Antarctic Ice Core Provides Clues on Global Warming

A 2-mile-long ice core laboriously drilled out of an Antarctic ice sheet shows that levels of heat-trapping greenhouse gases are higher now than at any time in the last 420,000 years.

As the longest ice core record of Earth’s weather history obtained to date, the core also shows that those gases--carbon dioxide and methane--play a big role in warming the planet when ice ages end.

But how this icy record will influence current theories about global warming blamed on human activity isn’t clear, the researchers said.

“The ice core gives us the past, not the future. But it adds to our thinking about the future, about the future of our climate,” said Jean-Robert Petit, director of research at the National Center for Scientific Research in Grenoble, France. “It’s clear that greenhouse gas levels are unprecedented compared with the previous 400,000 years.”

The core was extracted between 1992 and 1998 by a U.S.-Russian-French team at Russia’s Vostok research station, the coldest spot on the planet. Petit and team members endured readings as cold as minus 70 Fahrenheit, downright balmy compared to the station’s world record low: minus 129.

They stopped drilling about 120 yards short of a subterranean lake the size of Lake Ontario that has been trapped for perhaps millions of years beneath the ice sheet. Scientists want to send sterilized robots to explore the pristine lake and are protecting it from contamination until then.

Each cylinder-like chunk of ice drilled out of the ice sheet contains a record of snowfall, atmospheric chemicals, dust and bubbles of air. Those clues, trapped in icy layers like tree rings, enable scientists to reconstruct past climates.

The lengthy Vostok ice core is particularly significant because previous cores taken from Antarctica and Greenland dated back only about 150,000 years and showed just two ice age cycles. The new core reveals four ice ages at roughly 100,000-year intervals, shedding new light on how the icy interludes end.

All four appear to have given way to balmier times after levels of the heat-trapping gases carbon dioxide and methane rose by amounts smaller than the increase blamed on human industry in the last century.

The multinational team reported its findings in the June 3 issue of the journal Nature.

Petit and colleagues found carbon dioxide levels rose from about 180 parts per million during each ice age’s height to 280-300 ppm in the subsequent warm periods--far below the current levels of 360 ppm.

Methane levels, meanwhile, rose from 320-350 parts per billion during the icy interludes, to 650-770 ppb during the warm spells. Current methane levels are 1,700 ppb.

The levels of both greenhouse gases are expected to continue their rise in the next century as a result of continued burning of fossil fuels, such as coal, and other human activities.

“This study is probably the most convincing evidence to date that humans are making some really large changes to Earth’s climate system,” said Jonathan Overpeck, head of the paleoclimatology program at the National Geophysical Data Center in Boulder, Colo.

“What this says is we’re going well beyond the bounds of natural variation.”

Petit estimates that the rising greenhouse gas concentrations contributed to about 50% of the post-ice age warming. He attributes the remaining heat to periodic shifts in Earth’s orbit that increase the amount of sunlight warming the planet.

The natural rise in the gases was attributed to a variety of factors, including changes in oceanic processing of carbon dioxide, increased plankton activity and the return of methane-producing swamps.

But the core also appears to call into question previous research suggesting a 500- to 1,000-year lag time between the post-ice age temperature increase and the carbon dioxide increase.

Instead, the core suggests that temperatures rose in step with rising carbon dioxide levels, a finding of interest to scientists studying global warming.

“This core is telling us to get busy and understand the climate system because it really could change,” said Richard Alley, a professor of geoscience at Pennsylvania State University.