Educated Guess

Jim Hansen writes Science magazine about just how much he thinks the world can take. The abstract:

Paleoclimate data show that climate sensitivity is ~3^oC for doubled CO₂, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6^oC 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 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.

Full submitted paper: 'Target CO₂: Where should humanity aim?', document, [.pdf]
Supplementary data. [.pdf]

No kids, it's not standard operating proceedure for full professors of astrophysics to release submitted papers in press at Science to the public. But it's not standard operating procedure for Dear Leader to try to censor scientists and their findings, either.