The Proceedings of the (American) Nationa Academy of Science has just publi shed a paper giving new data on Paleocene-Eocene Thermal Maximum.
The Thermal Maximum - a roughly 6 degree Kelvin temperature rise over a tho usand years or so - seems to have been driven by a sudden surge of methane into the atmosphere, which produced fairly dramatic green-house warming
Vol. 113 no. 28 Alexander Gehler, 7739?7744, doi: 10.1073/pnas.1518116113
I read it at
The data is based on oxygen isotope ratios from small mammal tooth enamel, and needs quite a bot of interpretation, but the authors seem confident tha t it narrows down the options quite a lot.
The most plausible source of the extra methane is sub-sea methane clathrate s (and there's quite of lot of methane tied up in the same kid of reservoir today).