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Polar Heat Waves

The frozen polar regions provide a treasure trove of information on the distant past, preserving both snapshots of Paleolithic climates and long-extinct ecologies. Recent findings have unearthed surprising secrets from the planet's cold extremities, changing our understanding of how the global thermostat actually works.

polar heat


Some climatologists, their eyes fixated on the big picture, are in fact “forest people” and Pierre Taberlet, CNRS researcher at LECA,1 helps them, quite literally, see the individual trees. When a team of Danish scientists recovered biological material from a core that had been taken two kilometers below the Greenland ice sheet,2 they turned to the Grenoble-based ecologist for the techniques to identify what they found.

The samples, retrieved from deep below the ice, just two meters above the island's surface, were calculated to be at least 450,000 years old. Identifying the DNA found within the ancient ice could paint a picture of Greenland's ecology and, by extension, shine a light on the earth's climate during this period.

But creating a forest from fragments of Pleistocene-era DNA was no small feat. The strands that survived the rigors of millennia in the deep freeze were short–too short to identify through conventional means.

Taberlet was able to help them using a technique called “DNA bar-coding.” This relatively new concept can identify individual species of plants with only a short segment of chloroplast DNA (cpDNA)–a 100 base pairs long or less. By creating universal primers corresponding to cpDNA, and then amplifying them with a polymerase chain reaction (PCR), individual species can be identified from very short DNA fragments.

“We were able to identify tree species, meaning that Greenland had been covered with forests similar to what we have in Sweden today,” says Taberlet. And this leads to many questions about Greenland's climate history. “The dating was interesting: 450,000 years is older than the last interglacial period which occurred 125,000 years ago,” he adds. It is believed that this interglacial period was five degrees warmer than the present day. At those temperatures, the Greenland ice cap should have retreated, leaving room for the ancient forest or a similar ecology to reassert itself. Yet no biological material younger than 450,000 years old was recovered, suggesting that the ice cap remained present in an environment warmer than the present day. If confirmed, such results would challenge the current climate models that have the ice caps melting at these temperatures.

But things are also heating up at the other end of the globe. The Antarctic EPICA3 researchers recently released their analysis of ice cores from the Dome C region of the Antarctic, taken 3260 meters below the surface.4

 “These cores double the previous temperature records obtained from Vostok drillings, and go back 800,000 years,” explains Jean Jouzel, from LSCE5 who coordinated the EPICA project from 1995 to 2001. A diary of the earth's atmosphere, these cores marked the completion of EPICA's drilling. More than three kilometers thick–among the thickest of the Antarctic–the ice cap of Dome C is the result of repeated snowfalls. These frozen precipitations, and the air trapped within, is an archive of the state of the earth's climate at any given moment.

In addition to the usual determination of carbon and methane content of the atmosphere from this period, the method developed by Jouzel's team was capable of analyzing the presence and concentration of two isotopes: deuterium and oxygen 18. Their concentrations in precipitation vary according to ambient temperatures, and thus provide an accurate reading of oceanic and atmospheric temperatures through time.

The resulting curves of ice ages and interglacial periods derived from the EPICA cores show a series of 100,000 year temperature spikes and dips, including previously unknown ones at the further end of the scale. The curves show long, colder interglacial periods giving way to shorter, warmer ones. The variations in Antarctic temperature are remarkably correlated to sea-level change along the 800,000 years as recorded in submarine sediments. Finally, the EPICA data confirm the link that exists between climatic change in Antarctica and Greenland, emphasizing the active role that the high southern latitudes play in the dynamics of climate change. “Glaciologists are now looking to study older ice samples, in the hope of surveying the climate as it was a million years ago,” concludes Jouzel.


Mark Reynolds 

Notes :

1. Laboratoire d'écologie alpine (CNRS / Université Grenoble I et Chambery).
2. E. Willerslev et al., “Ancient biomolecules from deep ice cores reveal a forested Southern Greenland,” Science. 317(5834): 111-4. 2007.
3. European Project for Ice Coring in Antarctica.
4. J. Jouzel et al., “Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years,” Science. 317(5839): 793-6. 2007.
5. Laboratoire des sciences du climat et de l'environnement (CEA / CNRS / Université Versailles Saint-Quentin-en-Yvelines).

Contacts :

LCMD, Paris.
> Pierre Taberlet, LECA, Grenoble.
> Jean Jouzel, LSCE, Gif-sur-Yvette.


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