Target Atmospheric CO: Where Should Humanity Aim?

James Hansen1, 2, *, Makiko Sato1, 2, Pushker Kharecha1, 2, David Beerling3, Robert Berner4, Valerie Masson-Delmotte5, Mark Pagani4, Maureen Raymo6, Dana L. Royer7, James C. Zachos8
1 NASA/Goddard Institute for Space Studies, New York, NY 10025, USA
2 Columbia University Earth Institute, New York, NY 10027, USA
3 Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
4 Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109,
5 Lab. Des Sciences du Climat et l’Environnement/Institut Pierre Simon Laplace, CEA-CNRS-Universite de Versailles Saint-Quentin en Yvelines, CE Saclay, 91191, Gif-sur-Yvette, France
6 Department of Earth Sciences, Boston University, Boston, MA 02215, USA
7 Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459-0139, USA
8 Earth & Planetary Sciences Dept., University of California, Santa Cruz, Santa Cruz, CA 95064, USA

© 2008 Hansen et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the NASA/Goddard Institute for Space Studies, New York, NY 10025, USA; E-mail:


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 CO was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO 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.

Keywords: Climate change, climate sensitivity, global warming.