R.E. Zeebe, J.C. Zachos, G.R. Dickens, Nature Geosci.,Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming

Nature Geoscience, published online 13 July 2009; doi:10.1038/ngeo578

Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming

Richard E. Zeebe* (School of Ocean and Earth Science and Technology, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, MSB 504, Honolulu, HI 96822, U.SA.), James C. Zachos (Earth and Planetary Sciences Department, University of California, Santa Cruz, CA 95064, U.S.A.) and Gerald R. Dickens (Department of Earth Sciences, Rice University, Houston, TX 77005, U.S.A.)

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^{1, 2}. 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^{3, 4}. Concurrently, global surface temperatures rose by 5–9 ^°C within a few thousand years^{5, 6, 7, 8, 9}. Here we use published palaeorecords of deep-sea carbonate dissolution^{10, 11, 12, 13, 14} and stable carbon isotope composition^{10, 15, 16, 17} 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 -50. 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.

*Correspondence, e-mail: zeebe@soest.hawaii.edu