Every so often, the Earth’s slate is wiped nearly clean in cataclysmic “mass extinctions.” Over more than half a billion years, it has occurred five times, three of which were from massive volcanic events, and one from an asteroid (the second event’s cause remains unknown). Some have theorized that humanity is driving the sixth major die-off in the planet’s history, and that it is already underway.

Now, an MIT professor has attempted to quantify the tipping point of a mass extinction, to predict when the Earth’s point of no return, as presented in a paper in the journal Science Advances.

The forecast: humanity’s additions to the carbon in the ocean will reach that tipping point by the end of this century, if not sooner, concludes Daniel H. Rothman, in work supported partly by NASA and the National Science Foundation.

“The modern critical size for the marine carbon cycle is roughly similar to the mass of carbon that human activities will likely have added to the oceans by the year 2100,” concludes the paper.

The carbon fingerprints in the ocean sediment were compiled from isotope geochronology and astrochronology, especially carbon-12 and carbon-13. The figures and thresholds were plugged into an empirical database, the paper shows.

Of note were 31 carbon-cycle fluctuations during the last 542 million years. The mass extinction events determined a threshold of carbon maximum before a tipping point is reached (although Rothman points out that one of the five mass die-offs did not go over this threshold).

“The critical events are likely driven to their limit by the intrinsic dynamics of the carbon cycle over a time scale set by the characteristic relaxation time associated with a negative feedback,” Rothman writes. 

Put in other words, mass extinctions are caused by “either a critical rate at long time scales or a critical size at short time scales.”

The threshold currently stands at 310 gigatons of carbon, Rothman calculates.

That level is also roughly equivalent to the amount humans are expected to add to the oceans by 2100, as per projections by the Intergovernmental Panel on Climate Change, he adds.

The end of the century will thus be a tipping point into “unknown territory” – although it could be another 10,000 years for cataclysm to play out across the globe, he adds.

“This is not saying that disaster occurs the next day. It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict,” Rothman said, in a school statement. “In the geologic past, this type of behavior is associated with mass extinction.”

Rothman further told Laboratory Equipmeent that though there was "considerable uncertainty" in the conclusions, it was published with the best data and methods currently available to science.

"In essence, the paper reaches its conclusions based upon the best possible data and a theory based on assumptions that can be easily identified---and therefore subjected to critical evaluation," he said, in an email.

Rothman’s work on the mathematical modeling of the carbon cycle was previously awarded the 2016 Levi L. Conant Prize from the American Mathematical Society.

The disagreement over what constitutes a mass extinction continues. A look at the drastic uptick in vertebrate extinction over the last two centuries, published in the Proceedings of the National Academy of Sciences in July, contends that the sixth event is already underway – and is being driven by humanity.