title: Climate sensitivity, sea level and atmospheric carbon dioxide.
abstract: Cenozoic temperature, sea level and CO2 covariations provide insights into climate sensitivity to external forcings and sea-level sensitivity to climate change. Climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Pleistocene climate oscillations yield a fast-feedback climate sensitivity of 3±1(°)C for a 4 W m(-2) CO2 forcing if Holocene warming relative to the Last Glacial Maximum (LGM) is used as calibration, but the error (uncertainty) is substantial and partly subjective because of poorly defined LGM global temperature and possible human influences in the Holocene. Glacial-to-interglacial climate change leading to the prior (Eemian) interglacial is less ambiguous and implies a sensitivity in the upper part of the above range, i.e. 3-4(°)C for a 4 W m(-2) CO2 forcing. Slow feedbacks, especially change of ice sheet size and atmospheric CO2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered. Ice sheet response time is poorly defined, but we show that the slow response and hysteresis in prevailing ice sheet models are exaggerated. We use a global model, simplified to essential processes, to investigate state dependence of climate sensitivity, finding an increased sensitivity towards warmer climates, as low cloud cover is diminished and increased water vapour elevates the tropopause. Burning all fossil fuels, we conclude, would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.
authors.affiliation: The Earth Institute, Columbia University, , New York, NY 10027, USA.
authors.forename: James
authors.initials: J
authors.lastname: Hansen
authors.forename: Makiko
authors.initials: M
authors.lastname: Sato
authors.forename: Gary
authors.initials: G
authors.lastname: Russell
authors.forename: Pushker
authors.initials: P
authors.lastname: Kharecha
keywords: climate; climate sensitivity; palaeoclimate; sea level
references.citation: Philos Trans A Math Phys Eng Sci. 2010 May 28;368(1919):2395-415
references.pmid: 20403834
references.citation: Philos Trans A Math Phys Eng Sci. 2012 Sep 13;370(1974):4197-216
references.pmid: 22869797
references.citation: Glob Health Action. 2009 Nov 11;2:
references.pmid: 20052422
references.citation: Science. 2007 Aug 10;317(5839):793-6
references.pmid: 17615306
references.citation: Nature. 2009 Dec 17;462(7275):856-7
references.pmid: 20016585
references.citation: Science. 2012 Aug 17;337(6096):835-9
references.pmid: 22837384
references.citation: Nature. 2012 Nov 29;491(7426):683-91
references.pmid: 23192145
references.citation: Nature. 2009 Dec 17;462(7275):863-7
references.pmid: 20016591
references.citation: Nature. 2007 Dec 20;450(7173):1218-21
references.pmid: 18097406
references.citation: Nature. 2008 May 15;453(7193):383-6
references.pmid: 18480822
references.citation: Science. 2006 Apr 21;312(5772):428-30
references.pmid: 16627741
references.citation: Icarus. 1988;74:472-94
references.pmid: 11538226
references.citation: Nature. 2005 Jun 23;435(7045):1083-7
references.pmid: 15944716
references.citation: Science. 2011 Dec 9;334(6061):1385-8
references.pmid: 22116027
references.citation: Int J Hyperthermia. 2003 May-Jun;19(3):267-94
references.pmid: 12745972
references.citation: Proc Natl Acad Sci U S A. 2010 May 25;107(21):9483-4
references.pmid: 20483994
references.citation: Nature. 2012 Mar 14;483(7390):453-6
references.pmid: 22419155
references.citation: Geobiology. 2012 Jul;10(4):298-310
references.pmid: 22353368
references.citation: Science. 1982 Jul 23;217(4557):350-3
references.pmid: 17791515
references.citation: Science. 2001 Apr 27;292(5517):686-93
references.pmid: 11326091
references.citation: Philos Trans A Math Phys Eng Sci. 2007 Jul 15;365(1856):1925-54
references.pmid: 17513270
references.citation: Philos Trans A Math Phys Eng Sci. 2009 Jan 13;367(1886):157-68
references.pmid: 18854304
references.citation: J Chem Soc. 1947 Apr;:562-81
references.pmid: 20249764
references.citation: Proc Natl Acad Sci U S A. 2009 Oct 20;106(42):17655-60
references.pmid: 19805060
references.citation: Nature. 2012 Nov 22;491(7425):586-9
references.pmid: 23086145
references.citation: Proc Natl Acad Sci U S A. 2008 Oct 21;105(42):16065-70
references.pmid: 18809910
references.citation: Science. 2006 Mar 24;311(5768):1747-50
references.pmid: 16556837
references.citation: Science. 2000 Jan 14;287(5451):269-72
references.pmid: 10634774
references.citation: Proc Natl Acad Sci U S A. 2010 May 25;107(21):9552-5
references.pmid: 20439769
references.citation: Nature. 2008 Jan 17;451(7176):279-83
references.pmid: 18202643
references.citation: Proc Natl Acad Sci U S A. 2011 Jun 14;108(24):9770-5
references.pmid: 21628580
references.citation: Nature. 2005 Sep 1;437(7055):125-8
references.pmid: 16136140
references.citation: Science. 1982 May 7;216(4546):630-3
references.pmid: 17783310
references.citation: Science. 2005 Nov 25;310(5752):1317-21
references.pmid: 16311333
references.citation: Science. 1976 Dec 10;194(4270):1121-32
references.pmid: 17790893
references.citation: Nature. 2008 May 15;453(7193):379-82
references.pmid: 18480821
references.citation: Proc Natl Acad Sci U S A. 2000 Aug 29;97(18):9875-80
references.pmid: 10944197
references.citation: Nature. 2009 Oct 8;461(7265):776-9
references.pmid: 19812670
references.citation: Nature. 2012 Apr 04;484(7392):87-91
references.pmid: 22481362