As you all know I'm boringly mainstream in my views over climate change. The scientists tell us that we've got to do something, the economists that that something is a carbon tax so I say, great, let's have a carbon tax. And then we get information that rather changes this so far sterile debate:

It puts the overall cost at less than 2% of GDP for a 2.5 degrees Centigrade (or 4.5 degrees Fahrenheit) temperature increase during this century. This is vastly less than the much heralded prediction of Lord Stern, who said climate change would cost 5%-20% of world GDP in his influential 2006 report for the British government.

Here's what Stern did to reach his figure, the one that leads to that $80 a tonne carbon tax. He took the worst possible economic forecast of the next century, the one leading to the highest emissions path. Then he made that worse with a couple of other assumptions. Then he invented (on this, possibly rightly) a new method of calculating net present values. And he assumed thatt here was high sensitivity of temperature to emissions.

However, the essential heart of his argument was correct. We don't want to undertake actions that are more costly now that the damage they save in the future. The limit to our attempts to prevent climate change, and yes he does lay this out, must be the scale of that future damage. To spend more now than that damage then would be nonsensical.

Which is where the new numbers coming from the IPCC come in. We now think that climate senstivity is lower than Stern assumed. Thus the actions that we should take to deter future costs must cost less now. That is, we should rationally be doing less now than we were before.

And the thing about this finding is that it is the boringly mainstream finding too. Which is interesting really, because there seem to be only a handful of people (Matt Ridley and myself being among them, he shouting it from a much taller soapbox of ourse) who even grasp this point, let alone actively promote it.