James Hansen

What’s the optimum level of atmospheric CO2 for humanity and nature? It’s less than the amount in the air today and must be taken back:

If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that.

-- Dr. James Hansen et al, "Target Atmospheric CO2: Where Should Humanity Aim?"
The Open Atmospheric Science Journal, November 2008

The whole report is here. It’s worth a read. Its authors -- NASA climate scientist James Hansen and nine other scientists from the US, UK and France -- make the scientific case that most of the remaining fossil fuel carbon on the planet must never be emitted to the atmosphere, if we are to preserve a climate to which humanity is accustomed.