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Paris, April 16, 2008

Continental wetlands, a natural regulator of atmospheric methane

Researchers at the Laboratory for Environmental Geophysics and Glaciology (Laboratoire de glaciologie et géophysique de l'environnement)(1) as well as other researchers from the European ice-coring program EPICA(2), have identified the main reasons for the doubling of methane content between -18 000 years (the last glacial maximum) and -11 500 years ago (the current "warm" period). This appears to be due to the expansion of continental wetlands, such as boreal wetlands, whose methane emissions were practically non-existent during the glacial maximum. Even more surprisingly, methane emissions associated with biomass burning appear to have remained constant during this major climate transition. Published in the 17 April 2008 issue of the journal Nature, these results will enable us to get a better understanding of the mechanisms that may in the future amplify natural emissions of methane, a greenhouse gas which is thought to play an important role in global warming.

Ice cores form the only available record for the precise reconstruction of the composition of the atmosphere in the past, and especially of its content in greenhouse gases such as methane. Methane is the second most abundant greenhouse gas of human origin after carbon dioxide. The methane content of the atmosphere has increased by over 150% during the last 200 years, due to anthropogenic emissions (agriculture,exploitation of natural gas, etc).

On the basis of the analysis of two deep ice cores drilled in Antarctica, some of the scientists involved in the European consortium EPICA sought to get a better understanding of natural variations in atmospheric methane over time.  By analyzing the gas trapped in the ice cores, these specialists obtained the first ever detailed trend in the carbon-13/carbon-12 isotope ratio of the carbon contained in the methane molecule (written d13CH4), during the whole of the climate transition that took place between -18 000 years (glacial epoch) and -11 500 years (beginning of the Holocene(3)). This new signal provides additional information that can be used to explain the origin of natural methane variability.

This means that researchers now have a set of four unique types of data on methane: changes in atmospheric methane concentration, the difference in concentration between the two poles (measured in Greenland and Antarctica) – two results obtained by the team led by Jérôme Chappellaz, senior researcher at CNRS and deputy director of LGGE -, the deuterium/hydrogen isotope ratio of methane and, now, the d13CH4 ratio.

By combining these data with a simple model of the atmosphere, the researchers managed to identify the processes responsible for the doubling in atmospheric methane concentration during the last glacial-interglacial transition. Their first conclusion is that over half of this increase was due to a substantial increase in methane emissions from tropical wetlands. In addition, their analyses show that emissions of methane from boreal marshes and peatlands (i.e. at high latitudes in the northern hemisphere) were almost non-existent under glacial conditions. Such emissions only started to have an effect during the climate transition. Lastly, and surprisingly, biomass burning, which is responsible today for around 20% of natural methane emissions, did not cause significant variations in atmospheric methane content during this major change.

These results, which are of prime importance, underline the role played by continental wetlands as sources of methane, both in tropical and boreal environments.

methane 1

© CNRS Photothèque / AUGUSTIN Laurent (these images can be obtained from the CNRS photo library (photothèque du CNRS, phototheque@cnrs-bellevue.fr)

Ice core emerging from the drill pipe, drilled at Dome C as part of the EPICA program (European Project for Ice Coring in Antarctica).



methane 2

© CNRS Photothèque/IPEV / FRENOT Yves

Core library at the French-Italian base Concordia, Dome C (Antarctica).

Notes:

1) LGGE-OSUG, CNRS / Université Joseph Fourier.
2) Coordinated by the European Science Foundation (ESF), the EPICA project (European Project for Ice Coring in Antarctica) is funded by the 10 European countries taking part, including France (EPICA is backed in particular by various programs at the Paul-Emile Victor French Polar Institute and at INSU-CNRS, by the French Atomic Energy Agency CEA and by an ANR (National Research Agency) project), as well as by the European Union.
3) The warm period during which humans adopted a sedentary lifestyle.

References:

Changing boreal methane sources and constant biomass burning during the last termination. Hubertus Fischer, Melanie Behrens, Michael Bock, Ulrike Richter, Jochen Schmitt, Laetitia Loulergue, Jerome Chappellaz, Renato Spahni, Thomas Blunier, Markus Leuenberger & Thomas F. Stocker. Nature. 17 April 2008.

Contact information:

Researcher
Jérôme Chappellaz
T 04 76 82 42 64 / 06 72 83 12 69
jerome@lgge.obs.ujf-grenoble.fr

Public Information Officer
Priscilla Dacher
T 01 44 96 46 06
priscilla.dacher@cnrs-dir.fr

INSU/CNRS Communication
Dominique Armand
T 01 44 96 43 68
dominique.armand@cnrs-dir.fr

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