Target Atmospheric CO2: Where Should Humanity Aim?

James Hansen, Makiko Sato, Pushker Kharecha, David Beerling,
Valerie Masson-Delmotte, Mark Pagani, Maureen Raymo, Dana L. Royer,
James C. Zachos

Paleoclimate data show that climate sensitivity is ~3°C for doubled
CO2, including only fast feedback processes. Equilibrium sensitivity,
including slower surface albedo feedbacks, is ~6°C for doubled CO2 for
the range of climate states between glacial conditions and icefree
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 425±75 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.


Please see:

http://arxiv.org/pdf/0804.1126

For supporting materal see:

http://arxiv.org/pdf/0804.1135

For an article in the popular press, please see:

http://www.guardian.co.uk/environmen...arbonemissions