Periods during which large percentages of species died off occurred at the same time as big swings in carbon isotopes found in Earth’s geological record.

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Previous mass extinctions in Earth’s history can illuminate how much carbon human civilization can pump into the environment before risking a catastrophic climate change — and that point may be coming up within a century.

That’s the conclusion of Daniel Rothman, a geophysicist at the Massachusetts Institute of Technology, who built a database of fossil records going back half a billion years. Rothman found the periods in which large percentages of existing species died off coincided with big swings in the carbon isotopes found in those records, suggesting the planet’s carbon cycle was out of whack.

“It implies changes in the carbon cycle are likely both an indication of some kind of serious change and possibly a player in amplifying those changes,” Rothman told Seeker.

In a stable environment, carbon ebbs and flows from organic materials. Carbon dioxide fuels photosynthesis in plants, which store it as they grow; when they die and decay, it’s released back into the skies and the oceans.

But human civilization has been pumping more carbon into the environment by burning carbon-rich fossil fuels like coal, oil, and natural gas. On the current trajectory, the oceans are expected to absorb at least another 300 billion tons of carbon by 2100 — an amount that could end up producing long-term changes to the environment, Rothman concluded.

Credit: Shuzhong Shen

Passing the threshold Rothman has calculated could mean abrupt environmental change — abrupt in biological and geochemical terms meaning within 10,000 years, Rothman said.

“It’s not that the date 2100 is a magic date,” he said. “It’s that the projection of the amount of carbon that will have been added by anthropogenic means — fossil fuel burning — for the most part suggest that 300 gigaton limit will have been surpassed by end of the century. But it may happen sooner. The question in the end is: What happens next?” 

The study was published Wednesday in the research journal Science Advances.

Rothman isn’t alone in warning of a potential extinction. Some scientists argue a sixth such event is under way already, with about two species a year disappearing and thousands seeing their populations and ranges shrink. And scientists have long warned that an increase in global average temperatures beyond 2 degrees Celsius (3.6 degrees Fahrenheit) could have catastrophic consequences.

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The study doesn’t necessarily identify carbon dioxide or other greenhouse gases as a culprit — but he said four of the five mass extinctions in the fossil record have been accompanied by “serious disturbances” in how carbon flows through the environment.

In Rothman’s calculations, only the strictest projections of emissions cuts would come in under the line — and with a wide range of uncertainty, that margin isn’t guaranteed. But that’s all the more reason “for trying to slow down the additional carbon in the atmosphere and oceans,” he said.

“I personally do not subscribe to a one-cause explanation for even a single mass extinction,” he said. “It’s more that many processes are at play, each of which is coupled to the other. And when they start to go in a direction that is amounting to a disturbance, one disturbance begets another, begets another one.”

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