(ppmv) - (ppmv) 1.8 METHANE CONCENTRATION 340 CO2 CONCENTRATION IN THE SIPLE CORE IN THE SIPLE CORE 1.6

320 1.4 -+-

1.2 300 1.0

0.8 + 280 1 ±± ±

1700 1750 1800 1850 1900 1950 2000 (Age A.D.) 1750 1800 1850 1900 1950 2000 (Age A.D.) Figure 1. Measured mean carbon dioxide concentrations plotted Figure 2. Measured methane concentrations plotted against the against the estimated mean gas age. The horizontal axis indicates estimated mean gas age. The plus signs indicate measurements on the close-off intervals of 22 years. ("ppmv" denotes "parts per atmospheric air. ("ppmv" denotes "parts per million by volume:) million by volume: "CO2" denotes "carbon dioxide.") The atmospheric methane concentration is two orders of magnitude lower than the carbon dioxide concentration, but the present increase of 1.2 to 1.9 percent per year is fast, and a References continuation of the increase could enhance the climatic change expected for the rising carbon dioxide. Based on ice-core mea- Craig, H., and C.C. Chou. 1982. Methane: The record in polar ice cores. surements, Craig and Chou (1982), and Rasmussen and Khalil Geophysical Research Letters, 9(11), 1221 - 1224. Mayewski, P.A., and W.M. Lyons. 1985. Using an ice core to charac- (1984) reported a constant atmospheric methane concentration terize the climatic history of Antarctica. Antarctic Journal of the U.S., of about half the present value until about 300 years ago. Youn- 20(5). ger samples indicate an increase to the present value of about Neftel, A., E. Moor, H. Oeschger, and B. Stauffer. 1985. Evidence from 1.7 parts per million by volume. The measurements on ice polar ice cores for the increase in atmospheric CO 2 in the past two samples from the ice core from Siple Station fill the gap between centuries. Nature, 315, 45 - 47. previous measurements on ice samples and direct measure- Rasmussen, R.A., and M.A.K. Khalil. 1984. Atmospheric methane in ments on atmospheric air. The results are discussed by Stauffer the recent and ancient atmospheres: Concentrations, trends and et al. (in preparation) and are shown in figure 2. interhemispheric gradient. Journal of Geophysical Research, 89, 11599 - I would like to thank the principal investigator of the project, 11605. Hans Oeschger, and my colleague Henry Rufli, who did the Schwander, J., and B. Stauffer. 1984. Age difference between polar ice and the air trapped in its bubbles. Nature, 311, 45 - 47. field work during the 1984 - 1985 austral summer. I also thank Stauffer, B., C. Fischer, A. Neftel, and H. Oeschger. In preparation. rico for the good collaboration. The field operation was sup- Increase of atmospheric methane recorded in Antarctic ice core. ported by National Science Foundation grant DPP 83-12630 to H. Science. Oeschger. The laboratory work is supported primarily by the Stauffer, B., and J. Schwander. 1984. Core processing and first analysis Swiss National Science Foundation and the U.S. Department of of ice cores from Siple Station and from South Pole. Antarctic Journal of Energy. the U.S., 19(5), 59 - 60.

International antarctic glaciological and long-term changes of climate and atmospheric environment. program activities at South Pole The South Pole Station work consisted mainly of the recovery Station and Vostok and processing of samples from an electromechanical drill hole 143 meters deep, drilled the previous season. The field work also included the recovery of the French deep-drilling ("cli- matopic") equipment previously tested at South Pole Station C. LoRIus (Gillet and Legrand 1984). While two scientists remained at the station for 2 to 3 weeks, a group of three with scientific equip- Lahoratoire de Glaciologie et Geophysique de lEnvironnernent ment was flown to Vostok after a week of acclimatization at Grenoble, France South Pole Station. An LC-130 landed at Vostok on 31 De- cember. The LC-130 stayed at Vostok for a few hours to allow P. Wilkniss and Captain Shoemaker to visit the Soviet station. The The main objective of this program was to recover ice-core three French scientists remained in Vostok for about 6 weeks. amples from South Pole Station and Vostok to study current Two of them were flown back to Mirnyy by ski-equipped 11-14

985 REVIEW 73 aircraft about mid-February while the third one (J.R. Petit) had around 130,000 years ago, and (4) the last part of the previous the opportunity to participate in a Soviet traverse from Vostok glacial period. to Mirnyy. The French team, equipment, and ice samples were The logistic realization of this operation was made possible retrograded from Mirnyy on 13 March, on board the "Polar thorough the generous and efficient support of the National Bjorn" chartered by French Polar Expeditions. Science Foundations Division of Polar Programs, the Soviet The work in Vostok performed in cooperation with Soviet Antarctic Expeditions, and the Expeditions Polaires Francaises- scientists consisted mainly of surface sampling from pits and Terres Australes et Antarctiques Francaises. shallow cores and of processing samples from a 2,083-meter The field team consisted of D. Raynaud and C. Rado (South deep ice core obtained by Soviet scientists the previous seasons, Pole Station) and C. Lorius, M. Creseveur, and J. Petit for both using a thermal drill (Kudryashov et al. 1984). Solid and liquid South Pole Station and Vostok projects. conductivity measurements were performed in Vostok. Vol- canic fallout layers in these measurements suggest an ac- cumulation rate of about 2.1 grams per square centimeter per year over the last 170 years. The Vostok-Mirnyy traverse al- References lowed for the collection of samples to study surface geograph- ical changes of various geochemical tracers. Previous measure- Gillet, F., and M. Legrand. 1984. French glaciological activities at the ments performed by several laboratories, within a cooperative South Pole. Antarctic Journal of the U.S., 19(5), 61. Kudryashov, B. B., V. K. Chistyakov, E. A. Zagrivny, and V.Ya. Lipenkov. Soviet-French program, from about 100 samples collected along 1984. Preliminary results of deep drilling at Vostok station, Antarctica the 2,083-meter deep ice core by Soviet scientists, suggest that 1981-82. In G. Holdsworth, K. C. Kuivinen, and J. H. Rand (Eds.), Ice this record spans about the last 150,000 years (Lorius et al. in Drilling and Technology. (CRREL Special Report 84-34.) Hanover, preparation). This record should allow a detailed study of (1) the N.H.: Cold Regions Research and Engineering Laboratory. current Holocene climatic stage over the last 10,000 years, (2) Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. Barkov, Y.S. Korotkevich, the various glacial stages which occurred from about 18,000 and V.M. Kotlyakov. In preparation. A 160,000 year climatic record years ago to 110,000 years ago, (3) the last interglacial centered from Antarctic ice. Nature.

the timescales for the nonlinear growth of shear-heating in- Shear heating instabilities of large ice stability. The first problem we examined involves the imposi- sheets tion of finite-amplitude perturbations in the form of thickened ice layers (Yuen, Saari, and Schubert in preparation). The par- ticular form of this perturbation was chosen only for its mathe- D.A. YUEN matical simplicity which should capture the physics of more climatically realistic perturbations. We have conducted non- Department of Geological Sciences and CIRES linear calculations to determine quantitatively the simmering University of Colorado time before the onset of explosive instability in creeping ice Boulder, Colorado 80309 sheets. All instabilities are found to grow explosively after a prolonged period of simmering or relatively slow monotonic C. SCHUBERT growth. The explosion times are extremely sensitive to the activation energy and the pre-exponential constant of the ice Department of Earth and Space Sciences creep law. Sudden increases in ice sheet thickness of 1 to 2 University of California kilometers due to a rapid episode of climatic deterioration can Los Angeles, California 90024 lead to explosive instability and melting of the basal-shear layet in only thousands of years, if ice creep activation energies are M. R. SAARI lower than about 60 kilojoules per mole. Another form of perturbation we considered is that caused by Department of Geology climatic warming associated with the Holocene interglacial ep- Arizona State University och and with the present increase of carbon dioxide in the last Tempe, Arizona 85287 hundred years (Saari, Yuen, and Schubert in preparation). We calculated the time-dependent response of a steady-state solu- tion to a prescribed increase in the surface temperature wit Motions of large ice sheets represent an intrinsically ther- time. The time-scale for simmering is found to depend sen momechanical problem, because the shear-deformation of ice is sitively on the ice rheological parameters and could be as shor strongly controlled by its temperature-dependent rheology. Ac- as 1,000 years. Timescales of 10,000 years are produced by mor cordingly, viscous dissipation can play an important role in realistic parameter values. We also find that the present carbo modifying the movement of ice sheets. Our research efforts dioxide warming trend could lead to instability of the eas have been concerned with understanding the nonlinear, ther- antarctic ice sheet between 10,000 and 100,000 years from no momechanical responses of large ice sheets to different types of This work was supported by National Science Foundatio perturbations. In particular, we have focussed on quantifying grant DPP 82-15015.

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