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Global changes during the latestglacial‐interglacial cycleV. M. Kotlyakov & K. LoriusPublished online: 23 Dec 2008.

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GLOBAL CHANGES DURING THE LATEST GLACIAL-INTERGLACIAL CYCLE

V. M. Kotlyakov and K. Lorius

From: Izvestiya Rossiyskoy Akademii Nauk. Seriya geograficheskaya,No. 1, 1992, pp. 5-22.

Analysis: This review article summarizes the results of Soviet-French investigationsinto the ice core from a deep drillhole at Vostok Station in Antarctica. Changes in airtemperature, snow accumulation, greenhouse gases, aerosols and other chemical com-ponents in the environment are traced over 160,000 years, i.e., over a full climatic cycle.Orbital and atmospheric impacts on the climate and on the role of greenhouse gases inthese processes are analyzed. On the basis of these analyses, it is predicted that with adoubling of atmospheric CO2 (which many scientists believe to be highly probable)temperatures may rise by 3-4° C; this in turn could lead to a massive collapse of theworld's marine ice sheets and to a sea-level rise of 5-7 m; mountain glaciers in temperateand subtropical latitudes would almost entirely disappear.

INTRODUCTORY CONSIDERATIONS

The idea is steadily gaining strength that the future global climate depends onthe increase in the concentration of greenhouse gases in the atmosphere. Thesegases, primarily CO2, CH4, N2O, and chloroflurocarbon combinations, absorbtherrr al radiation from the Earth's surface more than solar radiation; part of thistherrral radiation is returned to the surface, leading to global warming [1-3].

Prediction of the future warming produced by the greenhouse gases depends onthe sensitivity of the global climate to the increase in atmospheric concentrationsof those gases. Experiments with global climatic models produce very indeter-minate results: the mean change in the equilibrium temperature, equivalent to adoubling in CO2, which corresponds to a radiation impact of 4.3 W/m\ variesbetween < 2° and >5°C. This indeterminate result depends, in particular, on thedifficulty in estimating the reverse connections, e.g., cloudiness, which mayrepresent a reaction to the controlling radiation effects produced by an increase inthe concentration of greenhouse gases. Under these conditions the study of actualchanges in global climate during the past several hundred thousand years repre-sents an important source of information, since global changes in temperatureduring this period are extremely large and correlate well with changes in thecontent of greenhouse gases.

The natural variability in the climatic system during the past 800,000 years isillustrated schematically in Figure 1 in the form of fluctuations in the mean

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Polar Geography and Geology, 1992,16, 2, pp. 89-113.Copyright © 1992 by V. H. Winston & Son, Inc. All rights reserved.

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atmospheric temperature at the earth's surface [19]. This period is characterizedby major climatic changes associated with the onset of the glacial epochs. Quanti-tative reconstructions of the climatic parameters at this time scale are clearlyreflected in the oceanological data, obtained mainly on the basis of isotopic andfaunistic studies of deep-sea sediments. The most characteristic peculiarity of thecurve in Figure 1 is the existence of a cycle with a wavelength of about 100,000years, which is difficult to explain by the peculiarities of orbital influences, follow-ing the Milankovich theory, even taking into account the nonlinear response ofthe glaciers [22, 29]. It is equally difficult to understand how relatively weakorbital influences, reaching less than 0.7 W/m2 over the course of a year, canprovoke a globally averaged change in annual temperature of 4-5° C, typical of thetransition from the glacial periods to the interglacials.

At the same time the pattern of temperature during the last few hundreds ofthousands of years provides evidence that the climatic changes were linked tochanges in the concentration of greenhouse gases, which of themselves gave rise tofluctuations in their radiative impact of approximately 2 W/m2; the fluctuations inclimate were extremely prolonged and, clearly, were able to attain equilibrium,something which is very unlike the present changes in the content of greenhousegases in the atmosphere.

New ideas as to the glacial epochs and as to the sensitivity of the climate,associated with changes in the composition of the atmosphere and in concentra-tions of greenhouse gases, have been derived from the study of deep glacier cores,especially the core from a drillhole at Vostok Station in Antarctica, which entirelyembraced a glacial-interglacial cycle [7, 8, 9, 14]. In addition to paleooceanologi-cal and glacial data, data on pollen and on lake levels provide important informa-tion for continental areas. Palynological data, which until very recently werelimited to the last 20,000 years, now extend usefully to much longer periods.

Although the data contained in the ice cores have their limitations, the possibili-ties for using them for paleoanalysis now seem almost limitless. Well-known

600 400Age, 103 years B.P.

200

Fig. 1. Changes in temperatures on earth over the past 8,000 years [9].

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limitations are associated with the fact that deep coring of ice is possible only inpolar ice sheets, and that due to the great difficulties associated with extractingthem, deep cores, extending back to the last glacial epoch are so far extremelyrare. Moreover, in the layers deposited prior to the Holocene, absolute dating ofthe ice is impossible; one has to determine its age on the basis of its dynamicmodels and of comparing it with other paleodata [30], which lowers the accuracyof the aj;e determination.

However, these limitations are more than compensated for by the unique possi-bilities of investigating the ice core. First, in a number of cases it has been possibleto obtai i high resolution: A considerable part of the Holocene has been investi-gated year by year from a core from DYE-3 in Greenland, while in the drillhole atVostok Station the last interglacial is represented by 300 m of ice. Second, on thebasis of the core data it is possible to obtain an uninterrupted series of quantitativedata on very important climatic parameters: air temperature, precipitation,sources of moisture, and wind strength. Third, the possibility arises of obtainingunique direct records of changes in the composition of the former atmosphere,including the all-important gases, chemical inclusions of both continental andmarine origin, traces of other components and isotopes and volcanic aerosols.Some of these elements may participate directly in the causes controlling climaticchanges (CO2, CN4, and other gases, aerosols, and volcanic activity), and these areall important for understanding biogeochemical cycles.

So far we have available only two deep glacier cores from Greenland (fromCamp Century and DYE-3) and three from Antarctica (from Byrd Station, DomeC, and Vostok). They all represent sources of long-period data on the last glacialepoch, a lthough only Vostok represents a complete, uninterrupted series spanningthe whole of the latest climatic cycle.

THE VOSTOK STATION CORE AND ITS PALEOGEOGRAPHICSIGNIFICANCE

Vostok Statiaon is located in the central part of East Antarctica, at 72°28'S,106°48'E, at a height of 3,490 m above sea level. The present mean annual temper-ature is -55.5° and the annual accumulation is 2.3 g/cm2. This is an ideal site forobtaining long-term data and more than 20 years ago the Soviet Antarctic Expedi-tion bej;an drilling a series of deep drillholes and extracting the cores. The first twodrillhohs (1G and 2G) reached depths of 950 m in 1974. A third deep drillhole(3G) was begun in 1980 and reached a depth of 2,083 m in 1985; thereafter it wascontinued to a depth of 2,200 m. Drilling of drillhole 4G was begun in 1985 andwas discontinued at a depth of 2,546 m. The drilling of a new drillhole is nowunder way; it has already considerably exceeded 1,000 m in depth. The main dataused in this article were derived from a study of the core from drillhole 3G.

The variation in the age of the ice with depth, established on the basis of thedynamic model, is illustrated in Figure 2. The fairly straight-line relationshipbetween depth and age in the vicinity of Vostok Station is associated with the factthat the bottom of the drillhole is still a long way from the basal layers of the icewhere any complications resulting from its movement and deformation would belikely. The range of accuracy in determining the age of the ice at a depth of 2,080

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500

-1

1000 /,mFig. 2. Section through the Antarctic Ice Cap from Vostok Station to the coast, showing changes

in temperature (7), in accumulation (A), and ice flow velocity (v) along that profile; h) height; /)distance from the coast. 1) Ice flow vectors; 2) isolines of age, 103 years.

m (160,000 years, according to our calculations) is 10,000-15,000 years. For iso-tope investigations a continuous column of samples was used, while the otheranalyses were made at specific intervals: ice samples 1.5-2 m long were taken every25 m.

What is the overall significance of the data obtained at Vostok Station? Thecurve of past temperature, based on isotopic data, confirms the existence of a linkbetween Pleistocene climate and orbital changes, and at the same time reveals aclose connection with variations in the concentration of CO2. This connection

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proves that the global climatic system and the carbon cycle are tightly interlinked.Although the mechanisms of this interlinkage are still not finally understood, astatistical comparison of the temperature record from the Vostok Station corewith a linear combination of various controlling factors leads one to the conclu-sion that CO2 played an important role in intensifying the relatively weak changesin insolation which, probably, led to the Quaternary glacial-interglacial cycles.Moreover, these data document very well the changes in the rate of accumulationand the iate of air transport.

CHANGES IN ISOTOPE TEMPERATURE AND SNOWACCUMULATION

The profile of the deuterium content in the ice [25], with maximum and min-imum values of about -420 and -480% with respect to the ocean water standard(SMOW) is linked to the isotopic fractionation which takes place at each changeof phase in the hydrological cycle. This occurs due to the minor differences inphysical properties (difference in vapor pressure during phase changes and molec-ular diffusion in the air) of the isotopic (HDO and H2

18O) and main (H216O)

components of water [5, 16].In the polar regions the isotopic composition of snow depends essentially on the

temperature at which it forms. In that there is a strict correlation, in terms ofAntarctica, between the temperature above the inversion layer where the precipi-tation is formed and the surface temperature, the isotopic composition of the snowmay be linked to that of the surface. This is the basis for the paleotemperaturereconstructions, although such a linkage has some limitations due to the impactupon it of parameters such as the peculiarities of the local temperature, isotopicnoise of different origins, and changes in the thickness of the ice. Nonetheless, thetemperature recommendations for the central part of East Antarctica according tothe ice core data allow one to state confidently that, first, a good linear connectioncan be observed between the mean annual surface temperature and the deuteriumcontent of the snow in East Antarctica, and, second, the ratio of 6%o to 1°C agreeswell with what was obtained by modeling. After correction of the data on theisotope changes experienced by sea water during the transition from glacial tointergla:ial epochs, the deuterium curve can easily be transformed into a record ofatmospheric temperature.

The uninterrupted profile of changes in deuterium content (which covers 85% ofthe length of the core) is illustrated in Figure 3a where every point represents 1-2m of ice. Stages A-H represent warm and cold periods, which are characterized ingreater detail by Komlyakov and Lorius [7] and by Lorius, Jouzel, Ritz, et al. [32].Stages A and G represent the Holocene and the last interglacial, and B through Fthe last glacial epoch, whereby stages C and E display relatively warm interstadialswithin the cold glacial conditions. Stage H is the last part of the previous glacialepoch.

Usinj; the relationship of 6u/oo/°C and making corrections for isotope changes insea water from cold conditions to warm, we converted the isotope curve into arecord of temperature and carried out a smoothing procedure, in order to elimi-nate oscillations of very high frequency. The resultant temperature curve (Fig. 3b)

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94 V. M. KOTLYAKOV AND K. LORIUS

demonstrates that the rise in temperature associated with the last deglaciation wasequal to approximately 9° C; this agrees well with the figure obtained for Dome C inAntarctica. One should add that the deuterium curve from Vostok Station differslittle from analogous data from Byrd Station and Dome C. Thus the Vostok datamay be considered fully representative for a considerable part of Antarctica.

500Depth, m

1000 1500 2000

100 150Age, 103 years

Fig. 3. Results of studies on the ice core from Vostok Station: a) deuterium profile, withclimatic stages after [32]; b) smoothed temperature profile; deviation of temperature atglacier surface from present values [25]; c) oxygen isotope profile based on deep-sea deposits[33]; d) CO2 content (parts per million by volume) (best estimates shown by thick line; thinlines display limits of accuracy [9]); e) 10Be concentration [37]. For stages of the Holocene,A-H, see text.

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POLAR GEOGRAPHY AND GEOLOGY 95

One should note several characteristic features of the curve of isotope tempera-ture over the duration of the 10,000-year epoch: a) the last deglaciation wasinterrupted by two coolings of about 2°C lasting about 1,000 years; b) over theduration of the last glacial epoch three temperature minima are clearly discernible,separated by two interstadials when the temperature was 4° and 6°C higher thanin the late Pleistocene, while conditions analogous to the late Pleistocene glacialmaximum are found only at the end of the preceding glacial around 150,000 yearsB.P.; c) the peak of the last interglacial was approximately 2°C warmer than theHolocene.

Now we shall compare the temperature curve from Vostok with the records ofthe oxygen isotope content in sea water (Fig. 3c) [33]. The latter value indirectlycharacterizes changes in the global volume of ice, which is reflected in the oxygenisotope composition of deep-sea deposits: During the glacial epochs the accumula-tion of isotopically light ice on the continents leads to a measurable isotopicenrichment of the ocean. Figures 3c and 4 demonstrate the excellent correspon-dence of the two isotope curves from Vostok Station and from the SouthernOcean, right back to 110,000 years B.P.; the coefficient of correlation betweenthese two curves r = 0.94 [25]; one should recall that the two curves were datedindependently.

The transition from the late Pleistocene culmination of the glacial epoch to theHolocene occurs in both curves around 15,000 years B.P. Both curves agree wellthrougli the duration of the entire glacial epoch; cold stages from Vostok corre-spond io marine stages 2, 4, and 5d. The first interstadial corresponds to stage 3,the second, with clearly marked temperature maxima, to stages 5 and 5c. In thatthe marine curve reflects changes in the volume of all continental ice, i.e., itrepresents a parameter of global significance, the high correlation with data fromVostok indicates that the latter also reveal global information, at least at a qualita-tive level.

This agreement is not observed prior to 110,000 years B.P. The last interstadialdiffers significantly in the two cores; it was almost twice as long at Vostok than theoceanic data would indicate. The cause lies most probably in the imprecision ofthe dating of the older part of the oceanic curve. But the divergence just noted inno waj reduces the global significance of the data from Vostok.

The temperature of the water surface, recently reconstructed at 55° S in theIndian Ocean, confirms the major role of temperature change in the high latitudeswith regard to the global volume of ice. The analogous pattern in the records ofthis isotope temperature and in the profile from Vostok Station allows one tosuggest common stratigraphic frameworks for the glaciological and oceanologicaldata.

In the dynamic model used for calculating the age of the ice at Vostok Stationwe took into consideration changes over time in the rate of accumulation inAntarctica, proceeding from the assumption that the rate of precipitation accumu-lation on the Antarctic Plateau depends on the amount of water vapor in theatmosphere circulating above the inversion layer. The amount of water is con-trolled mainly by the water vapor pressure during phase changes, i.e., the tempera-ture of the air masses; the vapor pressure decreases exponentially with tempera-ture. If one ignores possible changes in the air flows over the Antarctic Plateau, an

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120 140

Time scale for Vostok, 103 years

140103 years

Fig. 4. Temperature curve based on ice core from Vostok Station (a) compared to the curve ofsurface temperature in the Southern Ocean (b) and changes in sea level reflecting fluctuations in thevolume of continental ice (c) (derived from data from deep-sea deposits).

assessment of the changes in snow accumulation becomes directly dependent onthe isotope temperature; this allows one to hypothesize a 50% reduction in accum-ulation during the last glacial maximum as compared with the present value [7,32].

Although the dependence of the amount of precipitation on the air temperatureis also clearly confirmed by present-day data from Central Antarctica [4] and bycalculations using global climatic models, an independent confirmation is pro-vided by using 10Be data [26, 37]. The concentration of this cosmogenic isotope atVostok was absolutely identical during both interglacials and approxmately twiceas high during the glacial epochs (Fig. 3e). Two I0Be peaks around 35,000 and60,000 years B.P., each lasting 1,000-2,000 years, are superimposed on the generaltrend. They may be the result of 10Be production in the atmosphere when thegeneral glacial-interglacial changes probably reflected lower rates of accumulationduring a glacial. Apart from these two peaks there is good agreement between the

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rate of accumulation, estimated from the assumption as to the constancy of theflow of 10Be and that derived from data on the isotope temperature (cf. Fig. 3b, e).Although in both cases there is the possibility of an influence from changes in theatmospheric circulation, the general agreement between the data obtained by thetwo methods allows one similarly to estimate the changes in past accumulation.

One might say that determination of changes in the accumulation is of localrather than global interest. This is true over a short time period, but not on thescale cf the glacial epochs. The growth and decay of ice sheets depends strongly onthe rat e of accumulation and hence this relationship in the polar ice caps repre-sents £.n important parameter in long-term global changes. Apart from this theseresults illustrate the general trend toward increased continental aridity during thelast glacial epoch [15].

Gremhouse gases in the ice core. When firn is converted into ice, atmosphericair becomes locked in bubbles. Hence by extracting the gas from the bubbles oneis able to obtain data on the past composition of the atmosphere, and, in particu-lar, on the content of greenhouse gases. These data, obtained during the 1980s inGrenoble and Bern, have led to the following important results [24]: a) during thelast glucial maximum the content of CO2 in the atmosphere was 35-40% lower andthat o:* CN4 40-50% lower than during the Holocene; b) the increase in the amountof CO2 due to anthropogenic activity since the start of the 19th century is clearlyreflected in the ice; and c) rapid changes in CO2 were measured in the Greenlandcore between 30,000 and 40,000 years B.P. and abrupt changes in CN4 in theVostok core during the last deglaciation.

During the past 5 years these results have stimulated new research focusing onthe CO2 biogeochemical cycle, and these are leading to new ideas as to the role ofcarbon dioxide in climate and are emphasizing the importance of glaciologicaldata i i the study of global change [11]. The ice core from the drillhole at VostokStation has produced data which are new in principle. Measurements were madeat 66 depths, which corresponds to a resolution in terms of the time between twoadjacent samples of 2,000 to 4,500 years. The best estimates of the content of CO2and CN4 are plotted on Figure 5, with an indication of the corresponding accuracyof measurement [9,14]. The age scale on the CO2 profile was designed taking intoaccount the period involved in the enclosure of the bubbles in the process ofconversion of firn into ice.

The data on CO2 demonstrate the significant differences between the levelscenteied near 195 and 270 ppmv (parts per million by volume) with low and highvalues characteristic of the glacial and interglacial conditions respectively. Thehigh level equates with the preindustrial content of CO2 while the low one corre-lates with its generally low values during the last glacial maximum. Despite somenoticeable differences there is a good correlation (r = 0.89) between the content ofCO2 in the atmosphere and the temperature changes over the entire curve.

This correlation existed even during the preceding glaciation; this providesdirect proof of the close link between atmospheric CO2 and climatic changes overthe entire duration of the glacial-interglacial epochs. But there are still somedifferences between these two curves: No very low values of CO2 were recorded inconm ction with the extremely cold conditions 110,000 years ago. Apart from thisthe curves just cited allow one to hypothesize that while the CO2 content and the

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O 400

300 t -

V. M. KOTLYAKOV AND K. LORIUS

I—I—I—|—I—I—I—I—|—I—I—I r

100 150 103 years

Fig. 5. Variations in characteristics during the latest climatic cycle based onthe ice core from Vostok Station to a depth of 2,083 m: a) CO2 content inatmosphere [9]; b) air temperature in Antarctica [25]; c) CN4 content in atmos-phere [14]; for the CO2 and CN< curves the spread of data is shown; the temper-ature curve is smoothed.

temperature vary synchronously during the transition from a glacial epoch to aninterglacial, during the transition from the previous interglacial to the last glacialepoch (around 75,000 years B.P.) the changes in CO2 were delayed with respect tothe climatic changes.

Similar associations have been demonstrated for the concentration of methanein the former atmosphere. Analyses of samples from cores from the drillholes atDYE-3 in Greenland and Byrd Station in Antarctica have revealed an increase inthe methane content from 0.35 ppm during the last glacial maximum to 0.70 ppmduring the Holocene [41] and an analogous increase in methane in parallel withthe rise in temperature at Vostok Station during the transition from the previousglacial epoch to the last interglacial [7, 39]. These results are now fully confirmed

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by a detailed and uninterrupted profile of CN4 content from Vostok (see the lowercurve in Fig. 5), which demonstrates in particular four well-marked CN4 maximaduring ihe glacial epoch. These maxima appear during relatively warm inter-stadials and, although there are perceptible differences between the curves for CO2and CNi content, the correlation of the CN4 curve with the temperature profile isexactly :he same (r = 0.88).

There are also good prospects of obtaining essential data on N2O on the basis ofthe ice core. Results already available reveal that during the last glacial maximumthe concentrations of N2O were apparently less, but did not differ too much fromthat of the present period [42]. Thus we have the opportunity of assessing thedirect radiation impact of all the natural greenhouse gases, excluding possibly thechanges in ozone. From the glacial epoch to the interglacial that impact grew by~ 2 W/m2 (ATe« 0.7°C), i.e., the changes in CO2 and CN4 apparently played acritical role in the glacial-interglacial cycles.

The data on CO2 and CN4 from the Vostok drillhole are of fundamental interestfor a discussion of the long-term modifications in the carbon cycle. Presentattempts to explain the differences in the CO2 content during the last glacialmaximum and the Holocene concentrate on the role of the oceanic reservoir—changes in the marine biological productivity and oceanic circulation [12]. Men-tion is also made of the contribution of variations in the distribution of sea ice[13]; in the case of CN4 the distribution of bogs and flows of that gas from themare also examined [14].

It follows from the data from Vostok Station [9] that such mechanisms maydiffer depending on the period of examination. Only changes in the deep-seaoceanic circulation may be the cause of variations in CO2 between the last inter-glacial and the first part of the glacial epoch (110,000-70,000 years B.P.), whereasthe deep-sea and the surface circulations may be the cause of the lower values ofCO2 found during the second half of the glacial epoch (70,000-15,000 years B.P.).During the transition from the glacial epoch to the interglacial the increase inconcent ration of CO2 might initiate changes in the surface circulation, caused by areadjustment of the entire circulatory system, associated with a rise in sea level[24].

Thus present changes in the very important greenhouse gases are well docu-mented by the data from the polar glacier cores. As may be seen from Figure 6 theconcentration of atmospheric CO2 during the preindustrial era was 280 ppmv; inthe 1920s it rose to 300 and by the early 1960s to 320; the main causes were theburning of fossil fuels and the cutting of forests. Correspondingly the preindustrialconcentration of NjO was about 285 ppbv, whereas it is now 310 ppbv, and thecontent of CN4 in the atmosphere has doubled.

Aerosols and chemical inclusions. Over the past 200 years anthropogenic emis-sions have led to an increase in the content of nitrates and sulfates in glaciers.Thus, ciccording to the data from the Greenland core [34] since the start of theindustrial era deposits of SO4 and NOJ have increased by 3-4 and ~2 times,respectively.

Variations in the concentration of gases and other chemical elements in the iceare global in character; this permits us to use them in investigating the mutualcorrespondence between the chemistry of the atmosphere and the climate. Study

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1750 1800 1850 1900 1950 2000360

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. 340

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1750 1800 1850 1900 1950 2000 past 250 years.

Fig. 6. Increase in atmospheric content of CO2(a), methane (b), and nitric oxide (c), and also theincrease in human population on earth during the

of deep ice cores is so far the only means of reconstructing the past peculiarities ofthe atmosphere, especially the main biogenic cycles of carbon,- sulfur, and nitro-gen. In this respect the results of analyses of the ice core from Vostok Stationallow one to investigate the contribution of continental and marine aerosols, andalso the role of volcanic activity.

Figure 7 presents profiles of aluminum, sodium, and acidity from the core fromVostok Station [18, 28]. As an indicator of the continental contribution [5] thealuminum profile reveals a sharp increase during cold stages B, D, and F. Thisincrease may be explained by an expansion of arid areas and by the higher emis-sions of dust due to an increase in wind velocities over the continents, i.e., to amore intense atmospheric transfer due to an increase in the temperature differencebetween the tropics and the high latitudes. The increase in the dust content in the

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atmosp here during the last glacial epoch may be explained by the emergence of aconsiderable part of the continental shelves due to the lowering of sea level and anintensification of the aridity of the climate [35]. The reason for the absence of apeak in the aluminum profile during cold phase F is so far unclear, but one mightsuggest very large differences in the sources of dust and in atmospheric conditionsat the start of the glaciation as compared to the fully developed glacial epochs(stages B and H).

Depth, m

50 100 150103 years

Fig. 7. Results of studying the ice core from a drillhole at Vostok Station: a) smoothedcurve of deviation of isotope temperature from present values; b) aluminum content in ice;<:) sodium content in ice; d) acidity of ice. For stages of Holocene, A-H, see text.

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Despite the much greater extent of sea ice during the glacial epochs, the contri-bution of the marine components (sulfur was investigated as the best indicator) atthis time was more significant than during the interglacials. Admittedly whereasthe content of continental dust in the late Pleistocene exceeded that during theHolocene by 30 times, the concentration of marine salts was only 5 times greater.The cause apparently lies in the reduction of the rate of accumulation during theglacial epochs and the increase in wind speed over land areas, namely the sourcesof the dust, and the greater development of a longitudinal circulation.

In contrast to the continental and marine components, the content of acidicinclusions of HNO3 and H2SO4 in the ice does not show any trend throughout theentire duration of the climatic cycle investigated. This means that there is nolong-term correlation between volcanic activity and climate.

Thus all the chemical indicators lead us to the conclusion that there was astronger atmospheric circulation during the glacial epochs; this apparently waslinked to an increase at this time in the temperature gradient between the polesand the equator [18, 35].

A special feature of the Greenland data provides proof of rapid climaticchanges. During deglaciation, during the so-called Bolling-Allerod-Younger Driasoscillation, which has been well dated in the ice core from DYE-3 Station, onemay observe an increase in the mean temperature in Greenland of up to 7° C overonly 50-100 years. This event occurred approximately 10,700 years B.P. [1].Caused apparently by processes in the North Atlantic region, it appears to havebeen the last in the glacial epoch. Analogous events occurred around 30,000-40,000 years B.P. when, according to the 5I8O data temperatures in Greenlandchanged by 5-6° C over 100 years or over an even shorter time interval. The datafrom CO2 and from dust also correlate with these periods; for the colder condi-tions during these short time intervals the concentration of dust in the ice washigher and that of CO2 was lower, respectively.

ORBITAL AND ATMOSPHERIC IMPACTS ON CLIMATE

During the past two decades the astronomical theory of paleoclimates [10] andMilankovich's hypothesis, according to which the growth and destruction of theice sheets of the Northern Hemisphere are controlled by seasonal contrasts ininsolation at 65° N, have received considerable support, especially from an anal-ysis of 518O in deep-sea sediments [22]. But it is not easy to explain the intensifica-tion of this relatively weak action (the total insolation received by the planet hasvaried by less than 0.6% during the last million years), the predominance of a100,000-year cycle and the synchronous termination of the maj or glaciations in theNorthern and Southern hemispheres. In this regard the profile of temperature andCO2 obtained at Vostok Station for the past 160,000 years provides a uniqueopportunity to evaluate the link between climate change and astronomic andatmospheric processes.

Apart from the major 100,000-year fluctuations associated with the manifesta-tion of fluctuations in the eccentricity of the orbit, the temperature curve based onthe core from Vostok Station (Fig. 8b) clearly displays a signal with a periodicityof 40,000 years and with four minima, which agrees well with the minima in total

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Fig. 8. Profiles based on the deep drillhole at Vostok Station: a)smoothed profile of CO2 content; b) thick line indicates smoothed curve ofisotope temperature and thin line (b') calculated temperature taking intoaccount annual insolation at 78° S, July insolation at 65° N and contributionof atmospheric CO2 [20]; c) annual insolation at 78° S; d) July insolation at65° N. Both latter values were calculated as deviations in percent from run-ning values [10].

insolation at the latitude of that station (Fig. 8c), caused by a cycle of change inthe tilt of the planet's axis with a length of 41,000 years. There are also analogiesbetween the temperature curve based on the Vostok Station data and the patternof July insolation at 65°N (Fig. 8d) which plays a key role in the Milankovichtheory of glacial epochs, and is strongly influenced by changes in precession (T=19,000 and 23,000 years), the most important parameters in the Earth's orbit,which have a strong influence on the distribution of incoming energy by latitudeand season [10].

Speciral analysis of the data from Vostok confirms this qualitative hypothesis

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[25]. Apart from the 100,000-year oscillation a strong 40,000-year componentdominates in the temperature profile and to a lesser degree a 20,000-year signal(Fig. 9). In fact our analysis, using a precise nonparametric method, allows one toidentify two peaks at 23,000 and 19,000 years, respectively. These results fullyconfirm the role of astronomical impacts on the initial stage of the Pleistoceneglacial-interglacial cycles.

Strictly speaking the glacial-interglacial fluctuations are subject to rapid pro-cesses of a reverse relationship, produced by the presence of water vapor in theatmosphere, by the properties of the cloud cover, snow cover, and sea ice, as wellas more long-term processes associated with the slow changes in some limitingconditions and in the composition of the atmosphere, which extended the cold

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conditions of the glacial epoch to some degree into the interglacial. One mayderive some idea as to the role of such slow changes from the pattern of tempera-ture and of other parameters obtained at Vostok, while by examining the directradiation influences associated with changes in the concentrations of greenhousegases, one may also learn correspondingly about the role of rapid processes with areverse relationship.

One of the starting points is that our planet is close to a radiation equilibriumwith surrounding space. Thousands of years are required to attain this equilibriumand hence one may exclude from consideration rapid events that are completed ina few decades. Within certain limits we can hypothesize that the greenhouse gasesinfluence the glacial-interglacial changes in temperature through their direct radia-tion effects, associated with rapid processes with a reverse relationship. Thishypothesis is well confirmed by experiments with global circulation models [11,40]. Hence we may use the long-term records of climatic influences and climaticconsequences, reflecting equilibrium conditions, in order to calculate or at least toestimate the climatic role of the radiation effects of the greenhouse gases in thepast.

Of course this approach is fairly simplified. Naturally the interaction betweenthe different components of the climatic system are nonlinear. Apart from this, wehave not taken into account the role of the ocean which, clearly, links in a criticalfashion the climates of the Northern and Southern hemispheres, or of changes inthe extent of sea ice. Clearly, in the future we would have to examine a wider rangeof possible climatic actions, especially fluctuations in the optical thickness of theatmosphere associated with variations in the content of aerosols; changes in theconten: of other gases apart from CO2 and CN4; differences in the amount ofcondensation nuclei resulting from oceanic production of dimethyl-sulfides [24].

At the present stage of investigations we have carried out [20] multivariateanalysis of the temperature curve from Vostok Station, in an attempt to identifythree climatic effects in such a way as to minimize the impact of extraneous"noise" on the curve under examination. As parameters of climatic action we haveexamined the CO2 content, a component of the Southern Hemisphere representedby annual insolation at 78° S and a component of the Northern Hemisphereidentified either through the July insolation at 65°N or adopted from the marinedata on the basis of 618O, reflecting as a whole fluctuations in the volume ofcontinental ice in the Northern Hemisphere (Fig. 10). Our further calculationswere built on the basis of the following assumptions.

First, multivariate analysis assumes that the system is in a stable state. This doesnot hold true very well for the climatic system on which operate nonlinearitiesassociated with the growth and decay of the ice sheets. Hence we concentrated ourattenti Dn on those cases where the volume of ice in the Northern Hemisphere canbe reconstructed on the basis of the marine record of <518O [31]. Thus we are ableto estimate a considerable part of of the slow reverse relationships producingchanges in the limiting conditions and fluctuations in the area of the ice caps; dueto a change in albedo and terrestrial relief in this case the fluctuations in radiationeffects reach 3 W/m2.

Second, we have the opportunity of providing an estimate of the combinedradiation effects of two main greenhouse gases, CO2 and CN4; for this, on the

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106 V. M. KOTLYAKOV AND K. LORIUS

I I I I 1 I100 150 103 years

Fig. 10. Time series of climatic records from Vostok and of climatic elements usedin multivariate analysis: a) atmospheric temperature over Antarctica (thick curve)and reconstructed temperature (thin line); b) direct greenhouse radiation influence ofCO2 and CN4, as reflected in temperature; c) 518O data from deep-sea deposits as aresult of changing volume of continental ice, reflected in changes in sea level [33]; d)curve of aerosols in ice of Vostok Station core [36]; e) percentage change in annualinsolation at latitude of Vostok Station (78° S).

basis of the Vostok data we have calculated the direct radiation impact of thesegases according to the formula used by Hansen, Fung, Lacis, et al. [21], wherebythe impact of CN4 must be approximately doubled due to the chemical reverserelationship caused by the increase in stratospheric water vapor [14]. The totalgreenhouse radiation impact expressed in the form of changes in the equilibriumtemperature, i.e., without taking into account the climatic reverse relationship (seeFig. 10b) results in a glacial-interglacial amplitude of about 0.1° C.

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Third, we have investigated the resulting changes in the content of aerosols (Fig.lOd) affecting the amounts of condensation nuclei. Earlier the conclusion wasreached that the increase in the aerosol-controlled optical thickness in the atmos-phere might make a significant contribution to climatic cooling during glacialepochs, but so far it is difficult to make such estimates on the basis of investigationof pola:: ice cores. At the same time the data from Vostok Station [36] haverevealed a marked increase in dust at the end of the preceding glacial epoch andduring Ihe cold interstadial centered on around 60,000 years B.P., which suggestsa connection between cold periods and a high aerosol content. But the dust curvefrom Vostok is markedly peaked and is not closely linked with the smoothedtemperature curve; the correlation between dust content and the temperaturerecords (r = 0.6) is much lower than the similar correlation with greenhouse gases.

Worthy of attention in this connection is the hypothesis according to which aconsiderable number of condensation nuclei, mainly sulfates, produced by theoxidation of dimethyl sulfides picked up from the surface of the ocean, leads to theformation of marine stratus clouds with a characteristic albedo, which exerts adirect climatic influence [39]. The production of a changed content of condensa-tion nuclei under the influence of the sea may ultimately have an impact onclimate. The data from the Vostok drillhole on sulfates of a nonmarine origin,which indirectly characterize the number of condensation nuclei would indicatethat this process plays a role in global cooling [27]. The correlation between theabove-mentioned sulfates and temperature is relatively high (r = 0.8) but the roleof sulfates in the formation of condensation nuclei still needs to be proven.

A further step was multivariate analysis using the following five controllingfactors: greenhouse gases, the content of dust and sulfates of nonmarine origin,the volume of glacier ice, and the peculiarities of local insolation. The main resultsof this analysis may be summarized in similar fashion: the contribution of theimpact of greenhouse gases on the temperature never dropped below 40%; thelowest calculated value was 42%, while the data from Vostok coincide exactly withthe phase of marine data on 518O, providing evidence of a similar contribution(45%) from glacier ice, whereas the influence of each of the other three climaticfactors does not exceed 5%. Within the limits of reliable dating, i.e., back to110,00(1 years B.P., the contribution of the greenhouse effect never exceeds 65%whereas the total impact of greenhouse gases and the situation in the NorthernHemisphere as a whole is about 80%. Over 90% of the temperature fluctuations atVostok may be explained by the five factors examined, and Figure 10a (the upperthin line) shows the pattern of these reconstructed temperatures.

Thu 5, on the basis of the Vostok data the contribution of greenhouse gases totemperature change during the past climatic cycle may fluctuate between 40 and65%; proceeding from our multivariate analysis a figure of 50 ± 10% is a reason-able value. This means that ~3°C of the 6°C amplitude of glacial-interglacialtemperature change in Central Antarctica may be explained by the greenhouseeffect. One should note that the value obtained agrees well with the results of aglobal circulation model for the last glacial maximum [11]. And although this isonly one model, such agreement between two independent approaches is veryimportant.

If one takes into consideration the unique geographical position of Vostok

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108 V. M. KOTLYAKOV AND K. LORIUS

Station the data obtained there demand further examination in order to conferglobal significance to them. In this connection one should note that the glacal-in-terglacial warming by 6° C over Antarctica agrees well with the averaged globalvalue of 4-5° C. Moreover, experiments with global circulation models involving adoubling of CO2 reveal a 50% contribution from the greenhouse gases for the highlatitudes of continental regions [38]. This all confirms the global significance ofthe Vostok data and allows one to make the suggestion that greenhouse gasescontributed about 2°C to the global warming between the glacial and interglacialepochs.

We might also add that in experiments by Broccoli and Manabe [11] on increas-ing the CO2 content from 200 to 300 ppmv with accompanying rapid reverserelationships contributed about 40% of glacial-interglacial warming on a globalscale. These experiments also revealed that a large part of the remaining warmingwas due to the reduction in the ice sheets of the Northern Hemisphere, in accor-dance with our multivariate analysis. According to another model [40] the meanwarming by 2.5° may have been the result of increasing the concentration of CO2from 230 to 300 ppmv, which corresponds to at least a 50% contribution bygreenhouse gases to the glacial-interglacial warming.

An important characteristic of the spectral records of CO2 (Fig. 9c) is the factthat in addition to the 100,000-year signal one can clearly detect differences inconcentration of around 20,000 years [9]. Apart from its contribution to ourunderstanding of the carbon cycle itself, this allows one to suggest that astronomiceffects are being reflected in changes of both temperature and CO2, although therole of such processes is complex and so far not fully understood. Nonetheless, theclose correlation between the records of CO2 and temperature and their spectralcharacteristics confirm the idea that climate changes may be provoked by insola-tion effects associated with a relatively weak orbital influence which, however, wassharply intensified by orbitally stimulated changes in CO2. In particular theseresults give one the opportunity to propose that the glacial-interglacial oscillationwith a periodicity of 100,000 years may be associated with the observed fluctua-tions in the concentration of CO2, rather than with the postulated nonlineargrowth and decay of the ice sheets caused by fluctuations in insolation. Of coursethis so far is just a hypothesis requiring a clearer understanding of the essentialaspects of the operating mechanisms. But the results of studies of the core from thedrillhole at Vostok Station clearly reveal that there exists a close interactionbetween orbital and greenhouse influences and climate.

THE POSSIBLE FUTURE OF THE EARTH AND THE FATEOF THE CRYOSPHERE

Orbital and greenhouse influences clearly include slow and rapid reverse rela-tionships, respectively. The radiation impacts of the greenhouse gases, associatedwith glacial-interglacial changes amounted to 2 W/m2, which would produce atemperature change of 0.7° C without taking into consideration any reverse rela-tionships. The total effect, amounting to 2°C, represents an intensification byabout 3 times and was produced by rapid reverse relationships [31]. The sensitivityof the climate to a future increase in the greenhouse impacts according to

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present ideas and experiments with global circulation models, with a temperaturechange resulting from a doubling of CO2 corresponding to a radiation impact of4.3 W/m2; allowing for intensification factors ranging from 1.6 to 4.3 this gives anequilibrium warming of from 1.9 to 5.2° C [23]. Thus the factor of climatic sensi-tivity, according to our investigations and calculations using climate models, isabout 3.

The ice caps themselves are passive with regard to the main biogeochemicalcycles on which the biospheric, atmospheric, hydrologic, oceanologic, and sedi-mentary processes operate. But the ice profile records changes in these cycles inthe past on a detailed quantitative basis. Hence the study of ice cores has a criticalinfluence on the development of the biogeochemical models. This is true for thecarbon cycle already investigated and probably will be confirmed in the nearfuture for the nitrogen and sulfur cycles.

Although as yet we do not have a full understanding of the causes and conse-quences of climatic and atmospheric fluctuations, one may state that processes ofatmosplieric reverse relationships operate independently of the initial causes ofclimatic changes; these processes operating at present differ little from those oper-ating di ring the glacial epoch. Hence our data, derived from the past, may also beused for predictive purposes.

Ice cDres from Greenland and Antarctica provide proof of anthropogenicimpacts on the concentration of greenhouse gases in the atmosphere. During thepast 20(1 years the CO2 content has grown by 25%, that of N2O by 8%, and that ofCN4 by more than 200%. They reveal that the glacial-interglacial temperaturechanges at the surface of the glaciers reached approximately 10°C, whereby alower snow accumulation and a higher concentration of marine and continentalaerosol;: were associated with the cold stages; during the transition from the glacialepoch t3 the interglacial the content of CO2 and CN4 increased by 40 and 100%,respectively. The content of greenhouse gases played an important role in changesin the gacial-interglacial climates, intensifying the orbital impacts, along with theimpact of the growth and decay of the ice sheets in the Northern Hemisphere. Aclimatic sensitivity index o f / = 3, derived from paleoglacial and modeling data,allows one to conclude that with a possible future doubling of CO2 in the atmos-phere, the warming may attain values of 3-4° C.

According to world summary data, during the past 100 years the mean global airtemperature has risen by 0.5° C and the level of the ocean has risen by 15-16 cm (Fig.11). This perceptible climatic trend cannot fail to have an impact on the cryosphere.

Our analysis has shown [6] that "greenhouse" warming of the climate, thanks toan instability mechanism in the marine ice sheets, may lead to their catastrophiccollapse which would produce a rise of 5-7 m in sea level in only a few decades. Atthe sanr.e time mountain glaciers in the temperate and subtropical latitudes wouldhave a sharply negative mass balance of up to -3 to -5 m/yr and would almostentirely disappear.

Thus the "glaciological approach" is absolutely essential to understand thecauses ;ind mechanisms of global change, both those which occurred in the pastand those which can be anticipated in the near future. This approach has alreadyproduced unique data as to the properties and interactions in the global systemand as to its reaction to orbital influences.

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1860 1880 1900 1920 1940 1960 1980 2000

Fig. 11. Changes in global temperature (a) and in sea level (b) during past 130 years.

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