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RICE (US)—Something other than carbon dioxide caused much of the heating during a period of rapid climatic warming 55 million years ago, new study findings suggest.

“In a nutshell, theoretical models cannot explain what we observe in the geological record,” says oceanographer Gerald Dickens, earth science professor at Rice University and coauthor of a study that appears online and in Nature Geoscience. “There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models.”

During the period, known as the Palaeocene-Eocene thermal maximum, or PETM, global surface temperatures rose about 7 degrees Celsius—13 degrees Fahrenheit—in the relatively short geological span of about 10,000 years.

The amount of carbon during the period also rose rapidly, and for this reason, the PETM has been identified as the best ancient climate analogue for present-day Earth, but climate models explain only about half of the heating that occurred.

The climate models come from studies of core samples drilled from the deep seafloor over the past two decades, which show changes in the carbon cycle during the PETM.

“You go along a core and everything’s the same, the same, the same, and then suddenly you pass this time line and the carbon chemistry is completely different,” Dickens says. “This has been documented time and again at sites all over the world.”

Based on findings related to oceanic acidity levels during the PETM and on calculations about the cycling of carbon among the oceans, air, plants and soil, Dickens and coauthors Richard Zeebe of the University of Hawaii and James Zachos of the University of California-Santa Cruz determined that the level of carbon dioxide in the atmosphere increased by about 70 percent during the PETM.

The number is significant, researchers say, because it does not represent a doubling of atmospheric carbon dioxide. Since the start of the Industrial Revolution, carbon dioxide levels are believed to have risen by about one-third, largely due to the burning of fossil fuels. If present rates of fossil-fuel consumption continue, the doubling of carbon dioxide from fossil fuels will occur sometime within the next century or two.

Doubling of atmospheric carbon dioxide is an oft-talked-about threshold, and today’s climate models include accepted values for the climate’s sensitivity to doubling.

Using these accepted values and the PETM carbon data, the researchers found that the models could only explain about half of the warming that Earth experienced 55 million years ago.

The conclusion, Dickens explains, is that something other than carbon dioxide caused much of the heating during the PETM.

“Some feedback loop or other processes that aren’t accounted for in these models—the same ones used by the IPCC (Intergovernmental Panel on Climate Change) for current best estimates of 21st century warming—caused a substantial portion of the warming that occurred during the PETM.”

Rice University news: www.media.rice.edu