Greenhouse warming, climate sensitivity and Vostok data

Transcription

1 Glaciers-Ocean-Atmosphere Interactions (Proceedings of the International Symposium held at St Petersburg, September 1990). IAHS Publ. no. 208, Greenhouse warming, climate sensitivity and Vostok data INTRODUCTION C.LORIUS (1), J.JOUZEL (1, 2) & D.RAYNAUD (1) 1. Laboratoire de Glaciologie et Géophysique de l'environnement, BP 96, St-Martin-d'Hères 38402, Cedex, France 2. Laboratoire de Géochimie Isotopique, CEA - CEN, Saclay 91191, Cedex, France Y.S.KOROTKEVICH Arctic and Antarctic Research Institute, Beringa 38, St Petersburg , USSR V.M.KOTLYAKOV Institute of Geography, USSR Academy of Sciences, Staromonetny per. 29, Moscow , USSR ABSTRACT One of the main problems in estimating the possible climatic impact of doubled atmospheric CO from Global Circulation Models (GCMs) is to evaluate the sensitivity of the Earth's climate. In this respect valuable empirical data are provided by climate and greenhouse gases concentration records obtained from the Vostok ice core over the last climatic cycle. The temperature profile and CO -CH concentrations are closely related with significant increases of CO (200 to 280 ppmv) and CH (doubled) associated with glacialinterglacial transitions. A statistical approach shows that temperature changes can be essentially explained by a Milankovich type forcing associated with a greenhouse gas forcing. This result is in agreement with GCM simulations of the Last Glacial Maximum, both on a local (Vostok) and global scale. There is a growing concern about the future of the Earth's climate due to the continuing increase in concentration of anthropogenic "greenhouse" gases in the atmosphere. These gases, primarily CO, CH, N 0 and chlorofluorocarbons (CFC's), trap infrared energy within the atmosphere and are thus expected to lead to a global warming (Mitchell, 1989; Dickinson & Cicerone, 1986; Ramanathan, 1988). Human activities over the last hundred years have enhanced the radiative heating of the planet by ~ 2 W m and the radiative forcing corresponding to the often referred to doubling of CO (from 300 to 600 ppmv) is - 4 W rtf, a value to be compared with a mean flux of absorbed solar 29

2 C. Lorius et al. 30 radiation, s, of 240 W m (Mitchell, 1989; Dickinson & Cicerone, 1986). In terms of equilibrium surface temperature change, AT, the radiative forcing As of 4 W m 2 corresponds to - e 1.2 C (inferred from AT /T = 1/4 As/s which means that AT is practically proportional to As with AT * 0.3 As. The principal question is: how sensitive e is the Earth's climate to change of global forcing mechanism, such as abundance of atmospheric greenhouse gases? Climate experiments carried out with General Circulation Models (GCM's) for a doubled CO concentration, yield substantially higher surface warming (AT ), ranging for the most recently performed experiments between 1.9 and 5.2 C (Table 1). This amplification is due to fast feedback processes (i.e. those which respond rapidly to climate change and are thus important on decadal time scales) linked with water vapor, sea ice and clouds. Experiments listed in Table 1 correspond to a climate sensitivity f = AT /AT (as defined in Ha-nsen et al., 1984) ranging from e 1.6 to 4.3. But the uncertainty in climate sensitivity obtained from GCM's may even be somewhat greater than this as indicated by a recent intercomparison of 14 GCM's which showed a roughly threefold variation in climate sensitivity, attributable mainly to differences in the model's cloud feedbacks (Cess et al ). Some simple one-dimensional climate models suggest a sensitivity so small that essentially there is no net feedback (Ramanathan, 1988). Thus the net feedback factor f (i.e. the amplification of the no-feedback radiative equilibrium temperature change) is uncertain within the large range f One implication of the uncertainty in climate sensitivity is an even greater uncertainty in the climate response time. The time required for the climate system to approach a new equilibrium temperature after a change of radiative forcing varies at least linearly with f (Hansen et al ), because the feedbacks come into play in response to the change of temperature, not in response to the climate forcing. A consequence of the large uncertainty in climate response time, which may be anywhere from a decade to more than a century, is that the observed global temperature trend of the past century does not provide a very stringent empirical limit on climate sensitivity; indeed, allowing for the possibility of small unknown climate forcings, such as changes of tropospheric aerosols and solar irradiance, and for unforced climate variability, the temperature trend of the past century does not provide any useful bound on climate sensitivity (Hansen e_t al., 1984, 1985). A more promising source of empirical information on global climate sensitivity is provided by the paleoclimate changes on Earth during the past hundred thousand years, because: (a) the global temperature change was very great, about

3 31 Greenhouse warming, climate sensitivity and Vostokdata 4-5 C; (b) the climate change was linked with changes of greenhouse gas concentration, which by themselves changed radiative forcing by - 2 W m ; (c) unlike the present greenhouse gas changes, changes in the past were maintained sufficiently long for equilibrium to be achieved. TABLE 1 Mean surface temperature change (AT ) and net feedback factor (f) in recent CO doubling experiments adapted from Mitchell et 2 al. (1984) and in addition an extreme experiment of Mitchell e_t al. (1989) in which cloud cover and cloud radiative properties depend strongly on temperature cloud water content. Study Source AT f GISS NCAR GFDL MO OSU MO Hansen et al. (1984) Washington & Meehl (1984) Wetherald & Manabe (1986) Wilson & Mitchell (1987) Schlesinger & Zhao (1987) Mitchell et al. (1989) , Indeed, based on these considerations, there have been earlier attempts to estimate climate sensitivity (Harvey, 1989; Manabe & Broccoli, 1985) from conditions during the Last Glacial Maximum (LGM, years B.P.). Recently ice cores from the Greenland (Dansgaard et al , 1982) and Antarctic (Epstein et. al. : Lorius et al ) ice sheets, and, in particular the Vostok record spanning a full glacial-interglacial cycle, have allowed to document relationship between greenhouse radiative forcing (CO and CH ) and climate over a period of major climatic changes (Barnola e_t al., 1987, 1990; Dansgaard et al , 1982; Epstein e_t al ; Lorius et al., 1979, 1985; Jouzel et al ; Genthon et al., 1987; Raynaud et al ; Chappellaz gt al ). These new data add further support for the orbital theory of ice ages, but they are also of direct relevance to the problem of greenhouse gases and climate sensitivity. Our main approach in this article consists of comparing information about climate sensitivity derived from ice cores with GCM's results. Although a full understanding of the causes and sequences of climate fluctuations remains illusive much information is revealed from comparison of glacial-interglacial paleotemperature data with concurrent

4 C. Lorius et al. 32 changes in the planet's energy balance linked with atmospheric CO and CH concentrations. ] f Age (kyr B.P.) FIG. 1 Variation over the last climatic cycle as derived from measurements along the 2083 m Vostok ice core of: (a) the CO atmospheric concentration (Barnola et al., ); (b) the atmospheric temperature change over Antarctica (Jouzel e_ al ); (c) the CH atmospheric concentration (from Chappellaz et 4 al^, 1990). For CO and CH the envelope shown has been plotted taking into account the different uncertainty sources whereas the temperature record corresponds to a smoothed curve.

5 33 Greenhouse warming, climate sensitivity and Vostok data Interpretation of ice core data suggests that greenhouse gases have played a significant role in explaining the magnitude of past global temperature changes. The data suggest a net feedback for fast processes of f ~ 3, consistent with a climate sensitivity of - 3 to 4 C for a doubled atmospheric carbon dioxide. Studies of paleoclimate changes provide then a clue which helps to validate estimations of the impact of increasing greenhouse gases on climate. ICE CORE DATA Ice data are unique for our purpose in that one and the same core provides access to climate and climate forcings. The climate information includes an estimate of temperature changes at the atmospheric level where the snow has formed and at the surface, while the climate forcing information includes data on atmospheric aerosol amount and chemical composition. So far, there are only two deep ice cores from Greenland (Camp Century (Dansgaard e_t al., 1971) and Dye 3 (Dansgaard e_t al )) and three from Antarctica (Byrd (Epstein e_t al., 1970), Dome C (Lorius et al ) and Vostok) all of which provide long term climatic and environmental records extending over the last ice age. Only Vostok provides a complete undisturbed series over the last climatic cycle. Major findings obtained from these deep ice cores were recently reviewed at a Dahlem Workshop (Oeschger & Langway, 19 89). The Vostok temperature record is reconstructed from the continuous deuterium profile measured along the core (Joouzel et al ). Here, we refer to atmospheric temperature change, AT, just above the inversion layer because: (a) this is the parameter which is most directly accessible from the snow deuterium content which depends on the temperature of formation of precipitation and indeed such an interpretation is well supported by a theoretical approach; (b) this temperature is certainly more relevant to characterize the global atmosphere, as the existence of a strong inversion is restricted to Central Antarctica. The record (Fig. lb) shows the last two glacialinterglacial transitions with atmospheric temperature changes of about 6 C. The last ice age is characterized by three minima, around 20, 60 and 110 kyr B.P.,separated by slightly warmer episodes (interstadials). The three Antarctic ice cores yield remarkably similar isotope-derived temperature records over the last ~ 60 kyr (Jouzel e_t al ). The surface temperature warming associated with the last déglaciation is quite comparable

6 C. Lorius et al. 34 with values slightly below 10 C and the Vostok record can be considered to be representative of a large south polar area. The déglaciation warming for Dye 3 is similar to the one observed in Antarctica (while the Camp Century core suggests a somewhat larger change) and in fact, the main features of the Vostok climatic record are of global significance, at least qualitatively speaking. This significance is suggested by the comparison with the global ice volume changes derived from isotopic measurements in deep sea sediments (during glacial periods, the accumulation of isotopically depleted ice, over the continents, leads to a measurable isotopic enrichment of the ocean). In particular, the SPECMAP S 0 marine record (Martinson et. al ) is meant to essentially represent global ice volume (Fig. 2c), and the Vostok temperature record corresponds very closely (r = 0.87) down to 110 kyr B.P. (Jouzel et al ). Further back there is a discrepancy in the start and the duration of the previous interglacial which can probably be explained by dating uncertainties for this part of the record (see discussion below). The close association between greenhouse gases and glacial-interglacial climatic changes was first revealed from the analysis of air trapped in Greenland and Antarctic cores showing that the atmospheric CO content was around ppmv during the Last Glacial Maximum compared with an average close to ppmv during the Holocene (Delmas e_t al ; Neftel et al , 1985; Raynaud & Barnola, 1985). The detailed reconstruction from Byrd (Neftel et al ) for the 50 to 15 kyr B.P. period fully confirmed the association between cold climate and low atmospheric CO levels. The Vostok ice core allowed to confirm the existence of a strong CO -climate relationship over a full climatic cycle (Barnola et al ). This CO record exhibits large changes between two levels that are centered near and ppmv with the low and high levels associated with full glacial and interglacial conditions respectively (Fig. la). Moreover, despite some noticeable differences, there is a remarkable correlation (r = 0.79) between atmospheric CO and temperature change throughout the record. Such a correspondence between climatic change and radiatively active gases also holds for the concentration in methane. Analysis of Dye 3 ann Byrd samples showed an increase from 0.35 ppmv for the Last Glacial Maximum to Holocene values of 0.65 ppmv (Stauffer e_t al., 1988) and a similar increase which parallels the temperature change was revealed from the Vostok ice for the transition between the penultimate ice age and the last interglacial (Raynaud e_t_ al ). These results are now fully confirmed (Chappellaz e_ al ) by the detailed and continuous CH profile obtained along the Vostok core (Fig. lc) which in addition exhibits four well marked maxima during the glacial period. These maxima occur at

7 35 Greenhouse warming, climate sensitivity and Vostok data the time of relatively warm interstadials and although there are marked differences between the CH and CO record, the correlation with the temperature record is quite similar (r = 0.78 in the case of CH ). There is also a good prospect to obtain reliable N 0 data from ice core. Existing results suggest that during the Last Glacial Maximum N 0 concentrations were possibly smaller but not drastically different from those of the recent period (Zardini e_t al ). We thus have access to an estimate of the direct radiative forcing exerted by all naturally occurring greenhouse gases (except possible changes in ozone). In terms of radiative forcing such changes in the concentration of greenhouse gases are important, e.g.because of the logarithmic increase of CO and CH concentrations. From gla_cial to interglacial this forcing increases by about 2 W m~ (AT of ~ 0.7 C), i.e. half of that corresponding to a CO doubling from 300 to 6 00 ppmv. ICE CORE DATA AND THEORY OF THE ICE AGES The obtaining of the long Vostok ice core offered a unique opportunity to examine from a continental record the link existing between a time series for atmospheric temperature and the astronomical theory of the ice ages. Among the orbital parameters, the obliquity of the Earth's axis (period of - 41 kyr) and the precession of the equinoxes (periods at 23 and 19 kyr) are the most important as they strongly affect the distribution of available energy between latitudes and seasons (Berger, 1988); for example the range of variation of the summer insolation at 65 N, a key latitude in the astronomical theory of the ice ages, is of ~ 60 W m~ over the last climatic cycle. Spectral analysis shows that indeed the Vostok temperature record is dominated by a strong obliquity component and influenced to a lesser degree by the precession (Jouzel et al ). Aside from the kyr interval between interglacials, the main feature of the Vostok temperature record shows that the minima are in good agreement with those of the annual insolation received at the Vostok latitude (78 S) and that there are also similarities with the 65 N summer insolation. These results further support the role of orbital forcing in Pleistocene glacialinterglacial cycles convincingly demonstrated on the basis of deep sea core sediment records (Hays e_t al, 1976). However, the orbital forcing is relatively weak when considered on an annual globally averaged basis (the total insolation received by the planet gas varied by less than 0.7 W m" over the last 160 kyr). The amplification of this forcing, the observed dominant 100-kyr cycle and synchronized termination of major glaciations and their similar amplitude in the Northern and Southern Hemispheres cannot be explained despite developments including the non-linear response of ice sheets to orbital forcing. The

8 C. Lorius et al. 36 discovery of significant changes in climatic forcing linked with the composition of the atmospheric gas has now led to the idea that CO (Barnola ejt_ al ; Genthon et al ; Pisias & Shackleton, 1985; Saltzman, 1987; Broccoli & Manabe, 1987) and CH (Raynaud et al ; Chappellaz e_ al ) changes have played a significant role in the glacial-interglacial climatic changes in amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak global orbital forcing and in constituting a link between the Northern and Southern Hemisphere climates. Note that the observed changes in CO and CH imply modifications in their sources and links likely involving very different processes such as oceanic circulation and marine productivity of CO (see Broecker & Peng, (1989) for a recent review) and extent and fluxes of emission of natural wetlands for CH (Raynaud ejh al ; Chappellaz et al., 1990). The mechanisms behind modifications are not yet fully understood especially for those involving CO but one may note that orbital frequencies are present in each of these Vostok series with, in particular, a strong precessional signal (Barnola e_t al ; Chappellaz et al ). This suggests that those changes are in some way orbitally driven and even if in terms of radiative effect the orbital forcing is not the only one at work, this still supports the idea that orbital changes are one of the initial causes of the ice ages. ICE CORE DATA AND GREENHOUSE EFFECT The glacial-interglacial changes include not only the fast feedback processes associated with water vapor, cloud properties, snow cover and sea ice, but also longer term processes such as those linked with slow changes in several boundary conditions and in the atmospheric composition, which contribute to the maintaining of the ice age cold relative to the interglacial. We now propose to derive information about the respective role of these various slow changes from the Vostok temperature series and its comparison with other paleodata and climatic forcings. Taking then advantage of having access to the direct radiative forcing associated with changes in the concentration of greenhouse gases, information may be derived about the role of fast feedback processes. The important point to realize is that this does not require a solution to the "chicken and egg" problem, i.e. to fully address the question of causes of the glacial-interglacial cycles and of the sequence of possible forcing factors (Genthon e_t al., 1987). For example, it is not relevant for the study of fast feedbacks whether the temperature changes are in advance or behind the changes in CO or CH concentrations. There is however the important constraint that the planet must be near radiation balance with space. This may require several

9 37 Greenhouse warming, climate sensitivity and Vostok data thousand years which does not allow to address through this approach the question of transients or rapid events which may occur in a few decades (Dansgaard e_ al ). Within these limits, we may assume that greenhouse gases have contributed to the glacial-interglacial temperature change through their direct radiative forcing associated with fast feedback processes. As discussed below, such an assumption is well supported by the GCM's experiments described in Broccoli & Manabe (1987) and Rind e_t al. (1989) and we can take advantage of long records of climatic outputs which reflect equilibrium conditions to evaluate or, at least, reasonably bracket, the climatic role of greenhouse gas radiative forcing in the past. Genthon e_t al. (1987) followed such an approach in performing a multivariate analysis between the Vostok temperature record and three climatic inputs. This approach aims at decomposing the temperature variance into a part which can be counted from the inputs plus a residual due to noise not related to these inputs. The relative contribution of each input is calculated in such a way that the noise is minimized in a least square sense. It should be noted that the various climatic inputs must be independent, which otherwise introduces an uncertainty since the multivariate analysis algorithm will have more difficulties to partition the output series as a combination of the input series. As for climatic forcings, Genthon e_t al. (1987) used CO, a Southern Hemisphere component represented by the local (78 S) annual insolation (Fig. 2d) and a Northern Hemisphere component derived either from the 65 N July insolation or taken as 18 the S 0 marine record, a proxy of the Northern Hemisphere ice volume. We built on this approach and extended it in accounting for the following points. First such a multivariate analysis implicitly assumes that the system can be described by its steady state behavior. This is not the case for the climatic system affected by non-linearities associated with the growth and decay of the ice sheets. To account for this, we focus the discussion on the cases in which the ice volume reconstructed from the 5 0 record is taken as a proxy of the Northern Hemisphere forcing. This proxy combines the orbital insolation forcing and the above mentioned nonlinearities. We thus may assume that it represents a large part of the slow feedbacks associated with the changes in boundary conditions as much as the increased area of ice sheets provides a large forcing (- 3 W m mainly due to albedo effects but also to topographic changes). Second, we may now account both for CO and CH changes to evaluate the combined greenhouse radiative forcing. For this purpose we use the CO and CH Vostok records and calculate the associated direct radiative forcing through the formulas given by Hansen e_t al. (1988) which has for CH to be augmented by a factor of ~ 2 due to chemical feedbacks linked with the increase of stratospheric water vapor (Chappellaz e_t al ).

10 C. Lorius et al O -2 O AT a h- < -4 ~ 1 A A l\ / \ II/ 1/ 1-6 -\ * ^J raam *^~V _ AT e A A 7~v / \/ \ /» \ ~ / V \ < icevoiume Age(kyrB.P.) FIG. 2 Time series of the Vostok climatic record and of climatic forcings used in the multivariate analysis: (a) atmospheric temperature change over Antarctica (Vostok record), AT ( C) ; (b) the direct greenhouse radiative forcing accounting for CO and CH variations, AT g ( C); (c) the SPêCMAP record taken as a e proxy of ice volume change (normalized value from Martinson et al ); (d) percentage change in total insolation at the Vostok latitude (78 S) during the entire year. Vostok derived curves (a and b) have been redated for the period before 110 kyr B.P. in such a way as to put in phase the Vostok temperature and the marine record. j_

11 39 Greenhouse warming, climate sensitivity and Vostok data The total greenhouse radiative forcing expressed in terms of equilibrium temperature change (with no climatic feedback) given in Fig. 2b shows a glacial-interglacial amplitude of ~ 0.7 C. Third, we found useful to explore a wider range of possible atmospheric radiative forcings such as those resulting from changes in the aerosol loading (Harvey, 1989) or (Charlson e_t al., 1987), in the amount of Cloud Condensation Nuclei (CCN). A strong increase (up to a factor of 30) in the dust fallout has been recorded for the Last Glacial Maximum both in Greenland and Antarctic cores (Petit et al ). It has been claimed that the resulting increase in the atmospheric aerosol optical depth could make a significant contribution to the climatic cooling during this period (Harvey, 1989). This is still uncertain because it is difficult to evaluate the global aerosol loading from available data essentially coming from polar ice cores. Also change in optical depth and thus in direct radiative forcing strongly depends on the dust optical properties which are not known. Analysis of the Vostok core (Petit et al ) further showed a strong dust increase at the end of the penultimate ice age and during the cold interstadial centered around 60 kyr B.P. suggesting also a relation between cold periods and high aerosol loading. However the Vostok dust record is very spiky and not simply related with the smoothest temperature record. At this stage we may note that there is no dust peak during the cold period around 110 kyr B.P. and that the correlation (r = 0.36) with the temperature record is well below the corresponding one for greenhouse gases. On the other hand, the hypothesis that the number concentration of CCN, mainly the sulfate particles produced by the oxidation of dimethylsulfate emitted from the ocean, influences marine stratus cloud albedo and hence global climate was recently proposed (Raynaud et al ). In this hypothesis change in marine productivity may eventually act as a climatic forcing through the induced change in CCN concentration. Using the Vostok non-sea salt sulfate as a proxy of CCN concentration, Legrand et. al. (1988) suggested that this forcing may have contributed to the glacial cooling. The correlation between non-sea salt sulfate and Vostok temperature is in fact relatively high (r = 0.63), but it must be noted that the role of CCN and the CCN-sulfate link remain to be firmly established. Finally, we reexamined the previously noted influence of the dating (Genthon et al ). Indeed, the multivariate analysis must be done with all time series having a common time scale. There is no problem for the temperature, non-sea salt sulfate and dust Vostok series and, to the extent of the uncertainty associated with the correction due to the air-ice difference, for the greenhouse forcing. However there is a noticeable dating discrepancy between the Vostok and ice volume records

12 C. Lorius et al. 40 before 110 kyr B.P. which was shown to significantly affect the results of the multivariate analysis performed by Genthon et al. (1987). We further tested numerous redatings of the oldest part of the Vostok dust record which now suggests that the marine and Vostok records are roughly in phase before 110 kyr B.P. (Petit et. al ). The new multivariate analysis was then performed using five forcing factors: greenhouse gases, dust and non-sea salt sulfate concentrations, ice volume and local insolation. The results may be summarized as follows: (a) There is no case for which the contribution of the greenhouse effect is below 40%. The lowest calculated value is 42% when the Vostok record is exactly put in phase with the marine S 0 record with a similar contribution of the ice volume (45%) and with - 5% or less for each of the three climatic inputs. One may note that with this redating Genthon et. al. (1987) arrived to a CO contribution of 35%. Indeed, for all 2 the analyses performed, adding CH leads to a 10% increase in the contribution of the combined greenhouse effect with respect to that derived using CO alone. This may be related with the higher correlation of the Vostok temperature with the combined greenhouse fr = 0.84) than either with CO (r 2 2 = 0.79) or CH (r = 0.78). (b) Within the limits brought on the dating by the comparison of the Vostok dust record with marine records, the greenhouse contribution never exceeds 65% with the sum of greenhouse and Northern Hemisphere forcing being generally over 80%. Generally above 90% of the Vostok temperature variance is explained by the 5 climatic inputs. (c) The greenhouse contribution falls in the highest part of this 40-65% range (i.e. between 55 and 65%) when Labeyrie et al. (1987) used ice volume record instead of the SPECMAP curve to represent the Northern Hemisphere forcing. This alternative approach was followed because the Labeyrie et al. curve probably better represents the ice volume change than the more generally used SPECMAP record, because it is corrected for a part of the isotopic signal which comes from the change in ocean temperature. Also, there may be at least for CO (Barnola e_t al., 1987) some link between sea level (i.e. ice volume) and atmospheric concentration change and using different ice volume curves is one way to estimate the bias introduced in the multivariate analysis by two of the input series (greenhouse and ice volume) not being fully independent. To sum up, the contribution of greenhouse gases of the Vostok temperature change can be bracketed between a lower estimate of 40% and a higher estimate of 65%. This however holds true within certain limits that we now recall. These limits are associated first with the non-linearities

13 41 Greenhouse warming, climate sensitivity and Vostok data linked with the growth and decay of the Northern Hemisphere ice sheet, second, with all the series not having a common time scale and third with the possibility that there may have been other important forcings that we have not taken into account. We have attempted to account for the first two aspects. In addition, the introduction of a new climatic input which first must correspond to a process having a potential climatic input, will significantly alter the result of the multivariate analysis only if this new series is well correlated with the Vostok temperature and independent from the other inputs. One example of such forcings is that linked with vegetation change through its influence on the albedo, but model results (Hansen e_t al ; Broccoli & Manabe, 1987) show that the climatic role of the vegetation is relatively minor in maintaining the glacial climate. Although this may not be definitely ruled out, it is indeed difficult to imagine one additional climatic forcing which met these requirements. So and this will be sufficient for the present discussion, and we can say that, within the limits of our multivariate analysis, - 50 ± 10% is a reasonable estimate for the overall contribution of the greenhouse gases to the Vostok temperature change over the last climatic cycle. This means that 3 of the 6 C in the glacialinterglacial change at Vostok could be attributed to the greenhouse effect. Interestingly, this result well agrees with the GCM simulations for the Last Glacial Maximum performed by Broccoli & Manabe (1987). Using a model with prescribed cloud cover, these authors reduced CO concentration from 300 to 200 ppmv in experiments with and without changes in other boundary conditions. The CO change alone, which in terms of radiative forcing roughly corresponds to the combined CO and CH glacial-interglacial change measured in ice cores, results in a temperature drop of 2 to 2.5 C in Central East Antarctica but with variable cloud, the GCM sensitivity is increased by 31% (Manabe & Broccoli, 1985) implying a change of ~ 3 C over this part of Antarctica. Although it is based on one specific GCM and experiment, this agreement between two independent approaches is worth to be noted. ICE CORE DATA AND CLIMATE SENSITIVITY Despite the large geographical significance of the Vostok temperature record, it may certainly be argued that looking at one particular site or area is too restrictive. Other paleoclimate data and climate simulations are indeed helpful to examine the problem of greenhouse gases and climate sensitivity in a global perspective. First, we note that due to the general polar

14 C. Lorius et al. 42 amplification of climatic changes, the Vostok inferred glacial-interglacial warming of - 6 C over Antarctica is well consistent with a globally average value of C; for example, for doubled CO GCM experiments this amplification is on the average of ~ 50% for high latitude continental regions (Ramanathan, 1988). This suggests to extend the important conclusion that greenhouse effect has had a significant climatic contribution (of the order of 50%) over the last climatic cycle to a global estimate. This key step is clearly supported by the similarities observed between the Vostok temperature and paleorecords of a global character. This is illustrated through the multivariate analysis which shows that generally above 80% of the explained Vostok temperature variance is due to climatic forcings of global significance (ice volume and greenhouse gases). Similarities between the Vostok and other paleotemperature records further support this conclusion. For example the Vostok temperature is well correlated with the oceanic surface temperature at the Indian ocean site RC (Jouzel et al ) (with r 2 = 0.66 for the period before 110 kyr B.P. for which redating uncertainties are not critical). Indeed replacing the Vostok by the RC temperature as climatic output in the multivariate analysis, we still get a contribution of the greenhouse gases to the explained variance of ~ 50% or more. Such an approach would suggest that greenhouse gases have contributed for about 2 C to the global warming between glacial and interglacial periods. Estimating climate sensitivity then simply consists in dividing the estimated global warming associated with the greenhouse forcing by the corresponding AT (0.7 C from glacial to interglacial) to get the net e feedback factor. At this step, it is important to stress the independent character of this method with respect to that based of GCM's results. Although GCM's simulations are useful to go from a regional to a global scale this usefulness clearly does not introduce any circularity in our reasoning. Second, it is of interest to compare the relative contribution of the greenhouse forcing independently derived from the multivariate analysis and from GCM's. These results, only available for the GFDL and GISS models, suggest that a significant part of the glacial-interglacial warming may, at a global scale, indeed be attributed to the greenhouse effect. In the above mentioned Broccoli & Manabe (1987) experiments, the CO increase from 200 to 300 ppmv and associated fast feedbacks account for - 40% of the glacial-interglacial warming at a global scale. These experiments also showed that most of the remaining warming is due to the retreat of the Northern Hemisphere ice sheet in agreement with our multivariate analysis. Using the GISS model, Rind e_t al. (1989) recently mentioned that an average warming of 2.5 C would result from a CO increase from 230 to 300 ppmv. This corresponds to at least - 50% of the glacial-

15 43 Greenhouse warming, climate sensitivity and Vostokdata interglacial warming predicted by the GISS model for various Last Glacial Maximum simulations (Hansen e_ al ). Although consensus is not a perfect guide in science, this general consistency supports the belief that the net effect of the fast feedback processes associated with water vapor cloud and sea ice which contribute to the amplification of the direct greenhouse radiative forcing or at least their combined effect, are relatively correctly evaluated in these two models (or in models having similar climate sensitivity). To the extent that atmospheric feedback processes are independent of the detailed causes of climatic changes but governed by the overall climatic response, the feedback processes would operate now as during the Last Glacial Maximum (Harvey, 1989). Applying the net feedback factor of 3 deduced from ice core data to the current climate may be done within any supplementary assumptions (at least within the limits of uncertainty attributed to our method of estimating f ). As for greenhouse model experiments, climate sensitivity depends primarily upon fast feedback processes. Analysis of ice core results and paleodata over a full glacial-interglacial cycle suggests that a doubled CO induced temperature warming of 3 to 4 C (f - 3) may be a realistic figure which, although being in the middle range of values inferred from GCM's experiments, corresponds to a relatively high climate sensitivity. CONCLUSION In this study we have attempted to approach the evaluation of climate sensitivity to greenhouse gases (now a major problem) from the point of view of both the modellers and paleoclimatologists. Despite the relative agreement of the two approaches there is still much progress to be done and our assertion about the irrelevance of fully addressing the question of causes of the glacial-interglacial cycles for estimating climate sensitivity must not be misunderstood. We obviously attach the utmost importance to a full understanding of the physical, chemical and biological processes by which subtle changes in insolation are amplified to induce long-term changes in global climate. In this respect, the knowledge of the sequence of events and the exact timing of forcings and of their climatic responses in various parts of the Earth system is essential. A strong interaction between data acquisition and interpretation and modeling of paleoclimatic and paleoenvironmental changes, including atmosphere, ocean, cryosphere, hydrosphere and land processes and various biogeochemical cycles is also needed. Beyond GCM's which produce steady state simulations a hierarchy of time dependent models ranging from simple or even conceptual (Imbrie & Imbrie, 1980; Saltzman, 1987; Le Treut et al., 1988) to more complex two-dimensional models (Berger et al ), can certainly provide information on how

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