Global Warming

Ancient climate change catastrophe

The Paleocene Eocene Thermal Maximum is the time considered one of Earth's best analogues to this era of modern, human-caused global warming. The clues it left behind might help stop history repeating itself.

The PETM is the Paleocene Eocene Thermal Maximum — an ungainly name for the time that’s considered one of Earth’s best analogues to this era of modern, human-caused global warming. In a matter of a few thousand years, huge amounts of carbon were injected into the atmosphere, causing global temperatures to rise between five and eight degrees Celsius. The rapid climate change disrupted weather, transformed landscapes, acidified oceans and triggered extinctions. It took more than 150,000 years for the world to recover.

If history is allowed to repeat itself, the consequences for modern life could be similarly long-lasting.

The first major evidence for the PETM was uncovered in the early 1990s by scientists looking at the transition from the Paleocene, the epoch after the extinction of the dinosaurs, to the Eocene, when modern mammal orders first emerged. Chalk deposits at the bottom of the ocean began to dissolve as carbon dioxide made seawater more acidic. Fossils of tiny, deep sea-dwelling creatures showed evidence of an oxygen shortage — a sign that the water was getting warmer. Everywhere in the ocean, creatures adapted to the changed environment, or else they died out.

On land, mammals got smaller and smaller. Ancient ancestors of horses, tiny to begin with, shrunk 30 percent to the size of house cats.

Weather also got wilder. Geologists have uncovered huge rocks that were carried long distances by intense floods — something that happens when dry spells are followed by extreme rains.

The pace at which we are changing the climate outstrips anything in the geologic record. The carbon surge that set off the PETM unfolded over the course of as long as 5,000 years. At our current rate, humans will produce a comparable surge in a matter of a few centuries.