Average temperatures worldwide rose by about seven degrees Celsius -- about 13 degrees Fahrenheit -- in a short span of 10,000 years.

Many of the findings come from studies of core samples drilled from the deep seafloor over the past two decades. When oceanographers study these samples, they can see changes in the carbon cycle during the PETM.

'You go along a core and everything's the same, the same, the same, and then suddenly you pass this time line and the carbon chemistry is completely different,' Dickens said. 'This has been documented time and again at sites all over the world.'

Based on findings related to oceanic acidity levels during the PETM and on calculations about the cycling of carbon among the oceans, air, plants and soil, Dickens and co-authors determined that carbon dioxide levels in the air went up by about 70 percent during the PETM.

The study is slated for publication in Nature Geoscience.