New Research in Science Shows Highest CO₂ Levels In 650,000 Years

With the first in-depth analysis of the air bubbles trapped in the “EPICA Dome C” ice core from East Antarctica, European researchers have extended the greenhouse gas record back to 650,000 years before the present. This 210,000-year extension of atmospheric carbon dioxide and methane records — encompassing two full glacial cycles — should help scientists better understand climate change and the nature of the current warm period on Earth.

The record may also aid researchers in reducing uncertainty in predictions of future climate change and help to clarify when humans began significantly changing the balance of greenhouse gasses in Earth’s atmosphere.

EPICA is the European Project for Ice Coring in Antarctica. The new ice core, initially described in 2004, is from a site in East Antarctica known as EPICA Dome C. This work represents a long-term European research collaboration and appears in two studies and an accompanying “Perspective” article in the 25 November 2005 issue of the journal Science, published by AAAS.

One study chronicles the stable relationship between climate and the carbon cycle during the Pleistocene (390,000 to 650,000 years before the present). The second one documents atmospheric methane and nitrous oxide levels over the same period.

The analysis highlights the fact that today’s rising atmospheric carbon dioxide concentration, at 380 parts per million by volume, is already 27 percent higher than its highest recorded level during the last 650,000 years, said Science author Thomas Stocker of the Physics Institute of the University of Bern, in Bern, Switzerland, who serves as the corresponding author for both papers.

“We have added another piece of information showing that the timescales on which humans have changed the composition of the atmosphere are extremely short compared to the natural time cycles of the climate system,” Stocker explained.

The new work confirms the stable relationship between Antarctic climate and the greenhouse gasses carbon dioxide and methane during the last four glacial cycles. The new ice core analysis also extends this relationship back another two glacial cycles, to a time when the warm “interglacial” periods were milder and longer than more recent warm periods, according to the European researchers.

The fact that carbon dioxide and methane levels were lower during the relatively mild warm periods of the two additional cycles, compared to the warmer warm periods of the last 400,000 years, is especially interesting for the study of climate sensitivity, which is a measure of how the climate system reacts when atmospheric carbon dioxide concentrations double, explained Science author Dominique Raynaud from LGGE in Grenoble, France.

The new atmospheric and climate records from the EPICA Dome C ice core also indicate that the response of the natural carbon cycle to climate warming remains the same over time — in terms of the mechanism involved and the degree to which greenhouse gasses further amplify climate change, explained Science author Jean Jouzel from LSCE and Institut Pierre Simon Laplace in France.

The EPICA Dome C ice core contains hundreds of thousands of years’ worth of atmospheric air samples within tiny bubbles trapped in the ice. The air bubbles form when snowflakes fall, and they contain a record of global greenhouse gas concentrations.

The new ice core record described in the two Science papers provides some overlap with a similar record from the Vostok ice core — now, the second longest ice core record — and extends the Vostok record by 210,000 years.

The nitrous oxide record in EPICA Dome C is more fragmented and less clear than the carbon dioxide and methane records due to artifacts in the ice that appear related to the dust levels.

The new ice core analysis provides insights on our present interglacial warm period through a glimpse into Antarctic climate and greenhouse gas concentrations during the most recent warm period that is relatively similar to our current warm period. Known as Marine Isotope Stage 11 or MIS 11, this analog warm period occurred between 420,000 and 400,000 years and is not completely covered by the Vostok record.

The similarities between our current warm period and MIS 11 are primarily due to a similar configuration of the orbits of the Earth around the Sun: the relative positions of the Earth and Sun are thought to be the key driver of ice age cycles.

“MIS 11 shows us that the climate system can indeed reside in a warm period for 20,000 or 30,000 years, something that we can’t say based on the last three warm phases which are no longer than about 10,000 years each,” said Stocker.

We are now about 10,000 years into our current warm period.

The new papers also document MIS 13 and 15 — two warm periods more distant than MIS 11 that may have been about as long. The idea that MIS 13 and 15 were long warm periods contrasts the argument scientists have made in the past suggesting that our current warm period is exceptionally long.

The authors note, however, that the records for MIS 13 and 15 are not as clear as they are for MIS 11. One complicating factor is that the ice core records do not exactly match records from marine sediments that are used to help date the ice core data.

New insights important for understanding the impact early human activities such as land clearing and rice culture had on atmospheric greenhouse gas concentrations, the topic of several recent studies, are also now available, thanks to the methane and carbon dioxide records from the EPICA Dome C ice core. The new record shows that natural variability can result in significant oscillations in greenhouse gasses during some interglacial periods and raises the possibility that early human activities may not be responsible for the greenhouse gas variability seen as early as 10,000 years ago, writes Ed Brook from Oregon State University in Corvallis, Oregon in a related “Perspective” article.

The greenhouse gas record from EPICA Dome C during past ice ages also provides indirect evidence for abrupt climate change in the past, the authors found. This suggests that abrupt climatic events on time scales relevant to societies may be common features of the last climatic cycles.

The stable relationship between carbon dioxide, methane and Antarctic climate over the last 650,000 years highlights one of the major unsolved mysteries of climate change — the origins of climate-greenhouse gas relationships. Organic decomposition in subtropical wetlands remains a strong candidate for explaining the climate-methane relationship. On the other hand, oceans seem to play a critical role in the climate-carbon dioxide relationship; and the new work strengthens the idea that high latitude Southern Ocean processes are important for controlling glacial-interglacial variations in carbon dioxide, according to the “Perspective” author who says that retrieval and analysis of even older ice cores may provide more definitive answers.

“Stable Carbon Cycle-Climate Relationship During the Late Pleistocene,” by U. Siegenthaler, T.F. Stocker, E. Monnin, D. Lüthi, J. Schwander and B. Stauffer at University of Bern in Switzerland; D. Raynaud and J.-M. Barnola at Laboratoire de Glaciologie et de Géophysique de l'Environnement (CNRS) St Martin d'Hères Cedex, France; H. Fischer at Alfred-Wegener-Institute for Polar and Marine Research (AWI) in Bremerhaven, Germany; V. Masson-Delmotte and J. Jouzel at LSCE and Institut Pierre Simon Laplace in France.

“Atmospheric Methane and Nitrous Oxide of the Late Pleistocene from Antarctic Ice Cores,” by R. Spahni, T. Stocker, G. Hausammann, K. Kawamura, J. Flückiger and Jakob Schwander at University of Bern, in Bern, Switzerland; J. Chappellaz, L. Loulergue and D. Raynaud at Laboratoire de Glaciologie et de Géophysique de l'Environnement (CNRS) in St Martin d'Hères Cedex, France; V. Masson-Delmotte, J. Jouzel at LSCE and Institut Pierre Simon Laplace in France. K. Kawamura is now at Scripps Institution of Oceanography, University of California- San Diego in La Jolla. J. Flückiger is now at Institute of Arctic and Alpine Research, University of Colorado at Boulder.

The accompanying “Perspective” article “Tiny Bubbles Tell All,” is by E. Brook from Oregon State University in Corvallis, Ore.

The work described in the Siegenthaler et al. and Spahni et al. Science papers is a contribution to the “European Project for Ice Coring in Antarctica” (EPICA), a joint ESF (European Science Foundation)/EC scientific program, funded by the European Commission and by national contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the United Kingdom.

The researchers acknowledge long-term financial support by the Swiss NSF, the University of Bern and the Swiss Federal Agency of Energy, and EC Project EPICA-MIS. Support was also provided by the French program PNEDC (INSU-CNRS).

This news release is available in German and in French.

Daniel B. Kane

28 November 2005