Maybe not a greenhouse effect after all?
Could the Permian extinction (95% of all marine species,252 million years ago) been the result of ocean acidity levels provoked by volcanic activity?
http://www.nytimes.com/2012/05/01/science/new-studies-of-permian-extinction-shed-light-on-the-great-dying.html
Atmospheric CO2 levels monitored at Mauna Loa Observatory, over the last half-century:
http://www.esrl.noaa.gov/gmd/ccgg/trends/