During the middle of the Eocene, about 40 million years ago...

The Dependable Warmer

During the middle of the Eocene, about 40 million years ago, a transient
warming event interrupted the long-term cooling trend that had been in
progress for the previous 10 million years. Bijl et al. (p. 819; see the
Perspective by Pearson) constructed records of sea surface temperature
and atmospheric CO2 concentrations across the warming period. It appears
that vast amounts of CO2 were injected into the atmosphere, and a sea
surface temperature increase of as much a 6°C accompanied the
atmospheric CO2 rise.

http://www.sciencemag.org/cgi/conten...t/330/6005/819

Transient Middle Eocene Atmospheric CO2 and Temperature Variations
Peter K. Bijl,1,*, Alexander J. P. Houben,1,*, Stefan Schouten,2 Steven
M. Bohaty,3 Appy Sluijs,1 Gert-Jan Reichart,4 Jaap S. Sinninghe
Damsté,2,4 Henk Brinkhuis1

"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".