Brown asked Stern to perform a similar analysis to push forward the international effort on global warming. "Not to look for consensus, but to establish some of the basic issues and conclusions."
Chief among those is that a failure to cut emissions will be devastating. Critics such as Richard Tol of the Economic and Social Research Institute in Dublin complain that the report's conclusions are "alarmist and incompetent" and argue that it should be seen as a political document, not an impartial assessment of existing knowledge.
Stern says: "It's about providing an analytical basis for policy, while at the same time trying to be specific about policy. It's not a political document in the sense of making a narrow party case or a narrow case for a particular position. It's false to suggest there was any view on the way this should come out at the beginning."
Critics have focused on the way the report treats future generations, which will be most affected by decisions we make today because it takes time for the heat trapped by our carbon emissions to build up. Much of what is going to happen over the next 30 or 40 years is already determined.
Swords Drawn
So the policy decisions we take now in terms of reducing emissions will have implications in 50, 100 or 150 years. Stern believes the damage future generations will suffer at our hands is greater than generally perceived. Other economists, including Nordhaus, disagreed and an academic bun fight ensued.
Stern says: "Many other economists have taken [our] position so we're not peculiar in this at all." Some climate change skeptics tried to portray these disagreements as undermining the case for action. In fact, even the most strident of his academic tormentors also stressed the need to reduce emissions.
Stern says his team also took a different approach in the way they treated the scientific evidence. Rather than just working with the most likely scenarios, they took into account the smaller chances of far more severe events unfolding. And those events tend to be the most expensive. "You take into account the different probabilities and what kind of damages could follow," he says. "And at each stage where you've got a 'could,' you've got a probability distribution. So you have to build that into the story. We're starting to be able to do that."
One example is something called climate sensitivity — a measure of how much the atmosphere will warm. Most climate scientists assume that, if the concentration of carbon dioxide reaches double what it was before the industrial revolution, we would see an average rise of 3oC. But some have claimed the climate is much more sensitive, and temperatures under those circumstances could increase by 6oC, perhaps more. It's a small chance, but Stern has considered it.
He has also considered studies that warn of positive feedbacks in the climate system, where increasing temperatures force more greenhouse gases from natural sources such as the oceans and soils into the atmosphere. These processes are poorly understood but could accelerate global warming.
He calls this approach the economics of risk that, he says, "puts greater weight on downsides than upsides, because people are averse to risks."
Stern's legacy seems straightforward. If last year proves to be the year that the debate on global warming decisively shifted from whether it was real to what we should do about it, then future generations will give thanks. If not, his review will stand as a symbol of our folly.
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