Ice Age air reveals greenhouse gas story.

Ice Age air reveals greenhouse gas story

Geochemists have faced in recent years a seeming paradox concerning the atmospheric concentration of methane, the second most important "greenhouse" gas (behind carbon dioxide). While methane levels have clearly risen by nearly 250 percent during the last 200 years as a result of human activity, several experiments had suggested that before this industrial-age increase, the methane concentration had remained surprisingly constant, even during the last Ice Age -- a time that drastically affected other trace gases.

Now a group of researchers from the University of Bern in Switzerland seems to have resolved the problem by completing the first study documenting methane levels over the last 100,000 years. By measuring the gases trapped in ice layers from Antarctica and Greenland, the group found that methane concentrations have indeed fluctuated over time, especially during the Ice Age, they report in the April 28 NATURE.

"This is the first indication we've had that there really have been variations in atmospheric methane, going in and out of times of glaciation," says Ralph Cicerone of the National Center for Atmospheric Research in Boulder, Colo. "It sounds to me very important because it was rather suspicious that methane was just constant."

Based on numerous studies, scientists believe that before methane started accumulating in the atmosphere 200 years ago, its levels had remained near 650 parts per billion by volume (ppbv) for 3,000 years.

In contrast, the record for times much earlier is almost bare. Aside from the new study, only one previously published report looked at methane before the 3,000-year stable period. This work indicated that 70,000 years ago, the air's methane concentration was also 650 ppbv. Because this level matched the well-documented methane levels for the last few thousand years, the results suggested that methane concentrations in the atmosphere may have remained relatively constant over time.

The Swiss study, however, traced methane levels for a total of 24 times during the last 100,000 years at the two locations. They found that 60,000 years ago, methane was present at around 500 ppbv. At the height of the Ice Age, 20,000 years ago, the level dropped to 350 ppbv, or roughly half the preindustrial level.

The Swiss researchers used ice cores drilled from the thick caps covering Antarctica and Greenland. Over 2,000 meters in length, these cores contain a 100,000-year-long set of air samples trapped within bubbles. The researchers melted sections of the core in a vacuum chamber and analyzed the methane content of the released gas.

To explain the low methane levels during the last Ice Age, the researchers suggest that wetlands--a leading source of methane--were less biologically active because of lower temperatures and ice cover.

Cicerone believes the Swiss data will help scientists in the difficult task of determining how much specific modern and ancient sources of methane have contributed to the atmospheric levels. "These kinds of data are telling us something about the effects of glaciation on the terrestrial sources of methane," he says. "We can't say exactly what it's telling us yet, but without these data, we can't even start to think."