Target atmospheric CO2: where should humanity aim?

Exactly 20 years have passed since Dr. James E. Hansen of NASA first testified to Congress on June 23, 1988 that global temperatures had risen beyond the range of natural variability. Waiting another 20 years before taking decisive action is not an option. Since 1988, mainstream scientific thinking has caught up with Dr. Hansen`s declaration that our climate is being adversely affected by human activities, primarily the burning of fossil fuels-and the forecasts of climate change in the coming decades are increasingly dire. Dr. Hansen revisits the issue in the latest issue of the Open Atmospheric Science Journal. 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. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects., in Open atmospheric science journal, June 2008