Lord Lipsey, chairman of the Parliamentary Group on Statistics, has fired a shot across the bows of Damian Carrington, accusing him of making wild claims about the climate without citing any sources:
In what sense is 400 parts a million a milestone and what does the word add, save opinion, to the intro? By whom are these conditions 'expected to return in time' and who says the consequences will be 'devastating'? By whom is catastrophic warming 'thought to be unstoppable' at 2 degrees (and does this mean that at 1.99 degrees it is stoppable)? …it may well be that everything in this piece is true. But it is not enough for the reporter to assert that on his own authority without citing sources.
Carrington's response is essentially to argue that "everyone says so",
the entire world's governments, via the UN Framework Convention on Climate Change, agree that 2C is the safe limit."
Carrington's problem is that the UNFCCC document linked only asserts that temperature rises above 2°C would be dangerous. There is no citation. Fortunately a more complete record of the target is to be found in this recent paper from the Potsdam Institute, which notes that the 2°C target can be viewed in many ways, few of which are truly satisfactory. The authors conclude that the target actually has more to do with politics than science.
It seems that for decades European politicians—and more recently many of their partners from all over the world—have tried to orient their decisions on a guideline they perceived as expressing a scientific view, while scientists—who did introduce the 2° target into the climate debate—treat that guideline as a political issue.
I think Lord Lipsey had it right.