350James Hansen, NASA's chief climate scientist and the first to warn about global warming more than two decades ago, recently wrote: If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted,

http://crnano.typepad.com/crnblog/2008/12/350-or-bust.html

Target atmospheric CO2: Where should humanity aim?. J. Hansen (2008) arXiv [physics.ao-ph].
December 15, 2008