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GLOBAL WARMING

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Ice Core Records of Atmospheric CO₂ Around the Last Three Glacial Terminations

Science. 12 March 1999, Vol. 283. no. 5408, pp. 1712 – 1714; DOI: 10.1126/science.283.5408.1712
Authors: Hubertus Fischer, Martin Wahlen, Jesse Smith, Derek Mastroianni, Bruce Deck

ABSTRACT: Air trapped in bubbles in polar ice cores constitutes an archive for the reconstruction of the global carbon cycle and therelation between greenhouse gases and climate in the past. High-resolutionrecords from Antarctic ice cores show that carbon dioxide concentrationsincreased by 80 to 100 parts per million by volume 600 ± 400 yearsafter the warming of the last three deglaciations. Despite stronglydecreasing temperatures, high carbon dioxide concentrations canbe sustained for thousands of years during glaciations; the sizeof this phase lag is probably connected to the duration of thepreceding warm period, which controls the change in land ice coverageand the buildup of the terrestrial biosphere.

Scripps Institution of Oceanography, Geosciences Research Division, University of California San Diego, La Jolla, CA 92093-0220, USA.

Previous studies of Antarctic ice cores (1-3) revealed that atmospheric CO₂ concentrations changedby 80 to 100 parts per million by volume (ppmv) during the lastclimatic cycle and showed, together with continuous atmosphericmeasurements (4), that anthropogenic emissions increasedCO₂ concentrations from 280 ppmv during preindustrial times tomore than 360 ppmv at present, an increase of more than 80% ofthe glacial-interglacial change. Variations in atmospheric CO₂concentrations accompanying glacial-interglacial transitions havebeen attributed to climate-induced changes in the global carboncycle (5, 6), but they also amplify climatevariations by the accompanying greenhouse effect. Accordingly,the relation of temperature and greenhouse gases in the past derivedfrom ice core records has been used to estimate the sensitivityof climate to changes in greenhouse gas concentrations (7)to constrain the prediction of an anthropogenic global warming.This procedure, however, requires the separation of systematicvariations representative for all climatic cycles from those specificfor each event, as well as a more detailed knowledge of the leadsand lags between greenhouse gas concentrations and climate proxies.

To resolve short-term changes in the atmospheric carbon reservoir, to constrain the onset and end of major variations in CO₂concentrations, and to test whether these variations are temporallyrepresentative, we expanded the Antarctic Vostok CO₂ record overthe transition from marine isotope stage (MIS) 8 to MIS 7 [about210 to 250 thousand years (ky) before present (B.P.)] and analyzedthe time interval around the penultimate deglaciation (about 70to 160 ky B.P.) at a high resolution of 100 to 2000 years (8).This data set was supplemented by a CO₂ record recently derivedfrom the Antarctic Taylor Dome (TD) ice core (6, 9)covering the last 35,000 years. The internal temporal resolutionof ice core air samples is restricted by the age distributionof the bubbles caused by the enclosure process (10).This age spread is about 300 years for Vostok (11)and 140 years for the TD ice core (9) at present butabout three times higher for glacial conditions (11).The depth-ice age scale used for terminations II and III in theVostok core is a recently expanded version of the extended glaciologicaltime scale (12). The dating uncertainty (on the orderof 10,000 years for termination III) is considerable; however,the absolute time scale is not so important as long as we consistentlycompare Vostok CO₂ with the Vostok isotope temperature (D) record.

More important is the relative dating of ice and air at a certain depth. The ice age-air age difference (age) was calculatedwith a climatological firn densification model (11)and varies between about 2000 and 6000 years for warm and coldperiods, respectively. The accuracy of the model is better than100 years for recent periods but on the order of 1000 years forglacial conditions (11), which has to be kept in mindwhen interpreting the phase shift between ice and gas recordsof the ice core archive. In the case of termination I, recentlypublished age scales derived by synchronization of CH₄ variationsin central Greenland and Antarctic ice cores (13, 14)were used. The precision of the CH₄ correlation is about 200 yearsfor periods of substantial CH₄ change but is not very well constrainedin the interval between 17 and 25 ky B.P., when only subtle CH₄changes occurred. The uncertainty of age varies between 100 and300 years for central Greenland (13) and between 300and 600 years for TD (14) during termination I. Furtheruncertainty is added because the TD CO₂ record has been datedrelative to the Greenland Ice Sheet Project 2 (GISP2) core (14),whereas the Byrd and Vostok isotope temperature records have beensynchronized with respect to the Greenland Ice Core Project (GRIP)ice core record (13). This uncertainty is not relevantfor the interval between 10 and 15 ky B.P., for which dating ofGISP2 and GRIP is in good agreement; however, there is a shiftof up to 2000 years between the two Greenland reference coresat the age of 20 ky B.P.

In Fig. 1, our data and previously published CO₂ concentration records (1, 6, 9, 11, 15, 16) are compared with Antarcticisotope (temperature) ice core records (13, 17-19).Note that the CO₂ concentrations represent essentially a globalsignal. In contrast, the geographical representativeness of isotopetemperature records may vary from a synoptical to hemisphericalscale and accordingly within different cores with increasing variabilityfor shorter time scales. The Vostok and TD CO₂ data presentedhere are in good agreement with previous CO₂ values. On a 10,000-yeartime scale, CO₂ covaries with the isotope temperatures with minimumglacial CO₂ concentrations of 180 to 200 ppmv, glacial-interglacialtransitions accompanied by a rapid increase in CO₂ concentrationsto a maximum of 270 to 300 ppmv, and a gradual return to low CO₂values during glaciation. On a shorter time scale, however, amuch more complex picture evolves.

Fig. 1. Records of atmospheric CO₂ concentrations and isotope temperature records derived from the Antarctic Byrd, Vostok, and TD ice cores during the deglaciation and glaciation events around the last three glacial terminations. Error bars in CO₂ concentration data represent 1 of replicate measurements at the same depth interval. The long-term trend in CO₂ concentrations is indicated by a cubic spline approximation (P = 5 × 10⁹) of our data set. For convenience, marine isotope stages (22) are indicated as referred to in the text. [View Larger Version of this Image (48K GIF file)]

The onset of the atmospheric CO₂ increase during termination I recorded in the TD record is at 19 to 20 ky B.P. The rise inthe long-term trend in CO₂ concentrations seems to be about 1000years earlier than the rise in Vostok D values. In contrast,temperatures apparently started to rise at 20 ky B.P., as recordedin the Antarctic Byrd and the Greenland GRIP ice core (13).Again, CO₂ concentrations in the Byrd record increase ~2000 ±500 years later than those in the TD data. In view of the excellentagreement for the rest of the CO₂ records, these discrepanciescan be attributed to the insufficient age constraint during theonset of termination I induced by the different Greenland referencecores. No such dating uncertainties are encountered for the intervalbetween 10 and 15 ky B.P. Maximum CO₂ concentrations of 270 ppmvare reached at 10.5 ky B.P. (9), 600 to 1000 years afterthe isotope temperature maximum in the Byrd record (20).The CO₂ peak is followed by a decrease of 5 to 10 ppmv until 8ky B.P., after which CO₂ concentrations gradually rise to thepreindustrial value of 280 ppmv (9). A delay in theincrease of CO₂ concentrations with respect to the warming duringdeglaciation is also indicated by a brief 10-ppmv decline in CO₂concentrations found in seven samples during the interval 14 to13 ky B.P. This distinct feature lags the Antarctic Cold Reversal(ACR) in the Antarctic isotope temperatures (21) by300 to 500 years but occurs 1000 years before the Younger Dryascooling event.

A dip in CO₂ concentrations at 135 ky B.P. precedes the start of the increase in CO₂ concentrations during termination II,which reaches a maximum of 290 ppmv at 128 ky B.P. Like in theHolocene, CO₂ concentrations decrease after this initial maximumto ~275 ppmv. The onset of the major warming during terminationII is hard to define, but during the penultimate warm period,CO₂ concentrations reach their maximum 400 ± 200 years later thanAntarctic temperatures. In the following 15,000 years of the Eemianwarm period, CO₂ concentrations do not show a substantial changedespite distinct cooling over the Antarctic ice sheet. Not until6000 years after the major cooling in MIS 5.4 does a substantialdecline in CO₂ concentration occur. Another 4000 to 6000 yearsis required to return to an approximate in-phase relation of CO₂with the temperature variations.

Finally, termination III starts with a CO₂ concentration of 205 ppmv at 244 ky B.P., slightly higher than that for the beginningsof terminations I and II. At that time, temperatures had alreadyincreased since the glacial temperature minimum at ~260 ky B.P.CO₂ concentrations rise slowly from 244 to 241 ky B.P. and thenrapidly to more than 300 ppmv at 238 ky B.P. Keeping the rathercoarse resolution of the D record before 238 ky B.P. in mind,the major increase in CO₂ tends to lag temperature during thetransition, reaching a maximum CO₂ concentration 600 ± 200 yearsafter the peak in D. In contrast to the case for the Eemian,high CO₂ concentrations are not sustained during MIS 7 but followthe rapid temperature drop into MIS 7.4. Minimum CO₂ concentrationsas low as 210 ppmv are reached 1000 to 2000 years after the minimain isotope temperature during MIS 7.4. A short, warm event duringthe mild glacial interval at 224 to 228 ky B.P. appears to bereflected in a 30-ppmv increase in atmospheric CO₂ concentrationswith a phase lag of about 1000 ± 600 years relative to temperature.Another warm event at the beginning of the warm period MIS 7.3is accompanied by a 30-ppmv increase in CO₂ concentration, whichappears to be in phase with the temperature record. The variationsin CO₂ concentrations during these events are much larger thananticipated from the Vostok isotope temperature changes and donot have any counterparts during MIS 5.

Comparison of the sequence of events for the three time intervals described above suggests that the carbon cycle-climate relationshould be separated into (at least) a deglaciation and a glaciationmode. Atmospheric CO₂ concentrations show a similar increase forall three terminations, connected to a climate-driven net transferof carbon from the ocean to the atmosphere (6). Thetime lag of the rise in CO₂ concentrations with respect to temperaturechange is on the order of 400 to 1000 years during all three glacial-interglacialtransitions. Considering the uncertainties in age (between 100and 1000 years for recent and glacial conditions), such a lagcan still be explained by an overestimation of age for glacialconditions. The good agreement of the age model with the measuredvalue for the present supports the idea that at least the lagat the beginning of the warm periods is real. The size of thislag is on the order of the ocean mixing time (for a well-ventilatedocean like today), which is the major control for the time constantof equilibration within the deep ocean-atmosphere carbon systemafter climate-induced changes. In the case of a recent anthropogenicwarming, the external climate forcing by CO₂ emissions due tocombustion of fossil fuel leads climate variations, so the applicationof the CO₂-climate relation deduced from the past on a recentglobal warming seems not to be straightforward.

The situation is even more complicated for the interglacial and glaciation periods. During the extended Holocene and Eemianwarm periods, atmospheric CO₂ concentrations drop by ~10 ppmvafter an initial maximum, attributable to a substantial increasein the terrestrial biospheric carbon storage extracting CO₂ fromthe atmosphere. In the case of the Eemian, CO₂ concentrationsremain constant after the initial maximum in MIS 5.5 despite slowlydecreasing temperatures; during the Holocene, atmospheric CO₂concentrations even increase during the last 8000 years. Applicationof a carbon cycle model to CO₂ and ¹³CO₂ ice core data for the Holocene (9) shows that noequilibrium in the carbon cycle is established and that the waxingand waning of the terrestrial biosphere, possibly related to subtleclimate variations and early human land use, are the most importantfactors controlling atmospheric CO₂ concentrations over the last10,000 years.

During further glaciation in MIS 5.4, CO₂ concentrations remain constant, although temperatures strongly decline. We suggestthat this reflects the combination of the increased oceanic uptakeof CO₂ expected for colder climate conditions and CO₂ releasecaused by the net decline of the terrestrial biosphere duringthe glaciation and possibly by respiration of organic carbon depositedon increasingly exposed shelf areas. These processes, however,should terminate (with some delay) after the lowest temperaturesare reached in MIS 5.4 and ice volume is at its maximum at 111ky B.P. (22). In agreement with this hypothesis, CO₂concentrations start to decrease in the Vostok record at about111 ky B.P. Another possibility to explain this delayed responseof CO₂ to the cooling during MIS 5.4 would be an inhibited uptakeof CO₂ by the ocean. In any case, about 5°C lower temperatureson the Antarctic ice sheet during MIS 5.4 (17) aredifficult to reconcile with the full interglacial CO₂ forcingencountered at the beginning of this cold period and again questionthe straightforward application of the past CO₂-climate relationto the recent anthropogenic warming.

Another scenario is encountered during MIS 7, in which no prolonged warm period is observed. Although temperatures at theend of termination III are comparable to those at the end of terminationII and CO₂ concentrations are even slightly higher, a much shorterlag in the decrease of CO₂ relative to the Antarctic temperaturedecrease in MIS 7.4 is found. Comparison with the SPECMAP record(23) shows that during the preceding interglacial MIS7.5, ice volume was much larger than during the Holocene and theEemian warm periods. Accordingly, the buildup of the terrestrialbiosphere during MIS 7.5 is expected to be much less and sea levelchanges smaller, leading to a smaller net release of CO₂ intothe atmosphere during the following glaciation, which is not ableto fully counterbalance the CO₂ uptake by the ocean.

REFERENCES AND NOTES

J. M. Barnola, D. Raynaud, Y. S. Korotkevich, C. Lorius, Nature 329, 408 (1987) .

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D. Raynaud, et al., Science 259, 926 (1993) [Free Full Text] . Recent investigations in central Greenland have reported an in situ production of CO₂ in the ice, possibly related to carbonate or organic species reactions (or both), and have strongly compromised the validity of the determined CO₂ concentrations. However, Antarctic ice cores are (if at all) much less affected by this effect because of the very low abundance of reactive carbon species dissolved in Antarctic ice.

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M. Wahlen, D. Allen, B. Deck, A. Herchenroder, Geophys. Res. Lett. 18, 1457 (1991) . Air samples were extracted from Vostok 5G and TD ice with a dry extraction technique, and CO₂ concentrations were determined with laser spectroscopy. The accuracy of a single measurement (as essentially determined by the standard deviation of multiple frequency tunings of the diode laser) is better than 5 ppmv. The laser spectroscopic method enables the use of very small samples (~4 g), allowing us to pick crack-free ice and to measure replicate samples at the same depth interval. In general, all given CO₂ concentrations correspond to the average and standard deviation of at least three replicate samples. On average, the variability of such replicate measurements is 7.5 ppmv (1).

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Phase relations were determined by comparison of maxima and minima in the long-term trend of CO₂ concentrations and isotope temperatures as represented by spline approximations. Given errors reflect the uncertainty in the actual positions of the extrema, which are weakly dependent on the degree of smoothing. They do not take into account the uncertainty in age. This additional error is treated separately in the discussion of the data.

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We thank J.-M. Barnola and D. Raynaud for helpful comments and for sharing with us their unpublished Vostok CO₂ record of the last four glacial-interglacial cycles during our sample selection process. This study was funded by NSF grants OPP9615292, OPP9196095, and OPP9118534. Financial support of H.F. has been provided by Deutsche Forschungsgemeinschaft.

30 November 1998; accepted 29 January 1999

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