The easier of Adair Turner’s two main challenges is deciding how to save the entire UK banking system in his role as chairman of the FSA. As head of the Climate Change Committee, the more difficult job is persuading the government that it needs to take substantial action over greenhouse gases.

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Widespread relief greeted the publication of the Committee’s view that the UK should legislate to reduce emissions by 80% by 2050. Less attention has been paid to some of the stranger assumptions in the Committee’s letter to Ed Miliband, the cabinet member now responsible for climate change. While climate scientists have generally been getting more and more pessimistic, Turner’s figures convey a much more upbeat and cheerful tone.

Turner argues that the world needs to stop greenhouse gas concentrations rising above about 460 parts per million. (They are about 430 today and rising at over 2 ppm a year.) Two years ago, Lord Stern suggested that global emissions would need to fall to about 13 billion tonnes a year by 2050 for the world to stabilise at the figure of 450 ppm. Turner is far more optimistic, saying that we can cope with 20 billion tonnes and still keep to about this level. In other words, Stern’s maths was wrong by a large percentage – the world can emit about 50% more than his report indicated.

This divergence is followed by a much more significant variation. One of the charts in Stern’s first presentation indicates what would happen to temperature at various different levels of greenhouse gases. He says that a level of 450 ppm will produce an expected temperature rise of about 2.5 degrees Celsius. Once again, Turner is more buoyant than this, saying that his target for greenhouse gas concentrations will hold temperature rises to 2 degrees. He is also far more confident than Stern appeared to be about the risk of runaway global warming.

These variations are highly significant but Turner gives us no explanation of why he is more upbeat than other analysts. By contrast, since the publication of his report, Stern seems to have become more and more worried by climate change. And many others, like Jim Hansen of NASA, have suggested aiming for far lower greenhouse gas concentrations in the atmosphere. Turner has gone in the other direction, making the problems of global warming seem much less onerous. On the basis of Stern’s numbers, he should have asked for a 90% cut in emissions, not the relaxed 80% he proposes.

He has also suggested that the costs of reducing our emissions are trivial. His committee indicates it will be 1-2% of GDP in 2050. This number is so small and so far away that nobody will take much notice. This is a trifle disingenuous. The real costs to cutting emissions are going to be incurred in the next fifteen years as we switch from convenient and inexpensive fossil fuels to more costly sources of power. Energy costs represent about 5% of the economy now, and this may rise sharply for a few years as we go through the early pain of what Turner calls ‘decarbonisation’. Perhaps they will double for a few years. But by 2050, we can actually be reasonably confident that energy will actually cost less than it does now. Decades of going down the learning curve will give us the same sharp improvements in technology that we saw at the beginning of the coal and oil ages.

Adair Turner may look and talk like a boffin, but not far below the surface is an acutely political animal. Has he deliberately made climate change a less threatening and cheaper problem to deal with than Stern did? The last few years have seen an unending succession of bad news stories on climate change. It all seems too awful to contemplate. Perhaps he feels that by giving us a radically more optimistic assessment he is increasing the chances of getting action from politicians. He may be right.