Paleocene-Eocene Thermal Maximum (PETM): At the North Pole, summer temperatures reached 23 degrees Celsius, and ocean levels were 70 meters higher than today. Plankton shells disappeared from the mud on the seafloor, which turned from white to red. During the PETM, a mass of carbon of as yet unknown origin, equivalent to the contents of all present-day coal, oil, and gas deposits, flooded the atmosphere; temperatures rose by an average of 5°C. Seas and forests took more than 150,000 years to reabsorb it. Carbon dioxide also spread into the seas, acidifying them and dissolving shells. Ocean currents changed dramatically, the seas became acidic, and single-celled organisms in the deep ocean became extinct en masse. The source of the abnormally high carbon emission into the atmosphere may have been the release of enormous bubbles of methane and natural gas from the ocean floor, the craters of which are still visible. Note that all this was triggered by a smaller amount of greenhouse gases than humanity is currently causing.
