The bloke who called the 2degC limit was an economist, not climate scientist, Lord Nicholas Stern. When told this meant 450ppm CO2 limit, he proclaimed that world could not afford it, so 550ppm was his first pass. 12 months later, he retracted the lot, apologising that consequences of anything more than 350ppm were too extreme. Article(s) per link are James Hansen and colleagues, trying again, to get us to recognise what we risk.

>readmore> ThinkProgress