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Paleoclimatology

EPICA's Weather Report

It took 800,000 years in the making and just a few years to take it out of the ground. Scientists of the 10 European countries that make up the EPICA(1) team are analyzing a 3260-meter-long cylinder of ice and the gases trapped within to paint a portrait of the Earth's past climate.

epica

© C. Delhaye/CNRS Photothèque/IPEV

Part of the EPICA ice core wrapped and stored at Concordia Dome C.




There are few places on Earth more desolate than Dome-C, site of the joint Italy-France Concordia Antarctic research station. Yet it has proved to be fertile ground for climate scientists of the EPICA project. “The initiative has far surpassed initial hopes,” comments Jean Jouzel from LSCE,2 who was director of the EPICA project from 1995 to 2001. “We promised that we would get a core going back 500,000 years–we far exceeded that,” he adds. The collaboration was “truly European science,” says Jouzel, the EPICA cores have yielded some 200 collaborative papers, 14 of which have appeared in either Science or Nature. And there are many more to come.
One of the recent papers detailed the atmospheric content of carbon dioxide as far back as 800,000 years,3 extending climate records by 150,000 years. Dominique Raynaud, from LGGE,4 explains that the Earth’s carbon cycle works over 100,000 year periods with a strong link to Antarctic climate and atmospheric temperatures. “We found in the older cycle–800,000-650,000 years ago–the lowest CO2 values ever recorded in ice,” he adds. CO2 was measured at 172-180 parts per million during the coldest periods as compared to 240-300 parts per million during warm periods, and 380 parts per million today. This low level of CO2, compared to later levels, was not associated to abnormally cold temperatures, as could have been expected given the gas’s links to the greenhouse effect. That dissociation offers a tantalizing glimpse into the mysteries of paleoclimate to be investigated in the future.
These results on the carbon history of the atmosphere helped Jérôme Chappellaz, also from LGGE, in his own detailed analysis of methane (CH4) fluctuations over the same 800,000-year period.5 This analysis showed that methane–the third most important greenhouse gas after CO2 and water vapor–has never been as prevalent in the atmosphere as it is today (1800 parts per billion, versus historical highs of about 800 ppb).
What they also found was that methane levels in the atmosphere did rise dramatically between ice-ages. “The difference is very significant–interglaciation levels are twice those found during glaciation periods.” Accounting for this doubling required a lot of investigation. Analysis of the methane’s isotopic signature showed that during ice ages, production from northern Boreal wetlands shut down, accounting for part of the reduction associated to glaciation periods. Atmospheric conditions provided another part. Methane is removed from the atmosphere by hydroxyl (OH), a radical molecule formed from ozone that only exists in the atmosphere for a few seconds. During glaciation periods, tropical vegetation produced less of a compound called isoprene–a competitor of methane for OH through atmospheric chemical reactions. This left more OH in the atmosphere to oxidize methane.
“But the surprise came when we deduced from the isotopic measurements that no significant decrease in biomass burning took place during glaciation,” says Chappellaz. The distinct isotopic signature of biomass methane–released mainly from smoldering wildfires–remained at the same level, even when the northern woodlands were under ice.
This implies complicated changes in terrestrial climate dynamics, which Valérie Masson-Delmotte from LSCE is trying to further untangle. She explains that the methane data was correlated to known variations in the Earth’s orbit that lead to changes in the angle of the Earth’s position during solstices, and thus changes in exposure to solar radiation. “We know that there is a strong link between increased solar radiation and monsoons–whose intensities increase when the contrast between summer and winter is enhanced.” These tropical precipitations, in turn, pump more methane into the atmosphere. “The EPICA scientists are already planning further coring operations in the Antarctic with partners from Europe, China, the US, and Japan,” she adds. They hope to travel as far back as 1.5 million years in the past. “For future climate modeling, it is important to understand what controls natural variation. It’s really only the beginning,” concludes Masson-Delmotte.

Mark Reynolds

Notes :

1. EPICA project (European Project for Ice Coring in Antarctica).
2. Laboratoire des sciences du climat et de l'environnement (CEA / CNRS / Université Versailles Saint-Quentin-en-Yvelines).
3. D. Lüthi et al., “High-resolution carbon dioxide concentration record 650,000-800,000 years before present,” Nature, 2008. 453: 379-82
4. Laboratoire de glaciologie et géophysique de l'environnement (CNRS / Université Joseph Fourier-Grenoble).

5. H. Fisher et al., “Changing boreal methane sources and constant biomass burning during the last termination,” Nature, 2008. 452: 864-7
6. L. Loulergue et al., “Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years,” Nature, 2008. 453: 383-6

Contacts :

LSCE, Gif-sur-Yvette.
Jean Jouzel,
Jean.Jouzel@lsce.ipsl.fr
Valérie Masson-Delmotte,
Valerie.Masson@lsce.ipsl.fr
LGGE, Saint-Martin d'Heres.
Jérôme Chappellaz,
chappellaz@lgge.obs.ujf-grenoble.fr
Dominique Raynaud,
raynaud@lgge.obs.ujf-grenoble.fr


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