Could the best climate models—the ones used to predict global warming—all be wrong?

Maybe so, says a new study published online today in the journal Nature Geoscience. The report found that only about half of the warming that occurred during a natural climate change 55 million years ago can be explained by excess carbon dioxide in the atmosphere. What caused the remainder of the warming is a mystery.

“In a nutshell, theoretical models cannot explain what we observe in the geological record,” says oceanographer Gerald Dickens, study co-author and professor of Earth Science at Rice University in Houston. “There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models.”

During the warming period, known as the “Palaeocene-Eocene thermal maximum” (PETM), for unknown reasons, the amount of carbon in Earth’s atmosphere rose rapidly. This makes the PETM one of the best ancient climate analogues for present-day Earth.

As the levels of carbon increased, global surface temperatures also rose dramatically during the PETM. Average temperatures worldwide rose by around 13 degrees in the relatively short geological span of about 10,000 years.

The conclusion, Dickens said, is that something other than carbon dioxide caused much of this ancient warming. “Some feedback loop or other processes that aren’t accounted for in these models—the same ones used by the 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.”

Abstract:

The Palaeocene–Eocene Thermal Maximum (about 55 Myr ago) represents a possible analogue for the future and thus may provide insight into climate system sensitivity and feedbacks. The key feature of this event is the release of a large mass of 13C-depleted carbon into the carbon reservoirs at the Earth’s surface, although the source remains an open issue. Concurrently, global surface temperatures rose by 5–9 °C within a few thousand years. Here we use published palaeorecords of deep-sea carbonate dissolution, and stable carbon isotope composition, along with a carbon cycle model to constrain the initial carbon pulse to a magnitude of 3,000 Pg C or less, with an isotopic composition lighter than minus50permil. As a result, atmospheric carbon dioxide concentrations increased during the main event by less than about 70% compared with pre-event levels. At accepted values for the climate sensitivity to a doubling of the atmospheric CO2 concentration1, this rise in CO2 can explain only between 1 and 3.5 °C of the warming inferred from proxy records. We conclude that in addition to direct CO2 forcing, other processes and/or feedbacks that are hitherto unknown must have caused a substantial portion of the warming during the Palaeocene–Eocene Thermal Maximum. Once these processes have been identified, their potential effect on future climate change needs to be taken into account.