That period, known as the Paleocene-Eocene Thermal Maximum (PETM), was marked by a massive release of the Earth's natural carbon stores into the atmosphere. (It’s not clear what caused the PETM, but volcanic eruptions and methane gas release are suspects.) The excess carbon triggered a 5°C (9°F) temperature increase, along with drought, floods, insect plagues, and extinctions. (Read more about this period of “Hothouse Earth.”)