The Open Atmospheric Science Journal has published a new article by Jim Hansen and a distinguished group of climate scientists, Target Atmospheric CO2: Where Should Humanity Aim? in its latest issue. The PDF version is a free download.
The abstract reads:
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 ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 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 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.
That is about as clear a statement as we can get that we are already across a dividing line between planetary survival and oblivion. Consult your Hitchhiker's Guide under the first rule of holes. Don't Panic.