Institute of Directors were paying attention. It was a defining moment for the credibility of a movement once belittled as too counter-culture to be taken seriously. Stern became the grey hero of the greens - powerful precisely because he seemed such an improbable eco warrior.

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Since publishing the Stern Review in 2006, the professor has become the global authority on climate change. Commissioned by Gordon Brown, his study of the economics of climate change shifted the debate away from polar bears and unseasonal summers, and reframed it in the cold hard language of the balance sheet. Unless we invested 1% of global GDP per annum in measures to prevent climate change, the review warned, it would cost us 20% of global GDP. Suddenly, the CBI and the Institute of Directors were paying attention.

It was a defining moment for the credibility of a movement once belittled as too counter-culture to be taken seriously. Stern became the grey hero of the greens - powerful precisely because he seemed such an improbable eco warrior.Since then Stern has returned from the Treasury to the London School of Economics, been made a life peer, and is now about to publish a book - A Blueprint For a Safer Planet.

Guided by three principles of effectiveness, efficiency and fairness, it calls for an investment of closer to 2% of GDP, with rich countries leading the way in emissions reductions.

Proposing green technologies, international emissions trading, and financing to halt deforestation, it lays out the terms by which he believes we can avert catastrophe - and as such is fundamentally hopeful.But Stern navigates a delicate path between optimism and Armageddon, and at a recent climate change conference he was still exhorting world leaders to grasp the magnitude of the crisis.

"Do the politicians understand just how difficult it could be?" he appealed. "Just how devastating [a rise of] four, five, six degrees centigrade would be? I think not yet." With hindsight, he says he fears that even his own review underestimated the risks we face.

When it came out, people thought I'd over- egged the omelette. But all the things people were looking at turned out to be worse than they thought. Doing nothing looks even more reckless than it did even a few years ago." He pauses, as if uneasy with such an intemperate word, but keeps going. "Recklessness is the only word. I mean, we have to recognise the scale of the risk. If we go on at anything like business as usual, we'll be at concentration levels by the end of this century which will give us around a 50-50 chance of being above five degrees centigrade relative to, say, the 19th century. We humans are only 100,000 years old. We haven't seen that for 30 to 50 million years. We haven't seen three degrees centigrade for three million years. The idea that humans can easily adapt to conditions like these ..." He lets the proposition tail away, too foolish even for words."

What will we do? We'll move. People will move. Why? Because much of southern Europe will be desert. Other places will become underwater. Others will be hit by such severe storms with such frequency that they become almost uninhabitable.

So hundreds of millions of people will move. You're already seeing people moving in Darfur, where droughts devastated the grazing land of pastoralist people, and they moved, and come into conflict with people in the places they're moving to.

We're seeing that already on just a 0.8 degree rise. We're the first generation that has the power to destroy the planet. You're re-writing the planet. So you can only describe as reckless ignoring risks like that."At the heart of Stern's work is a simple calculation.

If the science on climate change is right, the transition costs incurred by switching to a low-carbon economy will - however daunting - be a fraction of what we will save by averting disaster. If the science is wrong, and we incur those costs unnecessarily, they would be "very far from disastrous", and we would still benefit, "because we will have a world that is more energy efficient, with new and cleaner technologies, and is more biodiverse as a result of protecting the forests".

The logic of the argument is compelling, but is there any part of Stern that believes the science could be wrong?

"As an undergraduate, I did maths and physics. That doesn't make me a scientist," Stern responds, with exaggerated patience. "So I try to read and understand and talk to scientists. I'm staggered by how many people who are lawyers, or politicians ..." Or former chancellors? "For example," he agrees drily. "Taxi drivers. People behind bars. People cutting hair. They all seem to be knowledgeable and expert on the science."In public policy we have to understand a little bit about nuclear physics, and biochemistry, and genetics. So what do you do if you want to understand about genetics? You talk to a geneticist. You don't turn to taxi drivers or politicians. Both respectable professions, but you don't go to them for the science of climate change, you go to scientists. And what do you hear? That this is basically simple physics. It's not as if it's something strange or mysterious that people can't explain to you. It's not something outside the experimental. The greenhouse effect is something you can observe experimentally - and most people have observed the greenhouse effect themselves, in greenhouses. Yes?"

Ultimately, he points out, the choice is quite simple: however difficult the challenge of action may be, the alternative is unthinkable. "If you say I'm not going to do that, what's left? What's left is you just reach for the suntan lotion and the big hat, and you say it's all too difficult, I'm signing off on this, and let's all fry. Why would you want to go there?" It seems more or less unimaginable to Stern that people would be stupid enough to allow a catastrophe to unfold, and his ultimate message is one of optimism."There are so many ideas out there, the pace of technological progress is so fast. It's a very optimistic thing about human nature; when humans focus on a problem, they're quite ingenious. And we have to recognise that this subject is young. It's only been deep in our understanding for two or three years. The scientists, of course, have been thinking about this for a long time, but in terms of politics and policy it's been big only for two or three years. I think if you look at it in that context, let's recognise what the government has done. We're ahead of the world on climate change legislation. I think the climate change bill is broadly along the right lines. If you ask yourself the question, 'How far have we got?', we've got a long way. It has to be faster, but let's not fail to recognise how far we've come."

Not even the world recession has dampened his optimism. "

This recession is seen as something that would prevent action on climate change only if we confuse ourselves.

If we think clearly, this is an opportunity to bring forward some of those investments, because resources are a bit cheaper at the moment. I've been struck that this climate change story has stayed very much on the agenda, the way that the green stimulus has been seen as part of the expansion package. In the next two or three decades, I think low-carbon technologies are going to be like the railways or IT - big drivers of growth.

"Stern won't live long enough to know if the world takes his advice. But if he had to go to the bookies now and place a £1,000 bet on whether we'll manage to do what's necessary in time, which way would he put it? "I would bet," he says cautiously, "we'll get there. But as in any bet, there's a risk of being wrong".'There are many half-baked attempts to naysay the science, but they always unravel on inspection'

by STEPHANIE LASCOMBES