The rate at which carbon is now accumulating in the atmosphere appears to be without precedent in the geological record. That makes it hard to find situations analogous to the current changing climate. Likely the closest analog occurred over 55 million years ago and has been termed the Paleocene/Eocene thermal maximum (PETM). During the PETM, there was a geologically rapid change in the amount of carbon in the atmosphere, which was followed by an equally sudden change in temperatures. The change upset ecosystems across the planet and led to a major extinction event in the oceans.

While these features are relatively easy to determine from the geological and fossil records, there’s one major aspect of the PETM that has remained uncertain: where the carbon came from. While various plausible ideas have been floated, there was no definitive evidence backing up any of them.

A few years back, a couple of researchers from Rutgers University suggested that the carbon literally showed up from outer space, delivered by a comet. While the idea was met with skepticism at the time, the same team is back with more evidence to back their idea: debris that they claim is likely to have been from an impact.