The levels of carbon dioxide found in the atmosphere in modern times have been found to be nearly ten times higher than any other time since the extinction of the dinosaurs, 65 million years ago. The event that came the closest to today’s CO2 levels, the Paleocene-Eocene Thermal Maximum (PETM), occurred 55.5 million years ago, where a spike in greenhouse gasses caused global temperatures to increase by 5–8 °C over what we’re experiencing today. While the existence and cause of the PETM is well established, the source of the massive amount of CO2 that caused the temperature spike has been a complete mystery to scientists. But researchers looking for the fossils of shelled organisms that lived during the era may have answered that question.

Researchers from the Rensselaer Polytechnic Institute, searching for Foraminifera fossils in New Jersey, came across a layer of sediment dated to 55.5 million years ago that contained what are called "microtektites": dark, glassy spheres that are formed by the exposure of soil to extremely high heat caused by a meteorite impact, melting the soil into glass globules. These particular microtektites also contained what is called “shocked quartz”, where the quartz crystals are deformed by intense pressures, implying that the impact that formed these particular microtektites was especially powerful.

This led the researchers to conclude that 55.5 million years ago, the Earth was struck by a meteorite, quite probably a comet, that caused the massive release of carbon dioxide into the atmosphere, that in turn caused the Paleocene-Eocene Thermal Maximum.

“This tells us that there was an extraterrestrial impact at the time this sediment was deposited – a space rock hit the planet,” said Rensselaer assistant professor Morgan Schaller, the study’s author. “The coincidence of an impact with a major climate change is nothing short of remarkable.”

The PETM is considered by climate researchers to be the closest known comparison, in terms of carbon dioxide levels, to what is being observed today, albeit only at one-tenth of today’s levels. Study of the PETM is giving climate researchers valuable clues as to what we might expect from the continued increase of greenhouse gasses in the atmosphere.