Dominique Raynaud,1 Frederic Parrenin,1 Patricia Martinerie,1 Jérôme Chappellaz,1 and Amaelle Landais2
With 1 Figure
The Antarctic ice contains the purest and most direct record of past atmospheric CO2 and can also provide a reliable record of Antarctic/austral high latitude temperature (see Jean Jouzel presentation). Ice cores drilled until now cover 800,000 years of history in Antarctica and allow us to better know glacial-interglacial cycles, which characterize this period. While the main driver of glacial-interglacial variations lies in changes in the Earth’s orbit around the Sun, the response of the climate system involves interplay between changes in ice sheets, lands, oceans, and the atmosphere, modulating natural variations in greenhouse gas concen-trations in the atmosphere. On the whole, Antarctic temperatures were warm during periods of high CO2 concentrations and cold during periods of low CO2 concentrations.
But this correlation does not allow us to disentangle the causal link between CO2 and temperature: was it the greenhouse effect due to CO2, which induced a warming, or was it the warming, which induced the increase in CO2? A supplementary clue comes from the sequence of events between CO2 and temperature. In particular, an essential prerequisite for understanding the role of atmospheric CO2 during past deglacial transitions is to know its phase relationship with climate.
The problem is not trivial because, while Antarctic temperatures are recorded at the sur-face of ice sheets, atmospheric gases such as CO2 are trapped at about 100 m depth – depend-ing on site conditions – at the base of the firn where the bubbles close off, this depth bedepend-ing dependent of past climatic conditions. Monnin et al. (2001), using a model of this lock-in depth during the past, concluded that CO2 had started to rise 800 ± 600 years after Antarctic temperature (as deduced from the EDC deuterium record) at the end of the last ice age. This result, which is in agreement with an earlier study on the Vostok and Taylor Dome ice cores, could support a scenario where the initial release of CO2 into the atmosphere has been a con-sequence of the initial warming at high southern latitudes.
Parrenin et al. (2013) have revised this CO2/Antarctic temperature lead/lag estimate by inferring the air lock-in depth from isotope 15 of nitrogen in air bubbles, which is enriched proportionally to the firn thickness. They also applied an innovative statistical method to
1 Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE), Le Centre National de la Recherche Scientifique (CNRS) / Université Joseph Fourier (UJF), Grenoble, France.
2 Institut Pierre Simon Laplace (IPSL), Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Com-missariat à l’énergie atomique et aux énergies alternatives (CEA) – Le Centre National de la Recherche Scienti-fique (CNRS) – l’Université de Versailles Saint-Quentin (UVSQ), Gif-sur-Yvette, France.
D. Raynaud, F. Parrenin, P. Martinerie, J. Chappellaz, and A. Landais
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determine the Antarctic temperature and its lead/lag to CO2. They determined that CO2 and Antarctic temperature increased at the same time at the end of the last ice age, within 200 years (Fig. 1). This finding now makes it likely that CO2 was responsible, at least partly, of the Antarctic warming at the end of the last ice age. New data and climate model experiments are necessary to disentangle the different contributors of this past natural global warming.
Fig. 1 Various climatic time series over termination 1. From top to bottom: δD from EDC (purple); Stack Antarctic temperature (dark blue); Atmospheric CO2 concentrations (light green); Atmospheric CH4 (taken as a proxy for Northern Hemisphere climate, red); Summer insolation (65°N, black) (adapted from Parrenin et al. 2013).
Landais et al. (2013) also investigated the changes in CO2 and Antarctic temperature during transition T2, the previous glacial-interglacial transition, roughly 136 ka ago. They found that CO2 and Antarctic temperature started increasing in phase around 136 ka ago but they also found a two-phase change in CO2, Antarctic temperature and global climate during the course of the deglaciation.
The Ice Core Record of CO2 – A Focus on the Climate/CO2 Phase Relationship
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Finally, when interpreting the ice core CO2 records in terms of atmospheric trends we should take into account the fact that a significant smoothing effect of the initial atmospheric signal can occur when the atmospheric perturbation occurs at the same time scale as the gas trapping duration at the base of the firn. This is likely the case for the four events occurring during the last glacial-interglacial transition, T1 (Fig. 1). We are now trying to quantify their correspond-ing smoothcorrespond-ing effect and we hope to be able to present the results durcorrespond-ing the symposium.
References
Landais, A., Dreyfus, G., Capron, E., Jouzel, J., Masson-Delmotte, V., Roche, D. M., Prié, F., Caillon, N., Chappellaz, J., Leuenberger, M., Lourantou, A., Parrenin, F., Raynaud, D., and Teste, G.: Two-phase change in CO2, Antarctic temperature and global climate during termination II. Nature Geosci. 6, 1062–1065, 10.1038/NGEO1985 (2013)
Monnin, E., Indermühle, A., Dällenbach, A., Flückiger, J., Stauffer, B., Stocker, T. F., Raynaud, D., and Barnola, J.-M.: Atmospheric CO2 concentrations over the last glacial termination. Science 291, 112–114 (2001) Parrenin, F., Masson-Delmotte, V., Köhler, P., Raynaud, D., Paillard, D., Schwander, J., Barbante, C.,
Landais, A., Wegner, A., and Jouzel, J.: Synchroneous change of atmospheric CO2 and Antarctic temperature during the deglacial warming. Science 339, 1060 –1063 (2013)
Dr. Dominique Raynaud
Laboratoire de Glaciologie et Géophysique de l’Environnement 54, rue Molière
BP 96
38402 Saint-Martin d’Hères cedex France
Phone: +33 4 76824252 Fax: +33 4 76824201
E-Mail: raynaud@lgge.obs.ujf-grenoble.fr
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