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Royal Swedish Academy of Sciences Palaeoclimate Sensitivity to CO₂ and Insolation Author(s): André Berger and Marie-France Loutre Source: Ambio, Vol. 26, No. 1, Arrhenius and the Greenhouse Gases (Feb., 1997), pp. 32-37 Published by: Allen Press on behalf of Royal Swedish Academy of Sciences Stable URL: http://www.jstor.org/stable/4314547 Accessed: 02/10/2009 17:58 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=acg. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Allen Press and Royal Swedish Academy of Sciences are collaborating with JSTOR to digitize, preserve and extend access to Ambio. http://www.jstor.org Andre Berger and Marie-FranceLoutre PalaeoclimateSensitivity and Insolation to CO2 105 to 125 m (2, 3) and the global average surfaceair temperaThe current Quaternary Ice Age is characterized by mul- turewas - 5?C below present(4). CO2levels were less thantwo tiple switches of the global climate between glacials and thirdstheirpresentvalue (5) and aerosol loading may have been interglacials. The Louvain-la-Neuve two-dimensional (alti- higherthanpresent(6). tude-latitude) climate model (LLN 2-D model) has been Over the last million years, the waxing and waning of ice used to simulate the long-term variations of the Northern sheets occurredin a more or less regularway. Reconstructions Hemisphere climate. The model is forced by both the C02 of long-term climatic variations, like the ice volume and sea and the insolation for each day and latitude. The period level, show a sawtooth shape with a 100-ka quasi-cycle over analyzed here is from 200 ka BP to 200 ka AP. The model which shorterquasi-cycles of roughly 41 and 21 ka are superprovides a total continental ice volume, which compares relatively well with the SPECMAP oxygen-isotope record, imposed. These kinds of broad climatic features are those exthe size and extent of the individual Northern Hemisphere plained by the astronomicaltheory of paleoclimates(7-9). Proponents of this theory claim that the changes in the Earth'sorice sheets being in fair agreement with their geological reconstructions. Seven simulations were done to quantify bital and rotationalparametershave been sufficiently large as the relative contribution of the insolation and C02 forcings to induce significantchanges in the seasonal and latitudinaldistributionsof irradiationreceived from the sun and so, to force and of the albedo and water-vapor temperature feedbacks to the last glacial maximum (LGM) cooling. This LGM glacials and interglacialsto recur in the mannerdeduced from cooling of 4.50C would be due to C02 alone for 20.8%, to geological records.In this process, feedbacksdue to the albedo, insolation and the albedo temperature feedback for 40%, the watervapor and other greenhousegases play a fundamental and to the water vapor feedback for 38.6%. Ifwe consider role. The aim of this paper is to explain (i) why and how the the low C02 concentration as the only forcing, but allow solar energy received by the Earthvaries on the geological time water vapor to change, 33% of the cooling are explained. scale; and (ii) the relative role played by this insolation forcing versus the feedbacks. QUATERNARY CLIMATE The currentIce Age, which the Earthentered2 to 3 million years ago is called the QuaternaryIce Age. It is characterizedby multiple switches of the global climate between glacials (with extensive ice sheets) and interglacials(with a climate similarto or warmerthantodayby a few degreesCelsius). The last cycle goes from the Eemian interglacialtimes centeredroughly 125 ka BP (ka = thousand years; BP: before present) to the present-day Holocene interglacialwhich peaked 6 ka BP, and includes the last glacial maximum(LGM) which occurred20 ka BP. In the NorthernHemisphere,the LGM world differed strikingly from the presentin the huge land-basedice sheets, reaching approximately2 to 3 km in thicknessand amountingto about 40 to 50 x 106 km3 of ice (1). Sea-level was lower by roughly BER78 0.06 0 c05 0.040.03 0.02 0.01 0.00 0.06 ' t eccentiity (a) 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 dcimatici precession( sin au) 25 obliquity(s) 24 23 22insolation 6-5NJune 500 450 2600 150o 100O 50 (kyrAP) 0 Time -50 -100 (kyrBP) -150 -200 Figure1. Long-termvariationsof the astronomicalelements (eccentricity,climaticprecession, obliquity)and of the insolationat 32 ASTRONOMICAL THEORYOF PALAEOCLIMATES Astronomical parameters and insolation The incoming solar radiationreceived over the Earthhas an annual periodic variationdue to the Earth's elliptic translational motion aroundthe sun. In addition,the seasonal and latitudinal distributionsof this solar radiationhave a long-period (tens of thousandsof years) variationdue to the so-called long-termvariations in the orbitalelements. Over the past 3 x 106 years (10), the eccentricityof the Earth's orbit has varied between near circularity (e = 0) and slight ellipticity (e = 0.07) at a period with a mean of about 100 ka. The tilt of the Earth'saxis (E)has variedbetween about22? and 25? over a period of nearly 41 ka. The climatic precession of the equinoxes (e sing is a motion in which the equinoxes and solstices shift aroundthe Earth'sorbitrelative to the perihelion, with a mean period of 21 ka. This period results actually from the existence of two periods which are close to each other: 23 and 19 ka. The combined influence of changes in e, ? and e sin X, and produces a complex patternof insolation variations(Fig. 1). A detailed analysis of the changes in daily solar radiation (11) shows that this daily radiationis principallyaffected by variations in precession,althoughthe obliquityplays an importantrole for high latitudes,mainly in the winter hemisphere.Changes in incoming solar radiationdue to changes in tilt are the same in both hemispheresduringthe same local season. The precession effect can cause warm winters and cool summersin one hemispherewhile causingthe oppositeeffects in the otherhemisphere. Pioneers of the Astronomical Theory and Arrhenius In 1842, J.A. AdhemarexplainedL. Agassiz's hypothesisregarding the existence of ice ages on the basis of the known precession of the equinoxes,therebyimplyingthattherehad likely been more than one (for details and referencesof early works on the astronomicaltheory see (7) and (12)). An astronomerin Paris ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997 at thattime, Urbainle Verrier,famed for discoveringthe orbital anomalies of Uranus, which led to the discovery of Neptune, immediatelycalculatedthe planetaryorbitalchanges of the Earth over the last IO'years. Within the next decades, largely because of the discovery of the repetitiveaspect of global glaciationin the Vosges, in Wales, and in the Americanrecordsof the Illinoian deposits, for example, glacial geology became stronglytied to astronomy.Around 1864, James Croll (13) initiateda series of importantworks that would continue to bear much fruit into modem times. He recognized three major astronomicalfactors: axial tilt, orbital eccentricity,and precession. A specific characteristicof his model lies, essentially, in its hypothesis that the critical season for the initiation of glacial stages is the NorthernHemisphere winter. He arguedthata decreasein the amountof sunlightreceivedduring the winter favors the accumulation of snow and that any small initial increase in the size of the area covered by snow would be amplifiedby the snowfields themselves (positive feedback). Croll's first theory predicts that one hemisphere or the other will experience an ice age whenever two conditions occur simultaneously:a markedlyelongatedorbitand a wintersolstice that occurs far from the sun. Later, Croll hypothesized that an ice age would be more likely to occur duringperiods when the axis is closer to vertical, for then the polar regions receive a smallerannualamountof heat. As time went on, many geologists in Europeand Americabecame more and more dissatisfied with Croll's theory, finding it at variancewith new evidence. Moreover,theoreticalarguments were advancedagainstthe theoryby meteorologistswho claimed that the variationsin solar heating describedby Croll were too small to have any noticeable effect on climate. These are precisely the argumentsused by Arrhenius (14) to refute Croll's theory. Arrheniusentirely agreedwith the following conclusion of de Marchi (15): "Now I (de Marchi) think I may conclude that from the point of view of climatology or meteorology, in the presentstate of these sciences, the hypothesisof Croll seems to be wholly untenableas well in its principles as in its consequences". Then Arrheniusconcluded: "It seems that the great advantagewhich Croll's hypothesispromisedto geologists, viz. of giving them a naturalchronology, predisposedthem in favor of its acceptance.But this circumstance,which at first appeared advantageous,seems with the advanceof investigationratherto militate against the theory, because it becomes more and more impossible to reconcile the chronologydemandedby Croll's hypothesis with the facts of observation". But this preconceived idea of a so resilient climate system is based upon a very partial analysis of the astronomicaltheory. Most of the opponents were considering averages of insolation calculatedover a large fractionof the year-6 monthsor a whole astronomicalseason-which smooth out large variations.Their statements were qualitative; they were not based on physical modeling or they consider a direct (linear) response of the climate system ignoringthe influences of the positive feedbacks. Milankovitch Theory A few decades after Arrhenius'paper,R. Spitalerrejected,also on theoreticalgrounds, Croll's theory that the conjunctionof a long cold winter and a short hot summer provides the most favorable conditions for glaciation. He adopted the opposite view, first put forwardby J. J. Murphy,that a long, cool summer and a short, mild winter are most favorable. This diminution of heat during the summer half year was also recognized by Bruckner,Koippen,and Wegener (16) as the decisive factor in glaciation. Milankovitch,however, was the first to complete a full astronomicaltheoryof Pleistocene ice ages, computingthe orbitalelementsand the subsequentchangesin the insolationand climate. MilutinMilankovitchwas a Yugoslavianastronomer,who was Ambio Vol. 26 No. 1, Feb. 1997 born in Dalj in 1879 and died in Beograd in 1958. He was a contemporaryof Alfred Wegener (1880-1930), with whom he became acquainted through Vladimir Koppen (1846-1940), Wegener's father-in-law(17). Milankovitch's first book dates from 1920, but his massive Special Publicationof the Royal Serbian Academy of Sciences was published in 1941 (18) and was translatedinto English only in 1969. Milankovitch'smain contributionwas to explore the solar irradianceat differentlatitudes and seasons in great mathematicaldetail, producingtabulations and chartsof classical and permanentimportance,and to relate these in turn with planetaryheat balance as determinedby the planetaryalbedo and by reradiationin the infraredaccordingto Stefan'slaw. He arguedthatinsolationchangesin the high northern latitudesduringthe summerseason were critical to the formation of continental ice sheets. During periods when insolation in the summerwas reduced,the snow of the previous winter would tend to be preserved-a tendency that would be enhanced by the high albedo of the snow and ice areas. Eventually, the effect of this positive feedback would lead to the formation of persistentice sheets. A simple linear version of the Milankovitch model would thereforepredict that the total ice volume and climate over the Earth would vary with the same regularpatternas the insolation;this meansthatthe proxy recordof climatevariationswould contain the frequencies of the astronomicalparametersthat are responsiblefor changingthe seasonalandlatitudinaldistributions of the incoming solar radiation.It happens that investigations during the past 20 years have indeed demonstratedthat the 19 ka, 23 ka and 41 ka periodicities,actually occur in long records of the Quaternaryclimate (8, 19). The geological observation of the bipartitionof the precessional peak, confirmed in astronomical computationsby Berger (20), was one of the first most delicate and impressive tests of the Milankovitchtheory. However, the same investigation identified also the largest climatic cycle as being 100 ka. As this eccentricity cycle is very weak in the insolation (11), it cannot be related to the orbitalforcing by any simple linearmechanism(9). TRANSIENTMODELSOF THECLIMATESYSTEM AND THELLNEXPERIMENTS Since the end of the 1970s, a numberof modelling efforts have attemptedto explain the relationbetween astronomicalforcing and climatic change. Most of these modelling studies have focused on the origin of the 100-ka cycle. Although these models are based on parameterizations which areconsideredto be physically plausible, they are all highly simplified. What these models do confirmis that the responseto orbitalforcing is nonlinear and that it involves some internalreactions in the climate system (feedbacks).Whetherthe externalorbitalforcing drives the internal behavior, phase-locks the oscillations of an internally driven system, or acts as a pacemakerfor the free oscillations of an internallydriven system remains,however, an open question. As a consequence, the discussion of how the climate system respondsto orbitalforcing calls for the constructionof a physically realistic model of the time-dependentbehaviorof the coupled climate system, including the atmosphere,the oceans, the cryosphere, the lithosphere and the biosphere. Unfortunately, such a model is likely to be too complex, given the constraints of computing power and speed and our lack of knowledge in the biogeochemistryof the climate system, in particular.This is why more simple 2-D coupled models of the climate system are now tentativelyused to performtransientsimulationsof global climatic changes at the thousandsof years time-scale for testing the astronomicaltheory of palaeoclimates(21, 22). At that time-scale, plate tectonics, mantle convection, mountainbuilding and sun evolution are kept constant.But the more rapidre- ? Royal Swedish Academy of Sciences 1997 33 sponse parts of the climate system, such as the cryosphereand the lithosphere,are included in the models in additionto the atmosphereand the ocean. Such a 2-D climate model (designatedby LLN) has been built in Louvain-la-Neuveand links the NorthernHemisphereatmosphere, ocean mixed layer, sea ice, ice sheets and continents; a full descriptionof the model is given in Gallee et al. (23). The atmosphere-oceanpartof the model is latitude-altitudedependent. In each latitudinalbelt, the surface is divided into, at most, seven oceanic or continentalsurfacetypes, each of which interacts separatelywith the sub-surfaceandthe atmosphere.The oceanic surface types are ice-free ocean and sea-ice cover, while the continental surface types are the snow-covered and snowfree land and the threepotentialice sheets. The atmosphericdynamics is representedby a zonally averaged quasi-geostrophic model, which includes a new parameterizationof the meridional transport of quasi-geostrophic potential vorticity and a parameterizationof the Hadley sensible heat transport.The atmosphereinteractswith the othercomponentsof the climatesystem throughverticalfluxes of momentum,heat and watervapor. The model explicitly incorporatesdetailedradiativetransfer,surface energy balances, and snow and sea-ice budgets. The vertical profile of the upper ocean temperatureis computed by a mixed layer model, which takes into accountthe meridionalconvergence of heat. Sea ice is representedby a thermodynamic model including leads (open cracks in the ice) and a new parameterizationof lateral accretion. Simulationof the present climate shows thatthe model is able to reproducethe main characteristicsof the generalcirculation.The seasonalcycles of oceanic mixed layer, sea ice and snow cover are also well reproduced. Sensitivity experimentsshow the importanceof the meridional sensible heat transportby the Hadley circulationin the tropics, the seasonal cycle of the oceanic mixed-layerdepth and sea-ice formationin latitudebandswherethe averagewatertemperatureis above the freezing point. The atmosphere-oceanmodel is asynchronouslycoupled to a model of the three main NorthernHemisphere ice sheets and theirunderlyingbedrock.The ice-sheet lithospheremodel is latitude and time dependent.The equationfor ice thickness represents the conservationof the ice mass and is a vertically integrated,nonlineardiffusion equationcomputedalong a meridian. The ice diffusivitydependson the ice thicknessandthe ice slope. The lateral ice flow is representedby a diffusivity term calculated for each latitudeby assuming that the east-west profile of the ice sheet is double parabolic.This parabolicshape has been calibratedagainst the 18-ka BP ice sheet. The net mass balance is the differencebetween the calculatedlocal snow precipitation and the local ablation computed from the balance of the heat fluxes at the snow or ice surfaces.The isostatic rebound,i.e. the bedrock-surfaceelevation respondingto the changing ice load, is calculated using a time-dependentdiffusive equation of the asthenospherealong latitude.Over the ice sheets, the model includes the elevation-desertfeedback, which reduces the accumulation to low values as the ice sheet grows high. The snow albedois dependenton the surfacetemperatureandthe snow age. This snow aging-albedofeedback is based on the fact that during daytime,the heat appliedto the snow cover by radiativeprocesses is used for recrystallizationprocesses and subsequentlyfor a decrease in the many facets of the minute crystals of fresh fallen snow, a decrease of the scatteringprocesses and an increase of absorption.Since metamorphismis an irreversibleprocess, the decrease in albedo due to metamorphismresults in an overall decrease in the albedo of the snow cover from day to day. As yet, thereis no carboncycle coupled to the model and CO2 is consideredas an externalforcing in additionto the insolation. The atmosphere-land-ocean seasonal model is run every day over 20 years to reach quasi-equilibrium;the outputis then used 34 to force the ice-sheets model which is run with a time step of 1 year for 1000 years, sensitivity tests having confirmed the adequacyof such choices. Daily insolationfor each latitudinalband is used through the seasons over the whole year to force the model. This model is thereforenot an annually averaged one, althoughannualaveragesof climatic variables,like the ice volume of each ice sheet, are built from the simulateddaily values and comparedto their geological reconstructionsto test the validity of the model. Being given the hemisphericcharacterof the model, it is interesting to note that the NorthernHemispherereceives annually - 10.8 MJ m-2 today, a value which varied by 0.1% over the last glacial-interglacialcycle because of the eccentricity.For the astronomicalseasons, this energy varied between 3.35 and 3.41 MJ m-2 (1.8%) during the summer i.e., between summer solstice and autumn equinox) and between 1.99 and 2.05 MJ m-2 (3%) during the winter i.e., between winter solstice and spring equinox), both with obliquity. But interestingly,the seasonal contrastfor the whole hemisphere,which amountsin average 1.36 MJ m-2 (over a summerplus winter total of 5.4 MJ m-2) has varied with an amplitudeof 0.12 MJ m-2 (- 10%) and a periodicitygovernedby obliquity.The daily value at the summer solstice variedmainly with precessionbetween440 and 520 WM-2 and at the winter solstice between 190 and 230 W m-2 (about20%). A first set of 2-D modelling experiments(21, 24) showed that the variationsin the Earth's insolation alone induce feedbacks in the climate system, which are sufficient to amplify the direct radiative impact and generate large climatic changes (25). On the contrary, if CO2 alone is allowed to vary, the induced feedbacks generate only small changes with no spectral signature at the time scale of tens to hundredof thousandsof years. Initiationand terminationof glacial cycles cannot be explained withoutinvoking both the fast feedbacksassociatedwith atmospheric processes (watervapor, cloud, snow and sea ice) and the slower feedbacks associated with coupling to other partsof the climate system, in particularthe land ice-sheetsbuild-upand disintegration.This confirms the Hays et al. (19) idea that the orbital forcing acts as a pacemakerof the ice ages. But sensitivity experiments to constant CO2 concentrations (Fig. 2) show that the ice volume simulated by the LLN 2-D model compares quite well with its geological reconstructions (in particular,its saw-toothed shape) only if this concentration is about210 ppmv. Because of the sensitivityof the LLN model to the atmosphericCO2concentration,less and less ice is indeed simulated for constant concentrations higher than about 260 ppmv, and the model can not reproduce the 100 ka cycle anymore. On the other hand, a detailed analysis of the model Northern hemisphere ice volume(10o6 km3) 50 0 5... ppmv .210 40L 5?2 250 ppmv.40 (kyrAP) 0 Time (kyrBP) Figure2. Long-termvariationsof the NorthernHemisphereice volume simulatedby the LLN2-D modelforced by the insolation(10) and three constant CO2concentrations(210, 250 and 290 ppmv)from200 ka BP to 200 ka AP. ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997 Northernhemisphere ice volume (10o6 km3) H 10 t / VBll ,1 200 -04--------------- 30 11 ~~~~~~~LLN .. . . . . . . . . . . . ..... _________________ 200 150 100 (ka AP) . . ........... ik [12 .... ... .. .. . . . .. . . . t. 2.0 _________________ 0 Time -2.0 1.2 ............................................................. .... . . . ................ _______ 50 0 -....l..'^---- 40~~~~~~~~~~~~~~~~~~~~~~~~~. --=-- --- - - -.. .. ... .. .. . .. . . 2D 50 . . . . . ........ . . . . . .... . . . . . . . . . 50 ................ f -5 -0 100 (ka BP) -150 -200 Figure 3. Full line: Long-term variations of the Northern Hemisphere ice volume (ice volume increases downward, left-hand scale) simulated by the LLN2-D model forced by insolation (10) and by CO2reconstructed by Jouzel et al. (5) for the past 200 ka and by a CO2scenario based upon this reconstruction for the next 130 ka. Dashed line: 6180 variations (right-hand scale) from SPECMAP (26), a proxy for ice volume changes over the whole Earth, compared to the present-day value. 518Q increases downward. The difference between the LGMand present-day is supposed to correspond to 48.6 x 106 km3 of ice which is equivalent to a sea level 125 m lower than to-day (40). If we assume that there was less ice at the LGM,the corresponding ice volume must be changed accordingly; a sea-level change of 105 m (2) would correspond to an ice volume change of about 41 x 106 km3. output shows that the LLN model is less sensitive to CO2 changes when the amplitudeof the insolation change is large. This is because the modelled climate system is highly sensitive to the albedo, which changes are mainly triggeredby insolation changes. Conversely, the LLN model is more sensitive to CO2 when the insolation changes are rathersmall. This is important becauseover the next 50 ka the insolationchangesareweak (Fig. 1) and, therefore,the climate system might be particularlysensitive to CO2changes. Because the CO2concentrationhas variedin the past, it is importantto see how the response of the LLN 2-D model to the orbitally induced forcing alone is modified by using the reconstructedvariable CO2as an additionalforcing. In Figure 3, the last 200 ka have been simulated using both the insolation and the CO2reconstructionby Jouzel et al. (5). Despite the simplicity of the model, the overall timing of ice volume changes fits, reasonablywell, the geological reconstructions(26, 27). However, discrepancies in the magnitude of the simulated ice volume still remains, like the too large ice melting around 170 ka BP and duringisotopic stage 5. For the future, the model forced by insolation and a natural CO2scenario based upon the Jouzel CO2reconstructionfor the past, simulates a Northern Hemisphere ice volume which remains almost the same as today up to 50 ka AP (27). The next glacial maximum is reached at 100 ka AP and is followed by a partial,but significant,melting of the ice sheets which peaks at 120 ka AP. Sensitivity experimentsto differentfutureCO2scenarios,includingglobal warmingscenariosresultingfrom man's activities,have demonstratedthatthe nonreappearanceof the ice sheets before 50 ka AP is a robustfeatureof the LLN model. C02, WATERVAPOR,ALBEDOAND INSOLATION AT THE LGM Palaeoclimate Sensitivity to CO2 Although it is unlikely that any period of the past would be a satisfactoryanalog for the future,the geological recordcan provide valuable informnation about certain processes operatingin the climate system (28) like the greenhouseeffect. AlthoughunAmbio Vol. 26 No. 1, Feb. 1997 certaintystill surroundsthe mechanismsby which CO2concentrationshave variedover the last interglacial-glacialcycle, there is little doubtthat such changes did occur. Moreover,modelling evidence supportsthe role of CO2as a positive feedbackmechanism. Using an energy balance model, Hoffert and Covey (29), for example, find that the decrease in atmosphericCO2 at the LGM enhanced global cooling by up to 1.5?C. Genthon et al. (30) estimate that the direct radiativeeffect of the CO2changes over the last glacial-interglacialcycle, as recordedin the Vostok core, is 0.6?C, this directeffect being amplifiedby variousfeedback mechanisms such as the water vapor feedback. Allowing for both direct and indirect effects, Chappellazet al. (31) estimate that CO2and CH4changes may have accountedfor about 2.3?C of the total global warming of 4.5 ? 1.0?C, which characterizesthe last glacial to interglacialtransition. Sensitivity experimentshave also indicated that most of the global sea-surfacetemperaturechangeestimatedby CLIMAP(1) is due to the changes of ice sheets and sea ice (32). But as argued by Loehle (33), to estimate the strengthof the greenhouse warmingeffect of CO2from historicaldata requirestaking into account strongmultiple correlationsbetween all the factors like CH4, CO2, water vapor, ocean currents,ice volume, dust and nonseasaltsulfate. The Last Glacial Maximum cooling, an LLN experiment with a I-D Model A similar analysis of the relative importanceof the feedback mechanisms involving CO2, water vapor and albedo has been made with the LLN 2-D model (34); its sensitivityto a doubling of CO2concentrationis a warmingof - 2?C. The cooling at the LGM calculatedby the LLN transientsimulationwith the seasonal cycle of insolation at 18 ka BP and a CO2concentration of 194 ppmv amountsto 4.5?C. When comparedto present-day values, this cooling is associated with an increase of the planetary albedo from 31 to 32.6%, an increase of the global-mean surface albedo from 16.2 to 19.3% and a decrease in the annually-averaged vertically-integratedwater vapor concentration from 2.44 to 1.99 g cm-2. At the LGM, in addition to the insolation and CO2-concentrationvariations,the hemispherically-averagedchanges in the surfacealbedo and in the verticaldistributionof watervaporare thereforeavailableas partof the LLN model diagnostics.These changes were insertedone by one or in combinationinto a 1-D radiative-convectivemodel (34) in orderto investigatethe physical processes responsible for the LGM cooling. As a consequence, the only feedback mechanismin this RCM is relatedto water vapor. The usual convective adjustmentprocedurewas used, but the mean temperatureprofile predictedby LLN was taken to be the critical lapse rate in the troposphere.The main contributionof each factor to the LGM cooling has been calculated using the classical (25) and Stein-Alpert(34) methods of feedback analysis. In this 1-D radiative-convectivemodel, when the CO2 concentrationis doubled (from 330 ppmv to 660 ppmv), the instantaneous perturbationof the net radiative budget (before the action of feedbacks) is estimated to be 3.72 Wm-2 at the tropopause.At equilibrium,the temperatureincreases by 1.8?C at the surface.Withoutallowing the watervaporfeedbackto operate, the warming is reduced to 1.2?C. The sensitivity of our RCM to CO2 change is thus very similar to that calculated in sensitivity studies using other RCMs with the same basic assumptions(35). In order to be able to compute the contributionto the LGM cooling of each parameter,seven simulations were performed. In additionto the present-dayclimate, three simulationsdid not allow the water vapor feedback to operate(Table la) and three did (Table lb). CO2and albedo were each time prescribedand assigned either their present-dayor LGM values. RCM experi- ? Royal Swedish Academy of Sciences 1997 35 ment 1 (Table la) is relatedto the present-dayclimate and leads to a mean temperatureof 288.32 K, 4.5 K above the LGM value of 283.8 K (exp. 5). In experiment3, only the surfacealbedois changedto its LGM LLN value, the CO2and water vapor concentrationbeing kept constantto theirpresent-dayvalue. In such a case, assumingwith Milankovitchthatthe insolationvariationsmodify the snow- and ice covers in the NorthernHemispherehigh latitudes,changing only ar,in the RCM is equivalent to looking for the "natural" cryosphericresponseof the climaticsystem to the insolationvariations. The simulatedcooling of 1.8?C relative to today is importantand correspondsto a direct radiativeforcing of -5.6 W m 2 (i.e. the net radiative budget at the tropopause has decreased-i.e. less trapping-by 5.6 W m2). If we allow the water vapor feedback to operate on this perturbation(exp. 6), the cooling amountsto 3?C. The smaller amountof water vapor in this cooler atmosphereleads indeed to a weakeningof the greenhouse effect. It combines with the larger planetaryalbedo induced by the increase of the surface albedo to produce a total cooling 66% larger than without the water vapor feedback. As a consequence, the water vapor induces a positive feedback mainly through its impact on the greenhouse effect and, in a much smaller amount, through an increase in the planetary albedo relatedto a smallerabsorptionof the solar radiation. If we decrease only the CO2concentrationfrom the assumed present-dayvalue (330 ppmv) to the LGM value (194 ppmv), the climatic response without (exp. 4) or with (exp. 7) the water vaporfeedbackis a cooling of 0.94?C or 1.57?C,respectively. This correspondsto an increase of 67%, a result very similarto the one obtained in experiment6 for the albedo increase. This might be a coincidence which must be tested using more sophisticatedmodels wheregeographicalcontrastsdo exist. The change of the surface albedo at the LGM (exp. 6) is indeed largely related to the change of the surface cover in middle and high latitudes,whereasthe CO2concentrationchange (exp. 7) is a worldwide phenomenon. The two experiments2 and 5 combine the changes in the CO2 concentrationand in the surface albedo. When the water vapor feedback is switched off (exp. 2), the combined effect of these forcings leads to a total cooling which is the sum of the individual coolings: -2.74 = -1.8 -0.94. This is expected througha linearization of the problem in the absence of the main nonlinearityrelated to the water vapor feedback. Allowing this water vapor feedback to amplify the direct initial perturbation leads to a total cooling of 4.5 1?C (exp. 5). Even in this case, CONCLUSIONS Although the seasonal and nonsynchronous nature of the Pleistocene warm periods seems to be mostly accounted for by Milankovitch variations, related feedbacks and changes in the hydrosphereand cryosphere, this result does not question the paradigm of climate sensitivity defined as the ratio of global mean radiativeforcing to global mean temperaturechange, the comerstone of the IPCC projections of future climate change from humanity'sgreenhousegases and aerosols (36). Some critics of the theory of anthropogenicglobal warming have indeed addressedthis question on groundsthat the annual and global mean radiativeforcing from the astronomicaltheory during the ice ages was near zero and yet the climate clearly changed. In fact, the two problems are conceptually totally different. On the one hand, the radiativeforcing of climate, related to the anthropogenicenhancementof the greenhouse effect, involves primarilythe longwave terrestrialradiationof the Earth, absorbedand then re-emittedby a numberof trace gases in the atmosphere.It is a global phenomenonwhere the latitudinaland seasonal componentsof the energy balance do not play a major role as in the Milankovitchtheory. On the other hand, the glacial-interglacialcycles are broughtabout by the long-termvariations of the latitudinaland seasonal distributionsof solar radiation, which is absorbedby the Earth.Then, slow feedbacks of the system cause the build up of ice sheets and the decrease of CO2during glacial cycle initiations, and the reverse during terminations.This means thatthe climate system is consequently forced by global mean albedo and greenhousegas changes. the global mean atmosphericsurface temperature respondingwith the appropriateclimate sensitivity. In other words, it is not the global mean insolation forcing that causes the ice ages, but changes in the insolation distributionover the annual cycle and with latitude that change planetaryalbedo and greenhouse gas compo......~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~... sition, making the astronomicaltheory of ice ages not inconsistentwith the IPCC paradigm of global mean forcing and climate sensitivity. .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~' ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..... This allows use of the feedback analysis technique to quantify the role played by C02, surface albedo and the water vapor feedback. Results show that the combined effect of two .'~~~~~~~~ -.*~~~~~~~~~~~~',. ~ ~ ~ ~ g~-v processes (CO2and albedo) which generatethe same feedback (water vapor) is about equal to the sum of the responses of the climate system when one or the other is kept constant. This type of linearresponsemight be due to the fact thatthe perturbationin each case remainssmall (a few degrees) as comparedto the basic state of reference(- 280 K) (Fig. 4). Cooling for an ice age, using only the astro... . .... ok.~. the total response (exp. 5) is more or less the sum of the individual responses (exp. 6 and 7): -4.51 _ -2.99 -1.57. Although the water vapor feedback introducesa nonlinearityinto the system, this kind of linear response might result from the fact that the magnitudeof the perturbationsis small comparedto the fundamentalstate (a few degrees against 288K). If we define the feedback parameter,X, as the radiativeforcing divided by the temperatureresponse expressed in Wm-2 K-l, it is equal to 3.1 in experiments2, 3 and 4 and to roughly 1.9 in experiments5, 6 and 7. This leads to a positive feedback of the water vapor equal to 1.2, a value close to the value 1.4 obtainedin greenhousetheory (42). It happensthereforethatthe watervaporfeedbackintensityis roughlyindependentof the specific characterof the prescribedchanges. This might be a characteristicof this simple RCM and must be tested with more sophisticatedGCMs. ... ......... . 36 ii ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997 nomical and albedo forcing-but allowing for the water vapor feedback-amounts to 3?C, 67% of the LGM 4.5?C cooling. The water vapor feedback is responsible for 27% of these 67% of the global cooling. In the CO2alone experiment,the watervapor AT AT (00) Figure 4. Summary of the contribution of _ _ _ (%) _ _ 4-5 C02,water vapor (WV)and insolationalbedo (AA) to the 4.50C cooling at the LGMdeduced from the LLN2-D model experiments. __ 1.8 < AA 40 WVF 27 < < L_ 1.2 IL > 1.5 1. > 0.6 WVF 13 0.9 c0 20 + OlN o ~~~~~~20 0 feedback (WVF) explains 13 of the 33% of the global cooling, the direct effect of CO2accountingfor the remaining20%. These results show the importanceof the albedo and of the water vapor, which was alreadybeing stressedby Arrheniusin 1876 (although less than by Tyndall (37)). Arrhenius indeed strongly disagreed with the de Marchi ice age hypothesis that "a lowering of the transparencyof the atmosphereattributedto a greaterquantityof aqueousvaporin the air would effect a lowering of the temperatureon the whole Earth".He argued that "de Marchi has not sufficiently considered the importantquality of selective absorptionwhich is possessed by aqueousvapor". He finally concluded "it is impossible to assume that the absolute humidity could have been greaterthan now in the glacial epoch",a conclusion which has been confirmedwidely over the last 20 years (38, 39). All the results discussed in this paper and obtainedfrom the LLN experiments,in particularabout CO2(41), albedo and water vapor,need to be confirmedusing more completegeneralcirculationmodels. References and Notes 1. CLIMAP Project Members. 1981. 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It seems that the carbon dioxide has been discovered by Jean-BaptisteVan Helmont (1577-1644), a belgian physician and chemist who was teaching at the Catholic University of Louvain. He named it "gaz sylvester"(woodland gaz). In Ortus medicinae (the dawn off medecine, Amsterdam, 1652) published by his son Francois-Mercure, he wrote (p. 86): "... hune spirritum,incognitumhactenus, novo nomine gas voco ... (this spirit,presentlyunknown,I give the name of gaz). 42. Berger,A. and Tricot, C. 1992. The greenhouseeffect. Surv. in Geophys. 13, 523-549. A. Berger has an MSc degree in meteorology from M.I.T.and a PhD from the Universite catholique de Louvainwhere he is professor of climatology and director of the Institute of Astronomy and Geophysics, Georges Lemaitre.His main scientific contributions are in the astronomical theory of palaeoclimates. He has been chairman of the International Commission on Climate of IUGGand of the Paleoclimate Commission of INQUA.He has received the Milankovitch medal from EGS and the quinquennial prize 1990-1995 from the National Fund for Scientific Research in Belgium. M.F.Loutre has a PhD from the Universite catholique de Louvain and is a member of the Institute of Astronomy and Geophysics, G. Lemaftre.Her main interests are the longterm variations of the astronomical parameters and related insolation and modeling Quaternaryclimatic changes. She is presently the secretary of the INQUAPaleoclimate Commission. Their address: Universite catholique de Louvain, Institut d'Astronomie et de Geophysique G. Lemaftre,2 Chemin du Cyclotron, B-1348 Louvain-la-Neuve,Belgium. ? Royal Swedish Academy of Sciences 1997 37