The long-term warmth of the Eocene (~56 to 34 million years ago) is commonly associated with elevated partial pressure of atmospheric carbon dioxide (pCO2). However, a direct relationship between the two has not been established for short-term climate perturbations. We reconstructed changes in both pCO2 and temperature over an episode of transient global warming called the Middle Eocene Climatic Optimum (MECO; ~40 million years ago). Organic molecular paleothermometry indicates a warming of southwest Pacific sea surface temperatures (SSTs) by 3° to 6°C. Reconstructions of pCO2 indicate a concomitant increase by a factor of 2 to 3. The marked consistency between SST and pCO2 trends during the MECO suggests that elevated pCO2 played a major role in global warming during the MECO.

Supplement to: Bijl, Peter K; Houben, Peter; Schouten, Stefan; Bohaty, Steven M; Sluijs, Appy; Reichart, Gert-Jan; Sinninghe Damsté, Jaap S; Brinkhuis, Henk (2010): Transient Middle Eocene atmospheric CO2 and temperature variations. Science, 330(6005), 819-821