The amount of greenhouse gas in the atmosphere is higher than an any other point in the past 800,000 years.
Greenhouse-gas concentrations are higher today than they have been at any point in hundreds of millennia, according to researchers who have analysed tiny air bubbles trapped in Antarctic ice that dates back 800,000 years.
Atmospheric levels of carbon dioxide are now more than 28% higher than at any other point in the time period covered by the samples, according to Thomas Stocker, one of the authors of two studies in this week's Nature1,2 and a climate scientist at the University of Bern in Switzerland. Levels of methane, another major contributor to climate change, are now 134% above prehistoric highs.
"It is clear that we have disturbed the atmospheric balance of greenhouse gases to an extent that will have consequences," Stocker says.
Antarctic ice cores provide a unique chance for scientists to obtain atmospheric samples from bygone eras. In normal climates, ice forms when snow melts and refreezes, but Antarctica is too cold for such a process to occur. Instead, upper layers of snow crush the lower ones until they compress into ice. The compression also traps tiny bubbles of air from the same period. By melting the ice and extracting the bubbles, researchers can obtain actual air samples from thousands of years in the past.
The cores obtained by Stocker and his colleagues came from 3 kilometres beneath Dome Concordia, also called Dome C, a plateau more than 3,000 metres high in East Antarctica. They are the last of a series of cores taken by the European Project for Ice Coring in Antarctica (EPICA) programme, an 11-year expedition by 10 European countries and the European Commission.
In addition to providing a basis of comparison for present-day concentrations of greenhouse gases, the samples also provide some intriguing details of past climate swings. For example, during the last ice age there seem to have been periods of rapid warming and cooling that were probably caused by changes in ocean currents. Nobody knew whether the cycles extended to previous ice ages, but the new measurements indicate that they do.
The new findings are likely to have a broader impact, says Eric Wolff, the science chair of the EPICA programme and a researcher at the British Antarctic Survey in Cambridge, UK. He thinks that the data obtained will allow climate scientists to further refine their models of the earth's system. "I'm sure these will be iconic papers for years to come," he says.
With EPICA reaching its end, Wolff says that an even larger international team is now on the hunt for Antarctic ice cores dating back 1.5 million years. But there is no need to hold the front page yet. "I don't suspect we'll start drilling for at least another five or six years," he says.
Lüthi, D. et al. Nature 453, 379-382 (2008).
Loulergue, L. et al. Nature 453, 383-386 (2008).