  Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2,
including only fast feedback processes. Equilibrium sensitivity, including
slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of
climate states between glacial conditions and ice-free Antarctica. Decreasing
CO2 was the main cause of a cooling trend that began 50 million years ago,
large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that
will be exceeded within decades, barring prompt policy changes. 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.
