Ice age study delivers blow to global-warming skeptics

Rising levels of carbon dioxide drove much of the global warming that thawed Earth at the end of the last ice age.

That's the conclusion a team of scientists has drawn in a new study examining the factors that closed the door on the last ice age, which ended about 20,000 years ago.

The result stands in contrast to previous studies that showed temperatures rising ahead of increases in atmospheric CO2 levels. This has led some skeptics of human-triggered global warming to argue that if warming temperatures came first, CO2 wasn't an important factor then and so can't be as significant a factor today as most climate scientists calculate it to be.

The measurements from the previous studies were taken from ice cores extracted from thick glaciers in Antarctica. The new work supplements that data with temperature evidence from 80 locations around the globe.

The results show that while temperature increases around Antarctica appear to have led increases in atmospheric CO2, the picture globally was the opposite – CO2 increases paved the way for temperature increases.

“The new work is a significant advance” in the study of the climate conditions surrounding Earth's cycle of ice ages, notes Richard Alley, a Penn State University geologist who specializes in studying glaciers and the climate records encoded in the ice.

It's the latest indication that researchers' understanding of CO2's effects on climate “is confirmed by the history of climate,” he writes in an e-mail.

The results also hold notes of caution for today, notes Jeremy Shakun, a climate researcher at Harvard University in Cambridge, Mass. 

He notes that during the 10,000 years from the end of the last ice age to the beginning of the current “interglacial” climate, atmospheric CO2 concentrations rose 40 percent, from 180 molecules per million in the atmosphere to 260 parts per million. During the past 100 years, concentrations have risen 34 percent, from 292 ppm to 392 ppm – and continue to rise.

“Clearly, it's not a small amount,” says Dr. Shakun, referring to the increases during the past century. “Rising CO2 at the end of the last ice age had a huge effect on global climate. We've raised it as much in the last century.”

That doesn't mean the full impact of these increases will appear during the course of this century, he explains. It takes much longer for the climate system to fully respond. The oceans are intercepting much of the current warming and additional CO2 humans have added to the atmosphere through burning fossil fuels and land-use changes. And Earth still hosts enormous ice sheets in Greenland and over Antarctica to keep things relatively cool.

“It will take many centuries and beyond to fully feel the effects,” Shakun says.

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