The amount of snowfall in any given region also governs the age and resolution of the record: Ice cores from regions of high snowfall provide greater resolution, or detail, of the short-term climate record. Ice cores from regions of low snowfall extend further back in time but tell less about any given year, Miller said.
The EPICA ice core extending back 740,000 years was collected on the eastern Antarctica plateau, a region of low snowfall. It is 1.9 miles (3 kilometers) long with each seasonal layer of ice about an inch (two centimeters) thick.
"In three kilometers of ice, you can stack more ice that is two centimeters thin than if it were twenty centimeters [eight inches] thick," Miller said.
This December the EPICA team intends to drill the final 330 feet (100 meters) to the Antarctic bedrock, extending the record back nearly a million years.
Scientists say ice cores are the only archives available to them that preserve information about changes in both Earth's past climate and its atmospheric composition. Analyzing these cores, they say, can show how our planet's climate has changed in relation to changes in the ratio of atmospheric gases.
For example, researchers can measure the relative abundance of hydrogen and oxygen that have atomic structures known as a stable isotopes. "The colder it is, the less abundance of stable isotopes with a high atomic weight we have," Miller said.
Scientists also analyze the atmospheric gases preserved in air bubbles in the ice. This can reveal the abundance of greenhouse gases, such carbon dioxide and methane during various periods.
Priscu, the Montana State University ecologist, said ice-core analysis has led scientists to a number of important findings. Among them, that atmospheric concentrations of carbon dioxide were high during each warm period, that more dust was in the air during glacial periods, and that ice ages occur very quickly.
"The recent EPICA core goes back seven glacial cycles and shows that the length of our current warm period may not be that unusual," Priscu added.
Preliminary analyses of the core, which is nearly twice as long as its closest rival, indicate that Earth has experienced relatively short warm periods between each ice age. At present our planet enjoys one of those warm periods.
Over the past 400,000 years, the warm periods have lasted about 10,000 years. The current warm period has already lasted 10,000 years, but the EPICA team says Earth is not headed for an imminent ice age.
Today Earth's orbit is similar to that of 400,000 years ago. The warm period at that time lasted about 28,000 years. Miller, the EPICA geophysicist, said that, based on this comparison, we can expect the current warm period to last at least another 15,000 years.
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