Professor Hans Joachim Schellnhuber CBE is a theoretical physicist, Director of the Potsdam Institute for Climate Impact Research, the former Chief German Government Advisor on Climate and Related Issues during Germany's EU Council Presidency and G8 Presidency and a forme lead author on theIPCC (see Wikipedia: http://en.wikipedia.org/wiki/Hans_Joachim_Schellnhuber ).
Professor Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research., Germany (2008): “It is a compromise between ambition and feasibility. A rise of 2oC could avoid some of the big environmental disasters, but it is still only a compromise…It is a very sweeping argument, but nobody can say for sure that 330ppm is safe. Perhaps it will not matter whether we have 270ppm or 320ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms" [280 ppm is the pre-industrial atmospheric CO2 concentration]. [1].
[1]. Professor Hans Joachim Schellnhuber quoted by David Adam, “Roll back time t safeguard climate, expert warns”, Guardian, 15 Septemebr 2008 : http://www.guardian.co.uk/environment/2008/sep/15/climatechange.carbonemissions .