Over the past 540 million years or so, there have been five mass extinctions on our planet. The largest one, 250 million years ago, saw 95 percent of all species wiped out. Understanding these mass die-offs gives scientists insight into what's going on today: Animals are disappearing at an alarming rate, largely driven by human-caused changes to the environment. Many scientists believe a sixth mass extinction is now underway.

Every ancient extinction happened at a time when Earth was also seeing disruption in the carbon cycle—the natural process that links cellular respiration (which releases carbon dioxide, or CO2) and photosynthesis (plants take up that CO2 to grow). Of course, humans are now pumping CO2 into the environment at astounding rates.

A new paper in Science Advances finds that a mass extinction period mirroring ones from our planet's ancient past could be triggered when humanity adds a certain amount of carbon to the oceans, which are home to the majority of all plants and animals on our planet. The paper pegs that amount at 310 gigatons. According to lead author Daniel Rothman of MIT, based on projections from the Intergovernmental Panel on Climate Change, we're on course to hit that number by 2100. After that, we enter "unknown territory."

Passing over this carbon threshold moves us "to the other side of the stability boundary"

Previous mass extinctions have happened over the course of thousands or millions of years, but the period of change we're in right now has lasted centuries at best, making it hard to compare them. Although plenty of experts say Earth is already experiencing a sixth mass extinction, that remains "a scientific question," Rothman, who is professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences, told me.

Once our planet hits the threshold he identified in this paper, he explained, it will kickstart changes that will "amplify" everything that came before. These same changes, to reiterate, have been associated with all previous mass extinctions on Earth.

"An interesting remark that could be made on the history of life and the carbon cycle, is that whenever there's a major event in one, there's a major event in the other," Rothman told me. Although every mass extinction has been marked by a disruption of the carbon cycle, there's also evidence that, at other periods in our planet's history, the carbon cycle was disrupted and most living things didn't go on to die.

Rothman argues that mass extinction happens when one of two thresholds are crossed. If the carbon cycle is disrupted over long timescales, extinctions follow, but only if the rate of change is faster than an ecosystem can adapt. For more abrupt changes, the pace doesn't matter; it's the magnitude of change to the carbon cycle that determines the likelihood of a mass extinction.

Read More: TV Weathercasters Are Being Recruited to Convince People Climate Change Is Real

Rothman derived a mathematical formula relating this critical rate and magnitude to the timescale over which it happens. Then he looked at 31 disruptions to the carbon cycle over the last 542 million years, and calculated the mass of carbon added to the oceans in each one. He found a common threshold that most of the 31 past carbon cycle disruptions seemed to stay under—those didn't provoke catastrophe and mass dying.

Four of the five mass extinctions did cross the threshold. Our planet's worst-ever extinction—the one that claimed 95 percent of all life—was farthest over the line. We will easily hit that 310-gigaton threshold by 2100, or sooner, unless something changes: Worst-case projections put 500 gigatons of carbon into the oceans by 2100.

Passing over this carbon threshold moves us "to the other side of the stability boundary," Rothman told me. It's not that on Jan. 2, 2100, all species on our planet begin to apocalyptically die off. "The next day, everybody wakes up and goes to work as normal," he said. It might take 10,000 years for a true disaster to play out.

There are still plenty of questions to answer. For one, the mechanisms that connect the carbon cycle to mass extinction events aren't clear right now. Carbon cycle upheavals "might be a sign or a symptom," Rothman said. The paper doesn't say anything about how extinction rates could change after 2100, or whether all of this is irreversible.

We may reach the point where we need to look at riskier carbon removal schemes, like geoengineering. "I think they certainly deserve attention," Rothman said.

We're rapidly moving towards a tipping point echoed in Earth's ancient past. If we continue at this pace, we'll reach it within our lifetimes, with implications that aren't yet clear.