By Sid Perkins
A kilometers-long ice core from Antarctica has recorded climate information for the past 800,000 years and has revealed a three millennia–long period when carbon dioxide levels in the air were lower than any previously measured.
The longest detailed records of atmospheric gases previously reported, from the uppermost sections of a 3.2 kilometer–long ice core drilled in eastern Antarctica, go back 650,000 years, says Thomas Stocker, a climate physicist at the University of Bern in Switzerland. Isotopic analyses of the ice in the deepest portions of that sample — at depths between 3,060 meters and 3,190 meters — have revealed how temperature in the region varied between 650,000 and 800,000 years ago. But researchers previously hadn’t assayed the gases trapped in bubbles in that portion of the core, Stocker notes. He and his colleagues have now performed those analyses and report their findings in the May 15 Nature.
Once snow piles up more than 80 meters or so deep, the pressure at the bottom of the heap converts the densely packed, somewhat porous snow into impermeable ice, thereby locking bubbles of air in place. As snow continues to accumulate, the mass of ice — whether a mountain glacier or a continent-wide ice sheet — becomes a chronicle of long-term variations in the atmospheric concentrations of various gases, including those such as carbon dioxide and methane that are linked to climate change.