A record of greenhouse gases spanning the past 650,000 years made headlines around the globe last week. The painstaking work proves that levels of carbon dioxide and methane in the atmosphere today massively outstrip those of the pre-industrial era. But it also reveals how little we understand about the way in which these gases influence global climate.

Researchers from the European Project for Ice Coring in Antarctica (EPICA) sampled air bubbles preserved inside a 3,000-metre ice core drilled at Dome C in eastern Antarctica. The data show previous fluctuations in levels of greenhouse gases in unprecedented detail (see Science 310, 1313–1317; 1317–1321; 2005).

Before the results were in, Eric Wolff, a physical scientist at the British Antarctic Survey in Cambridge, UK, set what came to be known as the ‘EPICA challenge’. From previously published data on the prehistoric temperature record, could anyone predict the EPICA gas record? He published the resulting efforts in September (Eos 86, 341–345; 2005).

Bubble whammy: gas in ancient ice shows global models need work. Credit: W. BERNER/UNIV. BERN

Now that the actual figures have been published, Wolff says all of the models correctly predicted that carbon dioxide levels in the period covering the four most recent ice ages would be higher than between previous ones. The surprise was that entrants who used global models, such as Peter Köhler of the Alfred Wegener Institute in Bremerhaven, Germany, did less well than those who considered only the Southern Ocean, such as Didier Paillard of the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette, France.

This highlights the degree to which the Southern Ocean influences global greenhouse-gas levels — owing to its size and the fact that its cold, deep waters put large amounts of dissolved carbon dioxide out of circulation. But the less successful performance of Köhler's model, which includes factors such as the effects of vegetation, shows that comprehensive models of the carbon cycle need improvement, says Wolff.

Difficulties in predicting future climate are compounded by the fact that greenhouse-gas levels are set to go off the scale relative to those in the EPICA record. Carbon dioxide did not exceed 290 parts per million in the 650,000 years before the advent of fossil fuels — it now stands at 375 parts per million, and many policy-makers are discussing strategies to stabilize it at an ultimate level of 550. “It seems like a dangerous experiment to me,” says Wolff.

The results of that ‘experiment’ are already starting to come in. In the past week alone, several groups have warned that the Atlantic Ocean is in trouble (see ‘Atlantic feels climate heat’). Researchers hope the findings will focus world leaders, who are currently gathered in Montreal, Canada, to plan emissions policies following the end of the first phase of the Kyoto Protocol in 2012.

“It's clear that we need to stabilize greenhouse gases at a level that allows food production but avoids dangerous interference with the climate,” says Thomas Stocker of the University of Bern, Switzerland, who led the latest EPICA analysis. “This cannot be done by quick fixes.”