Drilling deep into Antarctic ice, scientists have extended Earth's climate history by another 210,000 years. The new results, they say, drive home two key points:
• Today's atmosphere holds concentrations of carbon dioxide significantly higher than at any time in the last 650,000 years.
• The rise and fall in temperatures track the rise and fall in carbon-dioxide levels as tightly during this additional period as they have over the past 440,000 years.
The results add "another piece of information showing that the time scales on which humans have changed the composition of the atmosphere are extremely short compared to the natural time cycles of the climate system," notes Thomas Stocker, a researcher at the University of Bern in Switzerland and a member of the research team reporting the results.
For ice-core scientist Edward Brook, the results are likely to generate the next icon for long-term climate change. The most telling result, he adds, is the relationship between long-term CO2 trends and long-term temperature trends.
Dr. Brook, with Oregon State University in Corvallis, explains that in these latest results, the warm periods between glacial deep freezes are cooler than those over the past 440,000 years.
Yet the virtual lockstep pattern as temperatures and CO2 levels rise and fall, seen in previous ice cores, holds even for these more-modest swings in global climate.
"It's really striking. The link between temperatures and greenhouse gases is tight," he says.
The new data come from the European Project for Ice Coring in Antarctica, which drilled into the ice at a spot called Dome C in eastern Antarctica. The ice at that point is 3,309 meters thick. The research team pulled a series of ice cores reaching to a depth of 3,190 meters (1.98 miles).
Measuring CO2 and other gases is relatively straightforward. Determining temperatures is more round-about. In the absence of thermometers, scientists have found that shifts in the relative abundance of two forms of hydrogen provide a useful stand-in.
In addition to carbon dioxide, the cores also contain information on methane, a powerful greenhouse gas, and nitrous oxide.
In a separate study from the same cores, the rise and fall of methane also tracked closely with that of CO2 and temperatures. Both sets of results appear in Friday's edition of the journal Science.
The triggers for the changes in gas concentrations remain a mystery, Brook acknowledges. They tend to lag the temperature record by some 800 to 1,000 years.
Some have argued that this gap rules out a connection between rising CO2 and the warming climate.
But Brook explains that the gap most likely signals a "positive feedback" in the climate system. In short, warmth begets more CO2 in the atmosphere.
This raises temperatures further, which leads to more CO2 released into the air. The shift between these glacial periods and warm "interglacial" periods has been linked to long-term changes in Earth's tilt as it orbits the sun.