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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
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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.
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Korotkevitch,Y.S.and Kotlyakov V.M. 1987. Vostok ice core: the climate responseto
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insolationat the last glacial maximum.ScientificReport, 1996/6, Institutd'Astronomie
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Maskell, K., 1996. Climate change 1995-the science of climate change. Contribution of WG] to the Second AssessmentReport of the IntergovernmentalPanel on Climate Change. CambridgeUniversity Press, Cambridge,pp. 572.
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41. 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