<div><a href="http://www.350.org/home">http://www.350.org/home</a></div>
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<div><a href="http://www.giss.nasa.gov/research/briefs/hansen_13/">http://www.giss.nasa.gov/research/briefs/hansen_13/</a></div>
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<div><a href="http://arxiv.org/abs/0804.1126">http://arxiv.org/abs/0804.1126</a></div>
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<div>Target atmospheric CO2: Where should humanity aim?
<div class="authors"><span class="descriptor">Authors:</span> <a href="http://arxiv.org/find/physics/1/au:+Hansen_J/0/1/0/all/0/1">J. Hansen</a> (1 and 2), <a href="http://arxiv.org/find/physics/1/au:+Sato_M/0/1/0/all/0/1">M. Sato</a> (1 and 2), <a href="http://arxiv.org/find/physics/1/au:+Kharecha_P/0/1/0/all/0/1">P. Kharecha</a> (1 and 2), <a href="http://arxiv.org/find/physics/1/au:+Beerling_D/0/1/0/all/0/1">D. Beerling</a> (3), <a href="http://arxiv.org/find/physics/1/au:+Berner_R/0/1/0/all/0/1">R. Berner</a> (4), <a href="http://arxiv.org/find/physics/1/au:+Masson_Delmotte_V/0/1/0/all/0/1">V. Masson-Delmotte</a> (5), <a href="http://arxiv.org/find/physics/1/au:+Pagani_M/0/1/0/all/0/1">M. Pagani</a> (4), <a href="http://arxiv.org/find/physics/1/au:+Raymo_M/0/1/0/all/0/1">M. Raymo</a> (6), <a href="http://arxiv.org/find/physics/1/au:+Royer_D/0/1/0/all/0/1">D. L. Royer</a> (7), <a href="http://arxiv.org/find/physics/1/au:+Zachos_J/0/1/0/all/0/1">J. C. Zachos</a> (8) ((1) NASA GISS, (2) Columbia Univ. Earth Institute, (3) Univ. Sheffield, (4) Yale Univ., (5) LSCE/IPSL, (6) Boston Univ., (7) Wesleyan Univ., (8) Univ. California Santa Cruz)</div>
<div class="dateline">(Submitted on 7 Apr 2008 (<a href="http://arxiv.org/abs/0804.1126v1">v1</a>), last revised 15 Oct 2008 (this version, v3))</div>
<blockquote class="abstract"><span class="descriptor">Abstract:</span> 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, barring prompt policy changes. 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. 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. </blockquote>
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<td class="tablecell label">Comments: </td>
<td class="tablecell comments">20 pages, 7 figures; final version accepted by Open Atmospheric Science Journal; supporting article separately submitted to arXiv as "Target atmospheric CO2: Supporting material", arxiv:<a href="http://arxiv.org/abs/0804.1135">0804.1135</a></td>
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<td class="tablecell label">Subjects: </td>
<td class="tablecell subjects"><span class="primary-subject">Atmospheric and Oceanic Physics (physics.ao-ph)</span></td></tr>
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<td class="tablecell label">Journal reference: </td>
<td class="tablecell jref">Open Atmos. Sci. J. (2008), vol. 2, pp. 217-231</td></tr>
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<td class="tablecell label"><abbr title="Digital Object Identifier">DOI</abbr>: </td>
<td class="tablecell doi"><a href="http://dx.doi.org/10%2E2174/1874282300802010217">10.2174/1874282300802010217</a></td></tr>
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<td class="tablecell label">Cite as: </td>
<td class="tablecell arxivid"><a href="http://arxiv.org/abs/0804.1126v3">arXiv:0804.1126v3</a> [physics.ao-ph]</td></tr></tbody></table></div>
<div class="submission-history">
<h2>Submission history</h2>From: James Hansen [<a href="http://arxiv.org/auth/show-email/32472f58/0804.1126">view email</a>] <br><b><a href="http://arxiv.org/abs/0804.1126v1">[v1]</a></b> Mon, 7 Apr 2008 20:16:55 GMT (477kb)<br>
<b><a href="http://arxiv.org/abs/0804.1126v2">[v2]</a></b> Wed, 18 Jun 2008 21:38:01 GMT (454kb)<br><b>[v3]</b> Wed, 15 Oct 2008 21:51:55 GMT (550kb)<br> </div>
<div class="endorsers"><a href="http://arxiv.org/auth/show-endorsers/0804.1126">Which authors of this paper are endorsers?</a></div>
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<div class="endorsers">Vision2020 Post: Ted Moffett</div></div>