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


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 feedbacks1,2. 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 issue3,4. Concurrently, global surface temperatures rose by 5–9 C within a few thousand years5,6,7,8,9. Here we use published palaeorecords of deep-sea carbonate dissolution10,11,12,13,14 and stable carbon isotope composition10,15,16,17 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.

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Figure 1: PETM model simulations and palaeorecords.
Figure 2: Simulated CCD shoaling.
Figure 3: Deep-sea carbonate ion basin gradient during the PETM.
Figure 4: Simulated atmospheric CO2 during the main event.


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The research was supported by NSF grant EAR06-28719 to J.C.Z. and EAR06-28394 to R.E.Z.

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Correspondence to Richard E. Zeebe.

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Zeebe, R., Zachos, J. & Dickens, G. Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming. Nature Geosci 2, 576–580 (2009).

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