Earth's early Eocene period, also known as the Super Greenhouse period, was commonly believed to have concentration levels of carbon dioxide of as much as 2,000 parts per million (ppm). However, Dartmouth College researchers have discovered that it could have been less than 1,000 ppm ?" a difference of half! This could also mean that carbon dioxide is not the primary driver of planetary warming events, as other factors now have stronger weight on the final result.