The Open Atmospheric Science Journal, 2008, 2, 217-231 217 1874-2823/08 2008 Bentham Open Open Access Target Atmospheric CO2: Where Should Humanity Aim? James Hansen*,1,2, Makiko Sato1,2, Pushker Kharecha1,2, David Beerling3, Robert Berner4, Valerie Masson-Delmotte5, Mark Pagani4, Maureen Raymo6, Dana L. Royer7 and James C. Zachos8 1NASA/Goddard Institute for Space Studies, New York, NY 10025, USA 2Columbia University Earth Institute, New York, NY 10027, USA 3Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK 4Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA 5Lab. 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 6Department of Earth Sciences, Boston University, Boston, MA 02215, USA 7Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459-0139, USA 8Earth & Planetary Sciences Dept., University of California, Santa Cruz, Santa Cruz, CA 95064, USA Abstract: Paleoclimate data show that climate sensitivity is ~3°C for doubled CO2, including only fast feedback proc-esses. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO2 for the range of cli-mate 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 ir-reversible catastrophic effects. Keywords: Climate change, climate sensitivity, global warming. 1. INTRODUCTION Human activities are altering Earth’s atmospheric com-position. Concern about global warming due to long-lived human-made greenhouse gases (GHGs) led to the United Nations Framework Convention on Climate Change [1] with the objective of stabilizing GHGs in the atmosphere at a level preventing “dangerous anthropogenic interference with the climate system.” The Intergovernmental Panel on Climate Change [IPCC, [2]] and others [3] used several “reasons for concern” to es-timate that global warming of more than 2-3°C may be dan-gerous. The European Union adopted 2°C above pre-industrial global temperature as a goal to limit human-made warming [4]. Hansen et al. [5] argued for a limit of 1°C global warming (relative to 2000, 1.7°C relative to pre-industrial time), aiming to avoid practically irreversible ice *Address correspondence to this author at the NASA/Goddard Institute for Space Studies, New York, NY 10025, USA; E-mail: [email protected] sheet and species loss. This 1°C limit, with nominal climate sensitivity of °C per W/m2 and plausible control of other GHGs [6], implies maximum CO2 ~ 450 ppm [5]. Our current analysis suggests that humanity must aim for an even lower level of GHGs. Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback proc-esses not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less [7]. Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is ‘in the pipe-line’ [8], add urgency to investigation of the dangerous level of GHGs. A probabilistic analysis [9] concluded that the long-term CO2 limit is in the range 300-500 ppm for 25 percent risk tolerance, depending on climate sensitivity and non-CO2 forcings. Stabilizing atmospheric CO2 and climate requires that net CO2 emissions approach zero, because of the long lifetime of CO2 [10, 11].218 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al. We use paleoclimate data to show that long-term climate has high sensitivity to climate forcings and that the present global mean CO2, 385 ppm, is already in the dangerous zone. Despite rapid current CO2 growth, ~2 ppm/year, we show that it is conceivable to reduce CO2 this century to less than the current amount, but only via prompt policy changes. 1.1. Climate Sensitivity A global climate forcing, measured in W/m2 averaged over the planet, is an imposed perturbation of the planet’s energy balance. Increase of solar irradiance (So) by 2% and doubling of atmospheric CO2 are each forcings of about 4 W/m2 [12]. Charney [13] defined an idealized climate sensitivity problem, asking how much global surface temperature would increase if atmospheric CO2 were instantly doubled, assum-ing that slowly-changing planetary surface conditions, such as ice sheets and forest cover, were fixed. Long-lived GHGs, except for the specified CO2 change, were also fixed, not responding to climate change. The Charney problem thus provides a measure of climate sensitivity including only the effect of ‘fast’ feedback processes, such as changes of water vapor, clouds and sea ice. Classification of climate change mechanisms into fast and slow feedbacks is useful, even though time scales of these changes may overlap. We include as fast feedbacks aerosol changes, e.g., of desert dust and marine dimethylsul-fide, that occur in response to climate change [7]. Charney [13] used climate models to estimate fast-feedback doubled CO2 sensitivity of 3 ± 1.5°C. Water vapor increase and sea ice decrease in response to global warming were both found to be strong positive feedbacks, amplifying the surface temperature response. Climate models in the cur-rent IPCC [2] assessment still agree with Charney’s estimate. Climate models alone are unable to define climate sensi-tivity more precisely, because it is