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

• Published in: Nature geoscience Vol. 2; no. 8; pp. 576 - 580
• Main Authors: Zeebe, Richard E, Zachos, James C, Dickens, Gerald R
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-08-2009; Nature Publishing Group
• Subjects: Carbon cycle; Carbon dioxide; Carbon isotopes; Climate change; Climate system; Deep sea; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Eocene; Geochemistry; Geology; Geophysics/Geodesy; Global temperatures; letter; Paleocene; Surface temperature
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo578

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 −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 concentration, 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.