Hansen argues for CO2 level of 300 t0 350 ppm.

[cedric]

If humanity wants to preserve a climate resembling that in which civilisation developed, then the palaeoclimate evidence and ongoing climate change suggest CO2 must be reduced from its current level to between 300-350ppm. A 350ppm target is only achievable by phasing out coal use,” he says

http://www.timesonline.co.uk/tol/news/environment/article3341039.ece