Houston, Texas - Our best guesses about global warming are probably wrong: that's the conclusion of a report from scientists at Rice University.
The study found that climate models explain only about half the heating that occurred during a well-documented period of rapid global warming in Earth's ancient past. The study consists of an analysis of published records of a period known as the Palaeocene-Eocene thermal maximum, or PETM, about 55 million years ago.
"In a nutshell, theoretical models cannot explain what we observe in the geological record," said oceanographer Gerald Dickens, professor of Earth science at Rice University. "There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models."
During the PETM, for reasons that are still unknown, the amount of carbon in Earth's atmosphere rose rapidly. For this reason, say the researchers, the PETM is probably the best ancient climate analogue for present-day Earth.
In addition to rapidly rising levels of atmospheric carbon, global surface temperatures rose dramatically during the PETM - by about seven degrees Celsius in just 10,000 years.
Many of the findings come from studies of core samples drilled from the deep seafloor over the past two decades.
"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 said. "This has been documented time and again at sites all over the world."
Based on oceanic acidity levels during the PETM and on calculations about the cycling of carbon among the oceans, air, plants and soil, the team determined that the level of carbon dioxide in the atmosphere increased by about 70 percent during the PETM.
This is significant, as it does not represent a doubling of atmospheric carbon dioxide. If present rates of fossil-fuel consumption continue, the doubling of carbon dioxide from fossil fuels is expected to occur sometime within the next century or two.
Today's climate models include accepted values for the climate's sensitivity to doubling. Using these 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 said, 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 for current best estimates of 21st century warming - caused a substantial portion of the warming that occurred during the PETM."
The complete study can be seen atNature Geoscience.