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

Abstract

About 55 million years ago global surface temperatures increased by 5–9 ∘C within a few thousand years, following a pulse of carbon released to the atmosphere. Analysis of existing data with a carbon cycle model indicates that this carbon pulse was too small to cause the full amount of warming at accepted values for climate sensitivity. 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.