Abstract

Paleoclimate data show that climate sensitivity is ~3°C for doubled CO 2 , including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO 2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. 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 CO 2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO 2 forcings. An initial 350 ppm CO 2 target may be achievable by phasing out coal use except where CO 2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO 2 is not brief, there is a possibility of seeding irreversible catastrophic effects.