Évolution of greenhouse gases over the last 800,000 years

For the first time, they have reconstituted tthe evolution, over 800,000 years, of levels of carbon dioxide and methane, the two main greenhouse gases after water vapor. With these new numbers, the researchers now have access to data which will help them better predict future climate changes on earth. The results are published in two articles in the 15 May 2008 issue of /Nature/.*

In the absence of greenhouse gases (water vapor, carbon dioxide, methane…), the average temperature on earth would be -18°C, resulting in conditions unable to sustain life. The concentration of these gases in the atmosphere has substantially increased over time, due to human activity (fossil fuel combustion, development of agriculture). Studying the evolution of these concentrations allows us to better understand their interaction with the earth's climate, and this type of study is carried thanks to ice cores, which contain the only available records of greenhouse gas levels.

An ice core drilled in Antartica near the Franco-Italian base Dome Concordia, as part of the EPICA project, reached 3270 meters in December 2004, stopping a few meters above solid rock. At these depths, the ice dates back 800,000 years, or 8 glaciary-interglaciary climatic cyles. This is the oldest ice ever cored until now, and the analysis of gas bubbles trapped in the ice has allowed recordings of levels of carbon dioxide (CO2) and methane (CH4) in the atmosphere 800,000 years ago (previous recordings only went back as far as 650,000 years ago). In light of these new measurements, researchers have access, for the first time, to reference curves for levels of CO2 and CH4, showing the evolution of the gases in ancient times. This is precious information for scientists attempting to understand the correlation between climate change on earth and the carbon cycle. These results give hope for better predictions of future levels of greenhouse gases, and in theory, of the earth's climate.

This work has already enabled researchers to make major progress in certain areas. It confirms the close correlation between temperatures recorded in Antartica in the past and atmospheric levels of CO2 and CH4. Another significant observation is that never, in 800,000 years, have greenhouse gas levels been as high as they are today (current levels surpass 380 ppmv (5) for CO2 and 1800 ppbv (6) for CH4). The CO2 curve also shows that the lowest levels ever recorded were 172 ppmv, 667,000 years ago. Moreover, researchers have shown the existence of a modulation in atmospheric CO2 levels on a relatively long time scale, namely several hundreds of thousands of years. This unique phenomenon could stem from the more of less significant intensity of continental erosion which affects the carbon cycle over large time scales.

Thanks to the remarkably detailed records of atmospheric methane, researchers have noted an increase over time in the periodicity of a component called precession . This signal, which is correlated to monsoon intensities in South East Asia over millenia, probably reflects an intensification of the monsoon in tropical regions over the last
800,000 years. The methane curve shows rapid fluctuations at the millenial scale which are recurrent for each ice age. The mark of such events can also been seen in the CO2 signal from 770,000 years ago, when the earth entered a new ice after the magnetic reversal which occurred 780,000 years ago. This rapid climate variability is apparently related to fluctuations in the thermohalin (large scale circulation of water,

which helps to redistribute temperature around the globe). The issue of why this phenomemon appears at the beginning of the ice ages remains to be explained.

(1) Coordinated by the European Science Foundation (ESF) and the European Union, EPICA, or “European Project for Ice Coring in Antarctica”, is supported financially by the EU and the 10 countries participating in the drilling (Belgium, Denmark, France, Germany, Italy, Holland, Norway, Sweden, Switzerland and the UK). French researchers are supported by the Agence nationale de la recherche (ANR), the Institut
national des sciences de l'univers (INSU-CNRS) and CEA. Field logistics at Dom C are organized by Institut polaire français Paul-Emile Victor (IPEV), together with the National Italian Program for Antarctic research. EPICA was awarded the Prix Descartes for research in March 2008.
(2) Laboratoire de glaciologie et géophysique de l'environnement, CNRS / Université Joseph Fourier
(3) Laboratoire des sciences du climat et de l'environnement, CNRS / CEA/ Université Versailles Saint Quentin
(4) Institut de Physique and Centre Oeschger sur la recherche climatique of Université de Berne (Switzerland), among others.
(5) This means that for every million air molecules, 380 are CO2molecules. ppmv = part per million in volume.

(6) This means that for every billion air molecules, 1800 are CH4 molecules. ppbv = part per billion in volume.

*BIBLIOGRAPHY*
High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. /Nature/. 15 May 2008.

Orbital and millennial-scale features of atmospheric CH4 over the last 800,000 years. Loulergue, L., A. Schilt, R. Spahni, V. Masson-Delmotte,T. Blunier, B. Lemieux, J.-M. Barnola, D. Raynaud, T.F. Stocker, and J. Chappellaz. /Nature/. 15 May 2008.