Check out this interesting bit over at New Scientist:
THE Eocene ran from 56 million to 34 million years ago. Geological evidence from the early and middle part of this period offers troubling news: the average temperature in the tropics at this time could have been as high as 40°C while the poles were at temperatures of 15 or 20°C. None of our climate models accounts for how this “Eocene hothouse” might have arisen