420,000 years of atmospheric history revealed by the Vostok ice core, Antarctica.

Paris, June 3, 1999

French, Russian and American researchers have measured the temperature, the aerosol and greenhouse gas concentration and various other climate and environmental parameters over the last four climate cycles. The results confirm the idea that climate variations are caused by the Earth’s orbital changes and to a large extent amplified by greenhouse gases. The high concentrations of greenhouse gases, unprecedented in the past 420,000 years, underscore their role in the possible warming of the planet’s climate. This study was published in Nature by 19 researchers from three participating countries. They include thirteen glaciologists and climatologists from CNRS laboratories in Grenoble and the joint CEA-CNRS unit in Saclay (1).

The 3,623 meters of ice core bored in Vostok, Antarctica, are the result of a 10-year collaboration between Russia, France and the United States (2). The Vostok site is considered one of the least hospitable on Earth, since its altitude is 3,500 meters and its average yearly temperature is minus 55° C.
The analysis of this ice core provides a record of the atmosphere during the last four climatic cycles. For the first time, researchers were able to establish, over such a long period, a simultaneous record of temperature variations, carbon dioxide (CO2), methane (CH4) and oxygen proportions in the atmosphere, of the quantity of dust blown from the desert and aerosols from sea spray.
The climate and environment parameters show the existence of four major cycles, with a periodicity of 100,000, 40,000 and 20,000 years. During the four cycles, the atmospheric characteristics varied within rather stable limits, with a temperature amplitude variation in Antarctica of about 12°C at land level and 8°C in the troposphere. Between cold and hot periods, the greenhouse gas concentrations in the global atmosphere varied between 180 (ppmv (parts per million in volume) and 280 ppmv for carbon dioxide, and between 350 ppbv (parts per billion in volume) and 700 ppbv for CH4.
According to these findings, the greenhouse gas concentrations are correlated to the Antarctic temperature over the entire period under study, which confirms previous observations made for the last 150,000 years. This link also appears during the warmest interglacial periods, when greenhouse gas concentrations were at their highest (300 ppmv of CO2 and 750 ppbv of CH4). These values are nevertheless far below the level of present concentrations — 360 ppmv of CO2 and 1,700 ppbv of CH4. Such levels are unprecedented during the past 420,000 years.
Each of the four large glaciation periods was followed by a transitional interglacial period, towards the years – 310,000, -240,00, -135,000 and –15,000. The end of the glacial period was usually the coldest, and the transition towards a warmer climate took 5 to 10,000 years. In each of the four transitional periods, according to the analysis of the ice samples, the same sequence of events took place: the increase in the concentration of greenhouse gases (CO2 and CH4) was almost immediately (more or less 1,000 years) followed by the warming of the upper southern latitudes. And only several thousand years later, the Northern hemisphere warmed up and the ice caps that had formed massively merged. These findings show how the climate transmission system between the two hemispheres worked, and will serve as a basis for climate modelling systems.
The climate cycles, just as those observed for marine sediments, show the impact of the changes, however slight, in the Earth’s orbit on climate variations. But yet more has been discovered. Our warm period, which began 11,000 years ago, seems to be longest ever in 420,000 years. The sunshine variations are not significant enough to explain the amplitude of the observed climate changes. These findings confirm the idea, suggested a decade ago, that greenhouse gases, by amplifying the variations initiated by the orbital variations of the Earth, were also responsible for the glacial/interglacial changes. The actual mechanisms still remain to be determined.

(1) Climate and Atmospheric History of the past 420,000 years from the Vostok Ice Core, Antarctica, by Petit J.R., Jouzel J., Raynaud D., Barkov N.I., Barnola J.M., Basile I., Bender M., Chappellaz J., Davis J. Delaygue G., Delmotte M. Kotlyakov V.M., Legrand M., Lipenkov V.M., Lorius C., Pépin L., Ritz C., Saltzman E., Stievenard M., Nature, 3 June 1999.

(2) The project is supported by the Russian Ministry of Sciences, and in the United States, by the Office of Polar Programs of the NSF. In France, the project is coordinated by the Laboratoire de Glaciologie et Géophysique de l’Environnement du CNRS and carried out in the framework of a close collaboration between the CNRS/INSU, the CEA/DSM and the Institut français de recherche et technologie polaire in Brest. It is also supported by the National Programme for the study of climate dynamics of the European Commission, the Fondation de France, the Rhônes-Alpes region and the J. Fourrier University in Grenoble.

Researcher contacts:
Jean Robert Petit and Dominique Raynaud
Laboratoire de Glaciologie et Géophysique de l’Environnement – CNRS
Tel: 33 4 76 82 42 44/42 45
(Photos available at the laboratory)
Jean Jouzel
Laboratoire des sciences du climat et de l’environnement CEA-CNRS
Tel: 01 69 08 77 13
Press contact:
Séverine Duparcq
Tel: 33 1 44 96 46 06

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