Target atmospheric CO2: Where should humanity aim?

@article{Hansen2008TargetAC,
  title={Target atmospheric CO2: Where should humanity aim?},
  author={Jennifer C. R. Hansen and M. Sato and Pushker A. Kharecha and David J. Beerling and Robert. A. Berner and Val{\'e}rie Masson‐Delmotte and Massimo Pagani and M. E. Raymo and Dana L. Royer and J. C. Zachos Nasa Giss and Columbia Univ. Earth Institute and University of Sheffield and Yale Univ. and Lsceipsl and Boston Univ. and Wesleyan Univ. and Univ. California Santa Cruz},
  journal={The Open Atmospheric Science Journal},
  year={2008},
  volume={2},
  pages={217-231}
}
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… 
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