2009: Carbon dioxide forcing alone insufficient to explain Palaeocene-Eocene Thermal Maximum warming

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 years5–9. Here we use published palaeorecords of deep-sea carbonate dissolution10–14 and stable carbon isotope composition10,15–17 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 atmospheri