Increased Atmospheric CO2 During the Middle Eocene

Even without humans, there are many processes that can change the concentration of carbon dioxide (CO2) in Earth's atmosphere and affect global climate. On page 819 of this issue, Bijl et al. (1) provide the first direct evidence that very high CO2 levels occurred about 40 million years ago during the Middle Eocene Climatic Optimum (MECO), one of the hottest intervals in Earth's climate history. The hunt is now on for a geological cause for this event—and fingers are pointing at the Himalayan mountain belt