By analyzing data from deep-sea sediment cores to study an ancient global warming episode (the Paleocene-Eocene Thermal Maximum, PETM), researchers found a less-than two-fold increase (70%) in atmospheric carbon dioxide levels corresponding to the 5–9 °C (9-16 °F) warming of the PETM. Based on current knowledge and models of the Earth’s climate system, they had expected to find a three- to eight-fold increase in atmospheric carbon dioxide levels to drive that amount of warming.
In a paper published online in the journal Nature Geoscience, the team, led by Richard Zeebe of the University of Hawai‘i at Manoa’s School of Ocean and Earth Science and Technology, suggests 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 Paleocene–Eocene Thermal Maximum.
We were pretty surprised that the increase in atmospheric carbon dioxide turned out to be so small. To explain the entire warming, you would need a whole lot more carbon.—Richard Zeebe
The Paleocene-Eocene Thermal Maximum warming event occurred about 55 million years ago, and was marked by an increase in global temperature of 5-9 °C over about 10,000 years. A key feature of the event was the release of a large mass of 13C-depleted carbon into the atmosphere, possibly from the dissociation of oceanic methane hydrates, although the source remains an open issue.
Zeebe and his colleagues used data from the sediment cores and a carbon cycle model to calculate that the initial carbon pulse in the PETM was some 3,000 Pg C or less.
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 concentration, this rise in CO2 can explain only between 1 and 3.5 °C of the warming inferred from proxy records.—Zeebe et al. (2009)
[From the pre-industrial value of 280 ppm (parts per million), the current atmospheric concentration of CO2 is more than 380 ppm—about a 36% increase. (Earlier post.)]
Once these other processes have been identified, the authors wrote, their potential effect on future climate change needs to be taken into account.
In a nutshell, theoretical models cannot explain what we observe in the geological record. There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models...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.—Gerald Dickens, co-author and professor of Earth science at Rice University
There are a few ideas what may have contributed to the additional warming. But I don’t think we fully understand these events of intense and rapid global warming. By continuing to put these huge amounts of carbon dioxide in the atmosphere, we’re gambling with climate and the outcome is still uncertain.—Richard Zeebe
Zeebe, R. E., Zachos, J. C., and Dickens, G. R. (2009) Carbon dioxide forcing alone insufficient to explain Paleocene-Eocene Thermal Maximum warming. Nature Geoscience, Advance Online Publication, doi: 10.1038/ngeo578