Target atmospheric CO2: Where should humanity aim? - by James Hansen et al.
Last edited Tue Dec 20, 2016, 08:18 PM - Edit history (1)
This is the origin of the goal of 350 ppm.
http://dx.doi.org/10.2174/1874282300802010217
[font face=Serif][center][font size=1]The Open Atmospheric Science Journal, 2008, 2, 217-231[/font][/center][br][hr][font size=5]Target Atmospheric CO₂: Where Should Humanity Aim?[/font]
[font size=4]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[/font]
[font size=2]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[/font]
[font size=1]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.[/font]
[font size=2]Keywords: Climate change, climate sensitivity, global warming.[/font]
[font size=3]1. INTRODUCTION
Human activities are altering Earth’s atmospheric composition. 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 estimate that global warming of more than 2-3°C may be dangerous. 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 sheet and species loss. This 1°C limit, with nominal climate sensitivity of ¾°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 processes 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|.
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