The Open Atmospheric Science Journal, 2008, 2, 217-231 217 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 CO 2, including only fast feedback proc- esses. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO 2 fo r th e ran ge o f cl i- mate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 m illion years ago, the pl anet being nearly i ce-free unt il CO 2 fell to 450 ± 100 ppm ; barring prompt pol icy changes, th at c ritical l evel w ill b e p assed, in th e opposite direction, w ithin decades. If hum anity w ishes to preserve a planet similar to that on w hich civilization developed and to which life on Ea rth 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 sheet and species loss. This 1°C limit, with nominal climate sensitivity o f
°C per W/m2 and plausible control of ot her Human a ctivities a re a ltering Earth’s atmospheric com- GHGs [6], implies maximum CO ~ 450 ppm [5]. position. Concern a bout gl obal warming due t o l ong-lived 2 human-made gre enhouse ga ses (GHGs ) l ed t o t he Uni ted Our current analysis suggests that humanity must aim for Nations Framework Convention on Climate Change [1] with an even lower level of GHGs. Paleoclimate data and ongoing the obj ective of s tabilizing GHGs in t he a tmosphere at a global c hanges i ndicate t hat ‘s low’ c limate fe edback proc - level preventing “dangerous anthropogenic interference with esses not included in most climate models, such as ice sheet the climate system.” disintegration, vegetation migration, and GHG re lease from soils, t undra or oc ean s ediments, m ay be gin t o c ome into The Intergovernmental Panel on Cl imate Change [IPCC, play on ti me s cales as short as c enturies or l ess [7]. Rapid [2]] and others [3] used several “reasons for concern” to es- on-going climate changes and realization that Earth is out of timate that global warming of more than 2-3°C may be dan- energy balance, implying that more warming is ‘in the pipe- gerous. T he European Uni on adopted 2°C above pre - line’ [8], add urgency to investigation of the dangerous level industrial global temperature as a goal to limit human-made of GHGs. warming [4]. Hansen et al. [5] a rgued for a li mit of 1°C global w arming (re lative t o 2000, 1. 7°C re lative to pre - A probabilistic analysis [9] concluded that the long-term industrial time), a iming t o avoid practically i rreversible i ce CO2 l imit i s i n t he ra nge 300-500 ppm fo r 25 pe rcent ri sk tolerance, de pending on c limate s ensitivity a nd non-C O2

forcings. S tabilizing a tmospheric C O2 and climate r equires *Address correspondence to this author at the NASA/Goddard Institute for that n et CO2 e missions a pproach z ero, because of t he long Space Studies, New York, NY 10025, USA; E-mail: jhansen@giss.nasa.gov lifetime of CO2 [10, 11].

1874-2823/08 2008 Bentham Open 218 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al.

We use paleoclimate data to show that long-term climate Climate forcing in the LGM equilibrium s tate due to th e has high sensitivity to climate forcings and that the present ice ag e s urface p roperties, i .e., increased i ce area, d ifferent global mean CO2, 385 ppm, is already in the dangerous zone. vegetation distribution, and continental shelf exposure, was - 2 Despite rapid c urrent C O2 growt h, ~ 2 ppm /year, w e s how 3.5 ± 1 W/m [14] relative to the Holocene. Additional forc- that it is conceivable to reduce CO2 this century to less than ing due to reduced amounts of long-lived GHGs (CO2, CH 4, the current amount, but only via prompt policy changes. N2O), including the indirect effects of CH4 on t ropospheric S1 1.1. Climate Sensitivity ozone and stratospheric water vapor (Fig. ) was -3 ± 0. 5 W/m2. G lobal forc ing due t o s light c hanges i n t he E arth’s A gl obal c limate forc ing, m easured i n W /m2 av eraged orbit is a negligible fraction of 1 W/ m2 (Fig. S3). T he total over t he pla net, i s a n im posed p erturbation of t he pla net’s 6.5 W/m2 forcing and global surface temperature change of 5 energy balance. Increase of s olar irradiance (So) by 2% and ± 1° C re lative to t he Hol ocene [15, 16] yi eld a n e mpirical 2 doubling of a tmospheric CO2 a re each forc ings of about 4 sensitivity ~ ±
°C per W/m forcing, i.e., a Charney sen- 2 2 W/m [12]. sitivity of 3 ± 1 ° C for t he 4 W/ m forcing of doubled CO2. This empirical fast-feedback climate sensitivity allows water Charney [13] de fined an i dealized climate sensitivity vapor, clouds, aerosols, sea ice, and all other fast feedbacks problem, asking how much global surface temperature would that exist in the real world to respond naturally to global cli- increase if at mospheric CO were instantly doubled, assum- 2 mate change. ing that slowly-changing planetary surface conditions, such as ice sheets and forest cover, were fixed. Long-lived GHGs, Climate sensitivity va ries as Earth be comes w armer or except f or th e s pecified CO 2 c hange, we re also fi xed, not cooler. Toward colder extremes, as the area of sea ice grows, responding t o c limate c hange. The Charney proble m thus the p lanet approaches runa way s nowball-Earth c onditions, provides a measure of c limate sensitivity including only the and at high temperatures it c an approach a runaway green- effect of ‘fast’ feedback processes, such as changes of water house effect [12]. At its present temperature Earth is on a flat vapor, clouds and sea ice. portion of i ts fa st-feedback climate s ensitivity c urve (F ig. S2). T hus our e mpirical s ensitivity, a lthough s trictly t he Classification o f cl imate ch ange m echanisms in to f ast mean f ast-feedback s ensitivity f or climate s tates r anging and s low fe edbacks is us eful, e ven t hough t ime s cales of from th e ice ag e to th e cu rrent in terglacial p eriod, is a lso these ch anges m ay o verlap. We in clude as f ast f eedbacks today’s fast-feedback climate sensitivity. aerosol changes, e.g., of desert dust and marine dimethylsul- fide, that occur in response to climate change [7]. 2.1. Verification Charney [13] us ed climate m odels to e stimate fa st- Our e mpirical fa st-feedback c limate s ensitivity, de rived feedback doubled CO2 sensitivity of 3 ± 1. 5°C. Water vapor by c omparing c onditions a t two poi nts i n ti me, c an be increase and sea ice decrease in response to global warming checked ove r t he longer pe riod of ic e core da ta. F ig. ( 1a) were both found to be strong positive feedbacks, amplifying shows CO2 and CH4 data from the Antarctic Vostok ice core the surface temperature response. Climate models in the cur- [17, 18] and sea level based on Red Sea sediment cores [18]. rent IPCC [2] assessment still agree with Charney’s estimate. Gases are from the same ice core and have a co nsistent time Climate models alone are unable to define climate sensi- scale, but dating with respect to sea level may have errors up tivity more p recisely, b ecause it i s d ifficult to p rove that to several thousand years. models r ealistically in corporate a ll f eedback p rocesses. Th e We use the GHG and sea level d ata to calculate climate Earth’s history, however, allows empirical inference of both forcing by GHGs and surface albedo change as in prior cal- fast feedback climate sensitivity and long-term sensitivity to culations [7], but with two refinements. First, we specify the specified GHG c hange including t he s low i ce s heet f eed- N2O climate forcing as 12 percent of the sum of the CO2 and back. CH4 forcings, rather than the 15 percent estimated earlier [7] 2. PLEISTOCENE EPOCH Because N2O data are not available for the entire record, and its forcing is small and highly correlated with CO2 and CH4, Atmospheric composition a nd s urface prope rties i n t he we take the GHG effective forcing as late Pleistocene are known we ll enough for accurate assess- Fe (GHGs) = 1.12 [Fa(CO ) + 1.4 Fa(CH )], (1) ment of the fast-feedback ( Charney) climate s ensitivity. W e 2 4 first compare the pre-industrial Holocene with the last glacial using published formulae for Fa of each gas [20]. The factor maximum [L GM, 20 ky B P (be fore pre sent)]. T he pla net 1.4 accounts for t he higher efficacy of CH4 relative to CO2, was in energy balance in both periods within a small fraction which is d ue mainly to the in direct ef fect o f CH 4 on t ropo- of 1 W/m2, as shown by considering the contrary: an imbal- spheric ozone and stratospheric water vapor [12]. The result- ance o f 1 W/m2 maintained a few millennia would melt all ing GHG forc ing between the LGM and late Holocene is 3 2 ice on the planet or change ocean temperature an amount far W/m , apportioned as 75% CO2, 14% CH4 and 11% N2O. outside m easured va riations [T able S1 of 8]. T he a pproxi- The s econd re finement in our c alculations is t o surface mate e quilibrium characterizing m ost of E arth’s h istory is albedo. Based on m odels of i ce s heet s hape, we take t he unlike the current s ituation, in w hich G HGs ar e r ising at a horizontal a rea of t he ice s heet as proport ional t o t he 4/5 rate much fa ster t han the c oupled c limate s ystem c an re - power of vol ume. F ig. ( S4) co mpares o ur p resent a lbedo spond. forcing with prior use [7] of e xponent 2/3, showing that this Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 219

(a) CO2, CH4 and Sea Level CH4 SL CO2 (ppb) (m) (ppm) CO2 CH 300 4 700 Sea Level 0 250 500 -50 200 -100 300 400 350 300 250 200 150 100 50 0 (b) Climate Forcing 1 Surface Albedo 0 Greenhouse Gases ) 2 -1 -2 F (W/m -3 400 350 300 250 200 150 100 50 0 (c) Temperature Change 2 Observations Calculated Temperature 0

T ( ° C) -2

-4 400 350 300 250 200 150 100 50 0 Time (ky BP)

Fig. (1). (a) CO2, CH4 [17] and sea level [19] for past 425 ky. (b) Climate forcings due to changes of GHGs and ice sheet area, the latter inferred fro m sea l evel ch ange. ( c) Calculated global temperature change based on c limate sensitivity of
°C per W/m2. Observations ar e Antarctic temperature change [18] divided by two. choice a nd di vision of t he i ce i nto m ultiple ic e s heets ha s GHG and surface alb edo changes ar e mechanisms caus- only a minor effect. ing the large global climate changes in Fig. (1), but they do not in itiate th ese cl imate s wings. I nstead ch anges o f G HGs Multiplying the sum of GHG and surface albedo forcings 2 and sea level (a m easure of ic e sheet s ize) lag temperature by climate sensitivity
°C per W/m yields the blue curve in change by several hundred years [6, 7, 23, 24]. Fig. ( 1c). Vos tok temperature change [17] di vided by t wo (red cu rve) is u sed to cr udely estimate g lobal t emperature GHG and s urface a lbedo changes a re positive c limate change, a s typical gla cial-interglacial g lobal a nnual-mean feedbacks. Major g lacial-interglacial c limate s wings are in - temperature ch ange is ~5 °C an d is as sociated w ith ~1 0°C stigated by s low changes of E arth’s orbit, especially the tilt change on Antarctica [21]. Fig. (1c) shows that fast-feedback of Earth’s spin-axis relative to the orbital plane and the pre- 2 climate sensitivity
°C per W/m (3°C for doubled CO2) is a cession of t he equinoxes that i nfluences t he i ntensity o f good approximation for the entire period. summer insolation [25, 26]. Global radiative forcing due to 2.2. Slow Feedbacks orbital ch anges is s mall, b ut ice s heet s ize i s af fected b y changes of g eographical a nd s easonal insolation (e .g., ice Let us consider climate change averaged over a few thou- melts at both poles when the spin-axis tilt increases, and ice sand ye ars – l ong e nough t o a ssure e nergy ba lance a nd melts at one po le whe n p erihelion, t he closest a pproach to minimize effects of ocean thermal response time and climate the s un, o ccurs in late s pring [ 7]. A lso a w arming climate change leads/lags between hemispheres [22]. At such tempo- causes net release of GHGs. The most effective GHG f eed- ral r esolution th e temperature v ariations in F ig. ( 1) a re back is release of CO2 by t he ocean, due partly to tempera- global, with high latitude amplification, being present in po- ture de pendence of C O2 s olubility b ut m ostly to in creased lar ice cores and sea surface temperature derived from ocean ocean m ixing in a warmer cl imate, w hich ac ts to flush out sediment cores (Fig. S5). 220 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al.

Climate Forcing and Temperature Fe (W/m2) 6 0.5 × Vostok Temperature Global Temperature 3 GHG Forcing 3 All Forcings 2

1 Eemian Holocene 0 0

-1

-3 -2 Temperature Anomaly ( ° C) -3 400 350 300 250 200 150 100 50 0 1900 2000 Time (ky BP) Date

Fig. (2). Global temperature (left scale) and GHG forcing (right scale) due to CO2, CH4 and N2O from the Vostok ice core [17, 18]. Time scale is expanded for the industrial era. Ratio of temperature and forcing scales is 1.5°C per W/m2, i.e., th e temperature scale gives the ex- pected equilibrium response to GHG change including (slow feedback) surface albedo change. Modern forcings include human-made aero- sols, volcanic aerosols and solar irradiance [5]. GHG forcing zero point is the mean for 10-8 ky BP (Fig. S6). Zero point of modern tempera- ture and net climate forcing was set at 1850 [5], but this is also the zero point for 10-8 ky BP, as shown by the absence of a trend in Fig. (S6) and by the discussion of that figure.

deep oc ean C O2 a nd a lters oc ean bi ological produc tivity larger on P leistocene time scales (see Supplementary Mate- [27]. rial), bu t the sensitivity may b e r educed by ne gative f eed- backs on ge ologic time scales [29, 30]. The 6°C sensitivity GHG and surface albedo feedbacks respond and contrib- reduces to 3°C when the planet has become warm enough to ute to te mperature ch ange caused by any cl imate forcing, natural or hum an-made, g iven s ufficient ti me. The GH G lose its ice sheets. feedback i s ne arly linear i n g lobal temperature duri ng t he This l ong-term c limate s ensitivity i s re levant to GHGs late P leistocene (Fig. 7 of [6, 28]). Surface albedo feedback that r emain a irborne f or cen turies-to-millennia. The h uman- increases as E arth b ecomes co lder an d th e ar ea o f ic e in - caused atmospheric GHG increase will decline slowly if an- creases. Climate sensitivity on thropogenic emissions from fos sil fue l burni ng de crease enough, as we illustrate below using a simplified carbon cy- Pleistocene t ime s cales includes s low f eedbacks, an d i s cle model. On the other hand, if the globe warms much fur- larger th an th e Ch arney sensitivity, because th e dominant ther, carbon cycle models [2] a nd empirical data [6, 28] re - slow feedbacks are positive. Other feedbacks, e.g., the nega- veal a pos itive GHG fe edback on c entury-millennia t ime tive f eedback of increased weathering as CO2 increases, be- come important on longer geologic time scales. scales. T his a mplification of GHG a mount i s moderate if warming is kept within the range of re cent interglacial peri- Paleoclimate da ta p ermit evaluation of l ong-term s ensi- ods [6], but larger warming would risk greater release of CH4 tivity to specified GHG change. We assume only that, to first and C O2 from m ethane hydra tes in t undra a nd oc ean s edi- order, t he a rea of ic e is a func tion of gl obal te mperature. ments [29]. On s till longer, geological, time scales weather- Plotting G HG forc ing [7] from ice core d ata [18] a gainst ing of rocks causes a negative feedback on atmospheric CO2 temperature s hows that g lobal climate s ensitivity including amount [30], as discussed in section 3, but this feedback is 2 the slow surface albedo feedback is 1.5°C per W/m or 6° C too slow to alleviate climate change of concern to humanity. for doubled CO (Fig. 2), twice as large as the Charney fast- 2 2.3. Time Scales feedback sensitivity. Note that we assume the area of ice and snow on the planet to be predominately dependent on global How long does it take to reach eq uilibrium t emperature temperature, but some changes of re gional ice sheet proper- with specified GHG c hange? Response is s lowed by oc ean ties oc cur a s pa rt of t he Earth orbit al c limate forc ing (s ee thermal inertia and the time needed for ice sheets to disinte- Supplementary Material). grate.

This e quilibrium s ensitivity of 6° C for doubl ed C O2 is Ocean-caused de lay i s estimated i n F ig. ( S7) us ing a valid for s pecified GHG amount, a s in s tudies that e mploy coupled atmosphere-ocean model. One-third of the response emission scenarios and coupled carbon cycle/climate models occurs i n the fi rst fe w y ears, i n pa rt be cause of ra pid re - to determine GHG amount. If GH Gs are included as a feed- sponse over land, one-half in ~25 years, three-quarters in 250 back (with s ay solar irradiance as forcing) sensitivity is still years, and nearly full response in a millennium. The ocean- Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 221 caused delay is a strong (quadratic) function of climate sen- atmosphere [27, 28]. On longer time scales the total amount sitivity and it depends on the rate of mixing of surface water of C O2 in th e s urface r eservoirs v aries d ue to ex change o f and deep water [31], as discussed in the Supplementary Ma- carbon wit h the s olid e arth. C O2 t hus becomes a primary terial Section. agent of long-term climate change, leaving orbital effects as ‘noise’ on larger climate swings. Ice s heet r esponse time is ofte n a ssumed to be s everal millennia, ba sed on the broa d s weep of pa leo s ea le vel The Cenozoic era, the past 65.5 My, provides a valuable change (Fig. 1a) and primitive ice sheet models designed to complement to th e P leistocene f or ex ploring cl imate s ensi- capture that change. However, this long time scale may re- tivity. Cenozoic da ta on c limate a nd a tmospheric c omposi- flect the slowly changing orbital forcing, rather than inherent tion a re not a s pre cise, but larger c limate va riations oc cur, inertia, as th ere is no discernable lag between m aximum ice including an ice-free planet, thus putting glacial-interglacial sheet m elt rate and lo cal insolation that favors m elt [7]. P a- changes in a wider perspective. leo s ea lev el d ata w ith h igh tim e r esolution r eveal f requent Oxygen i sotopic composition of be nthic (de ep oc ean ‘suborbital’ sea level changes at rates of 1 m/century or more dwelling) fora minifera s hells i n a gl obal c ompilation of [32-34]. ocean sediment cores [26] provides a starting point for ana- Present-day obs ervations of Gr eenland a nd An tarctica lyzing Cenozoic climate change (Fig. 3a). At times with neg- show i ncreasing s urface melt [35], loss of but tressing i ce ligible ice s heets, oxyge n is otope change,
18O, provi des a shelves [ 36], ac celerating i ce streams [ 37], an d in creasing direct measure o f d eep o cean t emperature ( Tdo). T hus Tdo overall mass loss [38]. These rapid changes do not occur in (°C) ~ -4
18O + 12 between 65.5 and 35 My BP. existing ice sheet models, which are missing critical physics 18 Rapid increase of
O at about 34 My is associated with of ice sheet disintegration [39]. Sea level changes of several glaciation of Ant arctica [26, 40] a nd global cooling, as evi- meters per century occur in the paleoclimate record [32, 33], denced by da ta from Nort h Am erica [41] a nd As ia [42]. in response to forc ings slower and weaker than the present 18 From then until the present, O in deep ocean foraminifera human-made forcing. It seems likely that large ice sheet re- 16 is affected by both ice volume and T , lighter O evaporat- sponse will occur within centuries, if human-made forcings do ing p referentially f rom th e o cean an d a ccumulating in ice continue to increase. Once ice sheet disintegration is under- sheets. Between 35 M y a nd t he la st ice a ge (20 ky) t he way, decadal changes of sea level may be substantial. 18 change of
O was ~ 3‰, change of Tdo was ~ 6°C (from +5 2.4. Warming “in the Pipeline” to -1°C ) a nd ice vol ume c hange ~ 180 m sl (m eters of s ea level). Given that a 1.5‰ change of
18O is associated with a The e xpanded time s cale for t he industrial e ra (F ig. 2) 18 6°C Tdo change, we assign the remaining
O change to ice reveals a grow ing ga p be tween actual gl obal te mperature 18 volume linearly at the rate 60 msl per mil
O change (thus (purple cu rve) an d eq uilibrium ( long-term) t emperature re - 18 180 msl for
O between 1.75 and 4.75). Equal division of sponse ba sed on t he ne t estimated c limate forc ing (bl ack 18
O b etween t emperature an d s ea lev el y ields s ea l evel curve). Ocean and ice sheet response times together account change in th e l ate P leistocene in r easonable a ccord w ith for this gap, which is now 2.0°C. available sea level data (Fig. S8). Subtracting the ice volume 18 The forcing in Fig. (2) (black curve, Fe scale), when used portion of
O yields deep ocean temperature Tdo (°C) = -2 to drive a global climate model [5], yields global temperature (
18O -4.25‰) after 35 My, as in Fig. (3b). 3 change th at ag rees clo sely (Fig. in [5]) wit h observations The large (~14°C) Cenozoic temperature change between 2 (purple c urve, F ig. ). Th at climate m odel, w hich includes 50 My and the ice age a t 20 ky m ust have b een forced b y only fast feedbacks, has additional warming of ~0.6°C in the changes of atmospheric c omposition. Alternative dri ves pipeline today because of ocean thermal inertia [5, 8]. could c ome from out side (s olar irradiance) or t he E arth’s The remaining gap between equilibrium temperature for surface ( continental lo cations). Bu t s olar b rightness in - current atmospheric composition and actual global tempera- creased ~0.4% in the Cenozoic [43], a linear forcing change ture is ~ 1.4°C. This f urther 1 .4°C w arming s till to co me is of only +1 W/m2 and of t he wrong s ign to contribute to the due t o t he s low s urface albedo f eedback, s pecifically ice cooling tr end. Climate f orcing d ue to co ntinental lo cations sheet disintegration and vegetation change. was < 1 W/ m2, because continents 65 My ago w ere already S9 One m ay as k w hether th e c limate s ystem, as the Ear th close to p resent l atitudes ( Fig. ). Op ening or c losing o f warms f rom i ts p resent ‘ interglacial’ s tate, s till h as th e c a- oceanic gateways might affect the timing of glaciation, but it would not provi de t he c limate forc ing ne eded for gl obal pacity t o supply s low f eedbacks t hat double t he fa st- cooling. feedback sensitivity. This issue can be addressed by c onsid- ering longer time scales including periods with no ice. CO 2 concentration, in contrast, varied from ~180 ppm in glacial times to 1500 ± 500 ppm in the early Cenozoic [44]. 3. CENOZOIC ERA This change is a forcing of m ore than 10 W /m2 (Table 1 in P leistocene atmospheric CO2 v ariations o ccur as a cli- [16]), an order of m agnitude larger than other known forc - mate feedback, as carbon is exchanged among surface reser- ings. C H4 a nd N 2O, p ositively co rrelated w ith CO2 an d voirs: the ocean, atmosphere, soils and biosphere. The most global temperature i n the pe riod w ith a ccurate da ta ( ice effective feedback is increase of atmospheric CO2 as climate cores), likely increase the total GHG forc ing, but their forc- warms, t he CO2 tr ansfer b eing m ainly f rom o cean to ings are much smaller than that of CO2 [45, 46].

222 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al.

(a) Global Deep Ocean δ18O Antarctic Ice Sheet 12 0

N. Hemisphere Ice Sheets δ 8 ↑ 1 18 Paleocene-Eocene O ( Thermal Maximum 4 2 0 / 00 ) Deep Ocean 0 3

Temperature ( ° C) ↑ Ol-1 4 Glaciation 5 60 50 40 30 20 10 0 (b) Global Deep Ocean Temperature 16 -4 δ18O + 12 12 -2 (δ18O - 4.25)

8

4 Temperature ( ° C) 0 60 50 40 30 20 10 0 Time (My BP)

Fig. (3). Global deep ocean (a)
18O [26] and (b) temperature. Black curve is 5-point running mean of
18O original temporal resolution, while red and blue curves have 500 ky resolution.

3.1. Cenozoic Carbon Cycle Between 60 a nd 50 M y ago India moved north rapidly, 18-20 cm/year [50], through a r egion that long had b een a Solid Earth sources and sinks of CO are not, in general, 2 depocenter for carbonate and organic sediments. Subduction balanced at any given time [30, 47]. CO is re moved from 2 of car bon-rich cr ust w as s urely a lar ge s ource o f CO out - surface reservoirs by: (1) chemical weathering of rocks with 2 gassing and a prime cause of global warming, which peaked deposition of carbonates on the ocean floor, and (2) burial of 50 My ago (Fig. 3b) with the Indo-Asian collision. CO must organic matter; weathering is the dominant process [30]. CO 2 2 have then decreased due to a reduced subduction source and returns primarily via metamorphism and volcanic outgassing enhanced w eathering wit h upl ift of t he Him alayas/Tibetan at locations where carbonate-rich oceanic crust is being sub- Plateau [51]. Since then, the Indian and Atlantic Oceans have ducted beneath moving continental plates. been m ajor de pocenters for c arbon, but s ubduction of c ar- 12 13 Outgassing and burial of CO2 are each typically 10 -10 bon-rich crust has been limited mainly to small regions near mol C/ year [30, 47-48]. At t imes of unus ual p late tectonic Indonesia and Central America [47]. activity, such as rapid subduction of carbon-rich ocean crust Thus atmospheric CO declined following the Indo-Asian or s trong oroge ny, t he i mbalance be tween out gassing a nd 2 collision [44] and climate cooled (Fig. 3b) leading to Antarc- burial can b e a significant fr action of t he one -way carbon tic glaciation by ~ 34 My. Antarctica has been more or less flux. Although negative feedbacks in the geochemical carbon glaciated ever since. Th e rate of CO drawdown declines as cycle reduce the rate of surface reservoir perturbation [49], a 2 atmospheric CO de creases due to ne gative fe edbacks, i n- net i mbalance ~1012 m ol C/year c an be maintained ove r 2 cluding the effect of de clining atmospheric temperature and thousands of ye ars. S uch an imbalance, if c onfined to t he plant growth rates on we athering [30]. These negative feed- atmosphere, woul d be ~ 0.005 ppm/year, but a s C O is d is- 2 backs te nd t o c reate a ba lance be tween c rustal out gassing tributed a mong s urface re servoirs, t his i s onl y ~ 0.0001 and dra wdown of C O2, whi ch ha ve be en equal w ithin 1-2 ppm/year. T his r ate is n egligible co mpared to the p resent percent over the past 700 ky [52]. Large fluctuations in the human-made a tmospheric CO 2 increase of ~ 2 ppm/year, yet size o f the A ntarctic ice s heet h ave o ccurred in th e p ast 3 4 over a m illion y ears s uch a cr ustal im balance al ters at mos- My, possibly related to temporal variations of plate tectonics pheric CO by 100 ppm. 2 [53] a nd out gassing ra tes. The re latively c onstant a tmos- Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 223 pheric CO 2 amount of t he past 20 M y (Fig. S10) implies a We hinge our c alculations at 35 My for s everal reasons. near ba lance of out gassing a nd we athering ra tes ov er t hat Between 65 and 35 My ago there was little ice on the planet, period. so c limate s ensitivity i s de fined m ainly by fa st fe edbacks. Second, we w ant t o e stimate t he CO a mount that pre cipi- Knowledge of Cenozoic C O is li mited to imprecise 2 2 tated A ntarctic g laciation. F inally, the r elation b etween proxy m easures e xcept for re cent ic e core d ata. T here a re global surface air temperature change (
T ) and deep ocean discrepancies a mong di fferent proxy m easures, a nd e ven s temperature ch ange (
T ) differs for i ce-free and glaciated between di fferent investigators u sing t he s ame proxy do method, as discussed in conjunction with Fig. (S10). Never- worlds. theless, the proxy data indicate that CO2 was of the order of Climate m odels s how th at g lobal t emperature ch ange i s 1000 ppm in the early Cenozoic but <500 ppm in the last 20 tied c losely t o oc ean t emperature c hange [54]. De ep oc ean My [2, 44]. temperature is a function of high latitude ocean surface tem- 3.2. Cenozoic Forcing and CO perature, which tends to be amplified relative to global mean 2 ocean s urface t emperature. H owever, l and tem perature The en tire Cenozoic climate f orcing h istory ( Fig. 4a) is change exceeds th at o f th e o cean, w ith an effect o n g lobal implied by t he temperature reconstruction (Fig. 3b), assum- temperature that t ends to o ffset the latitudinal v ariation o f ing a fast-feedback sensitivity of °C per W/m2. Subtracting ocean temperature. Thus in the ice-free world (65-35 My) we the s olar a nd s urface a lbedo forc ings (F ig. 4b), th e la tter take
Ts ~
Tdo with generous (50%) uncertainty. In the gla- 18 from Eq. S2 with ice sheet area vs time from  O, we obtain ciated world
Tdo is limited by the freezing point in the deep the GHG forcing history (Fig. 4c). ocean.
Ts between the last ice age (20 ky) a nd the present

(a) Total Forcing 16 12 8 ) 2 ΟA 4 ↓ 0 ΟB F (W/m -4 ↓ -8 ΟC -12 ↓ 60 50 40 30 20 10 0 (b) Solar and Albedo Forcings ) 2 0 -4 Solar Albedo

F (W/m -8 60 50 40 30 20 10 0 (c) Greenhouse Gas Forcing 16 12 ) 2 8 4

F (W/m 0 -4 -8 60 50 40 30 20 10 0 Time (My BP)

Fig. (4). (a) Total climate forcing, (b) solar and surface albedo forcings, and (c) GHG forcing in the Cenozoic, based on Tdo history of Fig. 2 (3b) and assumed fast-feedback climate sensitivity °C per W/m . Ratio of T s change and Tdo change is assumed to be nea r unity in the minimal ice world between 65 and 35 My, but the gray area allows for 50% uncertainty in the ratio. In the later era with large ice sheets we take
Ts/
Tdo = 1.5, in accord with Pleistocene data. 224 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al.

(a) CO2 Forcing Converted to Mixing Ratio 3000 600 ppm at 35 My BP

2000 (ppm) 2 1000 CO

0 60 50 40 30 20 10 0 3000 450 ppm at 35 My BP

2000 (ppm) 2 1000 CO

0 60 50 40 30 20 10 0 3000 325 ppm at 35 My BP

2000 (ppm) 2 1000 CO

0 60 50 40 30 20 10 0 Time (My BP)

(b) Observed and Calculated CO2 in Last 425 kys 400

300 ppm) 2 200 CO

100 Petit et al.’s Data 600 ppm, 450 ppm, 325 ppm at 35 My BP 400 300 200 100 0 Time (ky BP)

Fig. (5). (a) Simulated CO2 amounts in the Cenozoic for three choices of CO2 amount at 35 My (temporal resolution of black and colored curves as in Fig. (3); blue region: multiple CO2 proxy data, discussed with Fig. (S10); gray region allows 50 percent uncertainty in ratio of global surface and deep ocean temperatures). (b) Expanded view of late Pleistocene, including precise ice core CO2 m easurements (black curve).

interglacial p eriod (~5°C) w as ~1.5 t imes l arger than
Tdo. and yields a good fit to the amplitude and mean CO2 amount In Fig. (S5) we show that this relationship fits well through- in the late Pleistocene (Fig. 5b). A CO2 threshold for Antarc- out the period of ice core data. tic gla ciation of ~ 500 ppm w as pr eviously i nferred from proxy CO2 data and a carbon cycle model [55]. If we s pecify C O2 a t 35 My, the GHG forc ing d efines CO2 at other times, assuming CO2 provides 75% of the GHG Indi vidual CO2 proxies (Fig. S10) clarify limitations due forcing, as in the late P leistocene. CO2 ~450 ppm at 35 My to scatter among the measurements. Low CO2 of some early keeps C O2 in the range of e arly Cenozoic proxies (Fig. 5a) Cenozoic proxi es, if va lid, woul d s uggest h igher climate Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 225 sensitivity. H owever, in g eneral the s ensitivities inferred slowed b y th e in ertia o f th e o cean ( Fig. S7) an d ice s heets. from the Cenozoic and Phanerozoic [56, 57, 58] a gree well However, E arth’s hi story s uggests that pos itive fe edbacks, with our analysis, if we account for the ways in which sensi- especially s urface albedo c hanges, can s pur ra pid gl obal tivity is defined and the periods emphasized in each empiri- warmings, including sea level rise as fast as several meters cal derivation (Table S1). per century [7]. Thus if humans push the climate system suf- ficiently f ar in to d isequilibrium, p ositive cl imate f eedbacks Our CO estimate of ~450 ppm at 35 My (Fig. 5) serves 2 may s et in motion dramatic climate change and climate im- as a p rediction to compare w ith n ew d ata o n CO 2 a mount. Model u ncertainties ( Fig. S10) i nclude pos sible c hanges of pacts that cannot be controlled. non-CO2 GHGs and the relation of
Ts to
Tdo. The model 4. ANTHROPOCENE ERA fails t o a ccount for c ooling i n the pa st 15 My i f CO in- 2 Human-made gl obal c limate forc ings now pre vail ove r creased, a s s everal proxie s suggest (F ig. S10). C hanging natural forc ings (Fi g. 2). E arth m ay ha ve e ntered t he An - ocean currents, such as the closing of the Isthmus of Panama, thropocene era [60, 61] 6-8 ky a go [62], but the net human- may have contributed to climate evolution, but models find made forcing was small, perhaps slightly negative [7], prior little effect on t emperature [59]. Non-CO GHGs also could 2 to the industrial era. GHG forc ing overwhelmed natural and have pl ayed a rol e, be cause lit tle forc ing woul d ha ve be en negative human-made forcings only in the past quarter cen- needed to cause cooling due to the magnitude of la te Ceno- tury (Fig. 2). zoic albedo feedback. 3.3. Implication Human-made climate c hange is d elayed by o cean (F ig. S7) and ice sheet response times. Warming ‘in the pipeline’, We infer from Cenozoic data that CO2 was the dominant mostly attributable to slow feedbacks, is now about 2°C (Fig. Cenozoic forcing, that CO2 was ~450 ± 100 ppm when Ant- 2). No additional forcing is required to raise global tempera- arctica glaciated, and that glaciation is reversible. Together ture to a t l east th e l evel o f th e P liocene, 2 -3 m illion y ears these inferences have profound implications. ago, a degree of warming that would surely yield ‘danger- ous’ climate impacts [5]. Consider three points m arked in F ig. (4): point A at 35 My, just before Antarctica glaciated; point B at recent inter- 4.1. Tipping Points glacial periods; point C at the depth of recent ice ages. Point Realization that today’s climate is far out of e quilibrium B is about half way between A a nd C in global temperature with c urrent c limate forc ings ra ises the s pecter of ‘t ipping (Fig. 3b) an d climate f orcings ( Fig. 4). The GHG for cing from the deepest recent ice age to current interglacial warmth points’, t he c oncept t hat c limate c an re ach a poi nt whe re, is ~3.5 W /m2. Additional 4 W/m2 forcing carries the planet, without additional forcing, rapid changes proceed practically out of our c ontrol [2, 7, 63, 64]. Arctic sea ice and the West at equilibrium, to the ice-free state. Thus equilibrium climate Antarctic Ice Sheet are examples of potential tipping points. sensitivity to GHG change, i ncluding t he surface a lbedo Arctic s ea i ce lo ss is m agnified b y th e p ositive f eedback o f change as a slow feedback, is almost as large between today increased absorption of s unlight as global warming initiates and an ice-free world as between today and the ice ages. sea i ce retreat [65]. West Antarctic ic e lo ss can be acc eler- The im plication is th at global cl imate sensitivity of 3°C ated by several feedbacks, once ice loss is substantial [39]. for doubl ed C O , a lthough va lid for t he idealized C harney 2 We define: (1) the tipping level, th e global cl imate forc- definition of c limate s ensitivity, is a considerable unde r- ing t hat, i f l ong m aintained, gi ves ri se to a s pecific c onse- statement of e xpected equilibrium gl obal warming i n re - quence, and (2) the point of no return, a climate state beyond sponse to imposed doubled CO2. Additional warming, due to slow climate f eedbacks in cluding lo ss o f i ce an d s pread o f which the consequence is inevitable, even if climate forcings are reduced. A point of no return can be avoided, even if the flora ove r the va st h igh-latitude land a rea in the Nor thern tipping lev el is tem porarily exceeded. Ocean an d ice sheet Hemisphere, a pproximately doubl es e quilibrium climate inertia permit overshoot, provided the climate forcing is re- sensitivity. turned b elow the tipping lev el b efore in itiating irreversible Equilibrium s ensitivity 6°C for doubl ed CO 2 is r elevant dynamic change. to the case in which GHG changes are specified. That is ap- Points of no return are inherently difficult to d efine, b e- propriate to t he anthropogenic case, provi ded t he GH G cause the dynamical problems are nonlinear. Existing models amounts are estimated from c arbon cycle models including climate feedbacks such as methane release from tundra and are more lethargic than the r eal w orld f or p henomena n ow ocean sediments. Th e eq uilibrium s ensitivity is ev en h igher unfolding, i ncluding c hanges of s ea ic e [65], ice s treams [66], ice shelves [36], and expansion of t he subtropics [67, if t he GHG fe edback i s i ncluded a s pa rt of t he c limate re - 68]. sponse, as is appropriate for analysis of the climate response to Earth orbital perturbations. The very high sensitivity with The tipping level is e asier to assess, be cause the paleo- both albedo and GHG slow feedbacks included accounts for climate quasi-equilibrium response to known climate forcing the huge magnitude of glacial-interglacial fluctuations in the is relevant. The tipping level is a measure of the long-term Pleistocene (Fig. 3) in response to small forcings (section 3 climate f orcing th at h umanity must a im to s tay b eneath to of Supplementary Material). avoid large climate impacts. The tipping level does not de - fine t he m agnitude or pe riod of t olerable ove rshoot. How - Equilibrium c limate re sponse woul d not b e r eached in decades o r even in a century, b ecause s urface w arming is ever, if overshoot is in place for c enturies, the thermal per- 226 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al. turbation will so penetrate the ocean [10] that recovery with- West Antarctica [75] he ighten concerns about ice sheet sta- out d ramatic ef fects, s uch as i ce s heet d isintegration, b e- bility. An initial CO 2 target of 350 ppm , to be reassessed as comes unlikely. effects on ice sheet mass balance are observed, is suggested.

4.2. Target CO2 Stabilization of Arctic sea ice cover requires, to first ap- proximation, re storation of pl anetary e nergy ba lance. Cli- Combined, GHGs other than C O cau se c limate f orcing 2 mate models driven by known forcings yield a present plane- comparable to th at o f C O [2, 6], but grow th of non-C O 2 2 2 tary e nergy i mbalance of + 0.5-1 W /m [5]. Obs erved he at GHGs is falling below IPCC [2] s cenarios. Thus total GHG increase in the upper 700 m of t he ocean [76] c onfirms the climate forc ing c hange is now d etermined mainly by CO2 planetary e nergy i mbalance, but obs ervations of t he e ntire [69]. C oincidentally, CO f orcing i s s imilar to the n et h u- 2 ocean ar e n eeded f or q uantification. CO a mount m ust be man-made forc ing, b ecause non-C O GHGs tend t o offs et 2 2 reduced t o 325-355 ppm t o i ncrease out going fl ux 0. 5-1 negative aerosol forcing [2, 5]. 2 W/m , if other forcings are unchanged. A furt her imbalance Thus we take future CO2 change as approximating the net reduction, and thus CO2 ~300-325 ppm, may b e ne eded to human-made forcing change, with two caveats. First, special restore sea ice to its area of 25 years ago. effort to reduce non-CO GHGs could alleviate the CO re- 2 2 Coral re efs a re s uffering from m ultiple s tresses, wi th quirement, allowing up t o about +25 ppm CO for t he same 2 ocean acidification an d o cean w arming p rincipal among climate e ffect, wh ile re surgent growt h of non- CO GHG s 2 them [77]. G iven additional w arming ‘in-the-pipeline’, 38 5 could re duce a llowed C O2 a s imilar a mount [6]. S econd, ppm CO2 is already deleterious. A 300-350 ppm CO2 target reduction of human-made aerosols, which have a net cooling would significantly relieve both of these stresses. effect, could force stricter GHG re quirements. However, an emphasis on reducing black soot could largely off-set reduc- 4.3. CO2 Scenarios tions of high albedo aerosols [20]. A large fraction of fossil fuel CO2 emissions stays in the Our estimated history of CO2 through the Cenozoic Era air a l ong ti me, one -quarter r emaining a irborne for s everal provides a sobering perspective for a ssessing an appropriate centuries [11, 78, 79]. Thus moderate delay of fossil fuel use target for future CO2 levels. A CO2 amount of order 450 ppm will not appreciably reduce long-term human-made climate or larger, if long m aintained, would push Earth toward the change. Preservation of a c limate r esembling that to whic h ice-free state. Although ocean an d ice sheet in ertia li mit the humanity is ac customed, th e cl imate o f th e H olocene, r e- rate of climate ch ange, such a CO2 level likely would cause quires that most remaining fossil fuel carbon is never emitted the pa ssing of c limate ti pping poi nts a nd i nitiate dyna mic to the atmosphere. responses that could be out of humanity’s control. Coal is the largest reservoir of c onventional fossil fuels The c limate s ystem, be cause of i ts i nertia, ha s not ye t (Fig. S12), exceeding co mbined r eserves o f o il an d g as [ 2, fully re sponded to t he re cent increase of hum an-made cli- 79]. The only realistic way to sharply curtail CO2 emissions mate forcings [5]. Yet climate impacts are already occurring is to p hase o ut co al u se ex cept w here C O2 is captured an d that allow us to m ake an in itial es timate for a tar get at mos- sequestered. pheric C O l evel. No doubt t he target will ne ed to be a d- 2 Phase-out of c oal e missions by 2030 (F ig. 6) k eeps justed a s climate da ta and knowl edge improve, but the ur - maximum CO2 close to 400 ppm , depending on o il and gas gency a nd di fficulty of re ducing the hum an-made for cing reserves and reserve growth. IPCC reserves assume that half will be less, and more likely manageable, if excess forcing is of r eadily ex tractable o il h as already b een u sed ( Figs. 6, limited soon. S12). EIA [80] estimates (Fig. S12) have larger reserves and Civilization is adapted to climate zones of the Holocene. reserve growth. Even if EIA estimates are accurate, the IPCC Theory and models indicate that subtropical regions expand case remains valid if the most difficult to extract oil and gas poleward w ith gl obal w arming [2, 67]. Da ta re veal a 4 - is left in the ground, via a rising price on c arbon emissions degree la titudinal shift a lready [68], larger than model pre - that discourages remote exploration and environmental regu- dictions, yielding increased aridity in southern United States lations that place some areas off-limit. If IPC C gas reserves [70, 71], t he Mediterranean re gion, Aus tralia a nd pa rts o f (Fig. S12) are underestimated, the IPCC case in Fig. (6) re- Africa. Impacts of this climate shift [72] support the conclu- mains valid if the additional gas reserves are used at facilities sion that 385 ppm CO2 is already deleterious. where CO2 is captured. Alpine glaciers are in near-global retreat [72, 73]. After a However, even with phase-out of c oal emissions and as- one-time added fl ush of fre sh wa ter, gla cier de mise w ill suming IPCC oil and gas reserves, CO2 would remain above yield summers and autumns of frequently dry rivers, includ- 350 ppm for m ore than two c enturies. Ongoing Ar ctic and ing ri vers ori ginating i n the H imalayas, Ande s a nd Rocky ice s heet changes, e xamples of ra pid pa leoclimate change, Mountains that now supply water to hundreds of millions of and other criteria cited above all drive us to consider scenar- people. Present glacier retreat, and warming in the pipeline, ios that bring CO2 more rapidly back to 350 ppm or less. indicate that 385 ppm CO2 is already a threat. 4.4. Policy Relevance Equilibrium sea level rise for today’s 385 ppm CO2 is at least several meters, judging from paleoclimate history [19, Desire t o re duce a irborne C O2 ra ises t he que stion of 32-34]. A ccelerating m ass lo sses f rom G reenland [ 74] an d whether CO2 could be drawn from the air ar tificially. There are no large-scale technologies for CO2 air capture now, but Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 227

CO2 Emissions and Atmospheric Concentration with Coal Phaseout by 2030 12 450 (a) (b) Observations EIA Oil & Gas Reserves + Growth EIA Reserves + Reserve Growth 10 IPCC Oil & Gas Reserves IPCC Oil & Gas Reserves

400 8 (ppm) 2

6 Forestry & Soil 350 4 Emissions (Gt C/yr) 2 Oil/Gas/Biofuel → Atmospheric CO CO 2 300

0 1900 1950 2000 2050 2100 2150 1900 1950 2000 2050 2100 2150

Fig. (6). (a) Fossil fuel CO2 emissions with coal phase-out by 2030 based on IPCC [2] and EIA [80] estimated fossil fuel reserves. (b) Re- sulting atmospheric CO2 based on use of a dynamic-sink pulse response function representation of the Bern carbon cycle model [78, 79]. with strong research and development support and industrial- avoiding a dangerously large, extended overshoot of the 350 scale pilot projects sustained over decades it may be possible ppm level will be dim. to a chieve c osts ~ $200/tC [81] or pe rhaps l ess [82]. A t 4.5. Caveats: Climate Variability, Climate Models, and $200/tC, the cost of re moving 50 ppm of CO2 is ~$20 tril- Uncertainties lion. Climate has great variability, much of wh ich is unforced Improved agricultural and forestry practices offer a more and unpredictable [2, 90]. This fact raises a practical issue: natural way to draw down C O . Deforestation contributed a 2 what is th e ch ance th at cl imate variations, e.g., a temporary net emission of 60±30 ppm over the past few hundred years, cooling trend, will affect publi c re cognition of climate of which ~20 ppm CO remains in the air today [2, 83] (Figs. 2 change, making it difficult to implement mitigation policies? (S12, S14). Reforestation could absorb a substantial fraction Also what are the greatest uncertainties in the expectation of of the 60±30 ppm net deforestation emission. a continued global warming trend? And what are the impacts Carbon sequestration in soil also has significant potential. of climate model limitations, given the inability of models to Biochar, produced in pyrolysis of re sidues from crops, for- realistically s imulate m any as pects of cl imate change and estry, and animal wastes, can be used to restore soil fertility climate processes? while storing carbon for centuries to millennia [84]. Biochar The a tmosphere a nd ocean e xhibit c oupled nonlinear helps soil retain nutrients and fertilizers, reducing emissions chaotic variability that cascades to all time scales [91]. Vari- of GHGs such as N O [85]. Replacing slash-and-burn agri- 2 ability i s s o large that the s ignificance of re cent de cadal culture w ith s lash-and-char an d u se o f ag ricultural an d f or- global temperature change (Fig. 7a) would be very limited, if estry w astes for bi ochar produc tion could provi de a CO 2 the da ta we re c onsidered s imply a s a ti me s eries, wit hout drawdown of ~8 ppm or more in half a century [85]. further i nformation. Howe ver, ot her knowl edge includes In the Supplementary Material Section we define a for- information o n th e cau ses o f s ome o f th e tem perature v ari- est/soil dr awdown s cenario t hat re aches 50 ppm by 215 0 ability, th e planet’s energy im balance, an d global cl imate (Fig. 6b). This scenario returns CO2 below 350 ppm late this forcings. century, after about 100 years above that level. The El Nino Southern Oscillation (ENSO) [94] a ccounts More rapid drawdown could be provided by CO2 capture for most low latitude temperature variability and much of the at power plants fueled by gas and biofuels [86]. Low-input global va riability. T he gl obal i mpact of E NSO i s c oherent high-diversity biofuels grown on degraded or marginal lands, from month to month, as shown by the global-ocean-mean with associated b iochar produc tion, could a ccelerate CO2 SST (Fi g. 7b), f or w hich th e o cean’s th ermal inertia m ini- drawdown, but t he na ture of a bi ofuel a pproach m ust be mizes the effect of weather noise. The cool anomaly of 2008 carefully designed [85, 87-89]. coincides w ith a n E NSO m inimum a nd doe s not i mply a A rising price on carbon emissions and payment for car- change of decadal temperature trend. bon s equestration i s surely n eeded to m ake dra wdown o f Decadal time scale variability, such as predicted weaken- airborne CO2 a reality. A 50 ppm drawdown via agricultural ing of t he Atlantic overturning circulation [95], could inter- and f orestry p ractices se ems p lausible. But i f most o f t he rupt global warming, as discussed in section 18 of t he Sup- CO2 in coal is put into the air, no s uch “natural” drawdown plementary M aterial. But t he i mpact of re gional dyna mical of CO2 to 350 ppm is feasible. Indeed, if the world continues effects on gl obal temperature is opposed by t he planet’s en- on a business-as-usual path for even another decade without ergy imbalance [96], a product of the climate system’s ther- initiating phase-out of unc onstrained coal use, prospects for mal inertia, w hich i s co nfirmed b y in creasing o cean h eat 228 The Open Atmospheric Science Journal, 2008, Volume 2 Hansen et al.

(a) Temperature Anomaly (°C): Seasonal Resolution (b) Global-Ocean Mean SST Anomaly (°C): Monthly Resolution

Dec-Jan-Feb .5 Dec-Jan-Feb .6 Mar-Apr-May Mar-Apr-May Jun-Jul-Aug Global .4 Jun-Jul-Aug .4 Sep-Oct-Nov Sep-Oct-Nov

.2 .3

.0 .2

-.2 .1 -.4 .0 .2

-.1 .0

-.2 -.2 Low Latitudes (23.6°N-23.6°S)

-.4 -.3 2 2 Nino 3.4 SST (°C) 1 1 0 0 -1 -1 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000 Fig. (7). (a) Seasonal-mean global and low-latitude surface t emperature anomalies relative to 1951-1980, an update of [92], (b) g lobal- ocean-mean sea surface temperature anomaly at monthly resolution. The Nino 3.4 Index, the temperature anomaly (12-month running mean) in a small part of the tropical Pacific Ocean [93], is a measure of ENSO, a basin-wide sloshing of the tropical Pacific Ocean [94]. Green tri- angles show major volcanic eruptions. storage [97]. This energy imbalance makes decadal interrup- principal dri ver of gl obal c limate. If we s tay our pre sent tion of global warming, in the absence of a negative climate course, using fossil fuels to feed a growing appetite for e n- forcing, improbable [96]. ergy-intensive life styles, we will soon leave the climate of Volcanoes a nd t he s un c an c ause s ignificant ne gative the Holocene, the wor ld of pri or human history. The even- tual re sponse t o doubl ing pre -industrial a tmospheric C O forcings. However, even if the solar irradiance re mained a t 2 likely would be a nearly ice-free planet, preceded by a period its v alue in the current solar m inimum, this reduced forcing of chaotic change with continually changing shorelines. would be offset by increasing CO2 within seven years (Sup- plementary Ma terial s ection 18). Hum an-made a erosols Humanity’s task of m oderating hum an-caused gl obal cause a gre ater ne gative forc ing, bot h di rectly a nd t hrough climate ch ange is u rgent. O cean an d ice s heet in ertias p ro- their effects on c louds. T he fi rst s atellite obs ervations o f vide a buffer delaying full response by centuries, but there is aerosols and clouds with accuracy sufficient to quantify this a danger that human-made forcings could drive the climate forcing are planned to begin in 2009 [98], but most analysts system beyond tipping points such that change proceeds out anticipate th at h uman-made a erosols w ill d ecrease in th e of our control. The time available to reduce the human-made future, rather than increase further. forcing i s unc ertain, be cause m odels of t he gl obal s ystem and c ritical c omponents s uch as ic e sheets are inadequate. Climate m odels h ave m any d eficiencies in th eir ab ilities However, c limate r esponse ti me i s s urely le ss t han t he a t- to s imulate climate change [ 2]. H owever, model u ncertain- mospheric lifetime of the human-caused perturbation of CO . ties cut both ways: it is at least as likely that models underes- 2 Thus remaining fossil fuel reserves should not be exploited timate effects of hum an-made GHGs a s ove restimate them without a plan for r etrieval and disposal of r esulting atmos- (Supplementary Material section 18). Model deficiencies in pheric CO . evaluating tipping points, the pos sibility that rapid changes 2 can occur without additional climate forcing [63, 64], are of Paleoclimate evidence and ongoing global changes imply special concern. Loss of Arctic sea ice, for example, has pro- that t oday’s CO2, about 385 ppm , i s a lready t oo hi gh to ceeded more rapidly th an predicted by climate models [99]. maintain th e c limate to w hich h umanity, w ildlife, an d th e There are re asons to e xpect t hat ot her nonl inear proble ms, rest of t he biosphere are adapted. Realization that w e must such a s ic e s heet di sintegration a nd e xtinction of i nterde- reduce the current CO 2 amount has a bright side: effects that pendent species and ecosystems, also have the potential for had be gun t o s eem inevitable, i ncluding im pacts of oc ean rapid change [39, 63, 64]. acidification, l oss of fre sh wa ter supplies, and s hifting o f 5. SUMMARY climatic zones, may be averted by the necessity of finding an energy course beyond fossil fuels sooner than would other- Humanity t oday, c ollectively, must fa ce the unc omfort- wise have occurred. able fa ct t hat i ndustrial c ivilization it self ha s be come t he Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, Volume 2 229

We suggest an initial objective of re ducing atmospheric [4] European Council, Climate change s trategies 2005; http://register. consilium.europa.eu/pdf/en/05/st07/st07242.en05.pdf CO2 to 350 ppm , with the target to be adjusted as scientific [5] Hansen J, Sato M, Ruedy, et al. Dangerous human-made interfer- understanding and empirical evidence of c limate effects ac- ence with climate: a GISS modelE study. Atmos Chem Phys 2007; cumulate. A lthough a c ase a lready c ould b e m ade that t he 7: 2287-312. eventual target probably needs to be lower, the 350 ppm tar- [6] Hansen J, Sato M. Greenhouse gas growth rates. Proc Natl Acad Sci 2004; 101: 16109-14. get is s ufficient to qua litatively c hange the dis cussion a nd [7] Hansen J , S ato M , K harecha P , R ussell G , L ea D W, Siddall M . drive fundamental changes in energy policy. Limited oppor- Climate change and t race gases. 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Received: May 22, 2008 Revised: August 19, 2008 Accepted: September 23, 2008

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