The final version of this paper from James Hansen and associates is now out:
The final version of "Target Atmospheric CO2: Where Should Humanity Aim?" in The Open Atmospheric Science Journal is available at http://dx.doi.org/10.2174/1874282300802010217
You can click on the main paper and supporting material individually. The two are combined in one pdf on the GISS web site at http://pubs.giss.nasa.gov/abstracts/2008/Hansen_et...
BTW, I think that the Supporting Material contains some interesting stuff.
NASA decided not to make a press release for the paper, but Yale did one http://www.eurekalert.org/pub_releases/2008-11/yu-... The draft press release that I wrote and "Q&A" about the paper are at http://www.columbia.edu/~jeh1/mailings/20081030_Ta...
http://www.columbia.edu/%7Ejeh1/mailings/20081114_...
I'll try to read the actual paper later tonight... The abstract:
Abstract: 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.[/quote]