Ice Cores Show Highest
Greenhouse Gases Now
In 420,000 Years
By Mark Schrope
Discovery News Brief
Studies of the deepest ice core ever taken reveal an ancient record of Earth's climate over the last 420,000 years that suggests current levels of greenhouse gases are at historical highs.
An international team analyzed the nearly 12,000-foot-long core taken from Russia's Lake Vostok. Their findings, published in this week's journal Nature, indicate the Earth has gone through an ice age about four times in the past 420,000 years.
Only about 11 degrees Fahrenheit of change in temperature generated such dramatic shifts, the team says.
Researchers reached their conclusions by analyzing the hydrogen in water molecules from the ice for changes in the abundance of different isotopes, or elements with slightly varying structures.
Levels of hydrogen isotopes vary according to the temperature at the time the water falls as snow, creating a kind of historical record. Remarkably, the low temperatures for all four glacial periods differed by only about 2 degrees Fahrenheit.
The team also measured carbon dioxide and methane -- key greenhouse gases -- in bubbles trapped in the ice core.
Increases in these gases were tightly linked with the rises in global temperature that led to the warmer periods, according to glaciologist Jean Robert Petit, lead author of the study.
The team reports that increases in greenhouse gases were probably responsible for about half of the temperature increase during climate cycles.
However, the researchers didn't speculate on what may have triggered the hike in greenhouse gases in ancient times.
The ice cores also revealed that, at least during the 420,000 years, methane and carbon dioxide levels have never been as high as they are now, the researchers say.
All of the findings confirmed previous ice core research, which only went back through two climate cycles.
"Extending the record to four climactic cycles is very important," says Bernhard Stauffer, an ice core specialist at the University of Bern in Switzerland. With only two cycles, he says, it was difficult to say whether findings indicated general rules or exceptions in the Earth's history.