Target Atmospheric CO2: Where Should Humanity Aim?

James E. Hansen
NASA Goddard Institute for Space Studies

James E. Hansen


2008 Joint Annual Meeting

Sponsored by The Geological Society of America
George R. Brown Convention Center, General Assembly Theater Hall B
Tuesday, 7 October 2008: noon–1:15 p.m.


Paleoclimate change implies 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, large scale glaciation occurring when CO2 fell to 450 ± 100 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, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced to at most 350 ppm, probably less. An initial 350 ppm CO2 target could be achieved by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.


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