At the onset of the Eocene (~55.5 Ma), the earth experienced a release of ~5,000 Pg of carbon and global warming of 4-8 ºC in ~10,000 years, a perturbation called the Paleocene-Eocene Thermal Maximum (PETM).  PETM research examines the source of the carbon, its rate and duration of release, the magnitude of changes in temperature and precipitation, and the effects of the perturbation on ecosystems.  Major tools in PETM research include stable isotopic work on carbon and oxygen from marine and terrestrial sources, biomarker abundances, and high-resolution paleontology and sedimentology.
Results/Conclusions

Continental deposits of PETM age are known in North America, the Arctic, northern Europe, Spain, China, and New Zealand; predictably, climatic and biotic responses vary.  On isolated New Zealand there were small increases in abundances of thermophilic plants and increases in terrestrial runoff.  This subtle response contrasts with dramatic floral changes in western North America, where mesic forests of angiosperms and deciduous conifers disappeared and were replaced by dry tropical plants, especially legumes. Some PETM species in Wyoming occur in older rocks to the south, suggesting rapid northward shifts in plant ranges.  Sedimentological data also indicate increased seasonality of rainfall. Fossil pollen and spores in the Arctic also show a decline in conifers during the PETM.  Sedimentological changes during the PETM in Spain suggest greater seasonality in precipitation and/or more storms.

Faunal composition in the Northern Hemisphere also changed at the onset of the PETM, with the first appearances of primates, artiodactyls, and perissodactyls in North America and Europe.  These mammals probably moved across Arctic land bridges from Asia to the other northern continents as climate warmed, because some have been found in pre-PETM rocks in China.  In western North America, there is evidence for northward intra-continental migration in turtles.  In the northern Rocky Mountains, where the faunal record is best, several lineages of mammals show a transient decrease in body size during the PETM, possibly a response to warmer climate and/or decrease in plant protein content.

PETM records have lower temporal resolution and less geographic coverage than do records of Quaternary climate change.  In spite of these limitations the PETM is the perturbation of carbon cycle and climate that most nearly matches our future.  If we are to use the record of Earth’s past as a guide to the future we must study the events that are most relevant as well as those that are best recorded.