Target Atmospheric CO2: Where Should Humanity Aim?

Paleoclimate d ata show that climate sensitivity is ~3°C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, i s ~6°C for doubled CO 2 fo r the range o f cl imate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 m illion yea rs a go, the pl anet bei ng ne arly i ce-free unt il CO 2 f ell to 450 ± 100 ppm ; ba rring pr ompt pol icy changes, th at c ritical l evel w ill b e p assed, in th e opposite di rection, w ithin de cades. If hum anity w ishes t o pr eserve a planet similar to that on w hich civilization developed and to which life on Ea rth is adapted, p aleoclimate 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. 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.