Ocean Acidification Could Drive Mass Extinction Without Rapid Drop in CO2 Emissions

Ocean acidification driven by ever-increasing carbon dioxide levels could take on a life of its own and begin driving a sudden, mass extinction if emissions are not brought under control by the year 2100, according to a new paper in the Proceedings of the National Academy of Sciences.

“Once we’re over the threshold, how we got there may not matter,” said Massachusetts Institute of Technology geophysicist Daniel Rothman, who first warned of the phenomenon in 2017. “Once you get over it, you’re dealing with how the Earth works, and it goes on its own ride.”

But in a step back from the “completely terrifying” headline in one widely-circulated news report last week—quoting a single reaction on social media, not the study itself—Rothman’s mathematical model pinpointed CO2 emissions as the known cause of the problem and suggested a long enough time scale for a solution to take hold. (That makes the paper yet another proof point on the need for fast, global decarbonization in response to the climate crisis. But not cause for the panic and despair we’ve heard from at least three Energy Mix readers since the original headline appeared last week—because if the panic slows down the action we have to take, the headline becomes its own self-fulfilling prophecy.)

“It’s difficult to know how things will end up given what’s happening today,” Rothman said in an MIT release. “But we’re probably close to a critical threshold. Any spike would reach its maximum after about 10,000 years. Hopefully that would give us time to find a solution.”

After publishing his initial paper two years ago in the journal Science Advances, Rothman, co-director of the Lorenz Center in MIT’s Department of Earth, Atmospheric and Planetary Sciences, took a closer look at a geological record dating back 540 million years. He traced dozens of instances when the oceans’ carbon stores “changed abruptly, then recovered,” MIT News reports.

“This ‘excitation’ of the carbon cycle occurred most dramatically near the time of four of the five great mass extinctions in Earth’s history.”

Until now, scientists have assumed the changes in ocean carbon were proportional to the cause. Rothman found that, beyond a certain tipping point, the reaction begins feeding on itself. And while human activity has “only been pumping carbon dioxide into the atmosphere for hundreds of years, versus the tens of thousands of years or more that it took for volcanic eruptions or other disturbances to trigger the great environmental disruptions of the past,” today’s oceans “are absorbing carbon about an order of magnitude faster than the worst case in the geologic record—the end-Permian extinction,” MIT states.

So Rothman developed a mathematical model that captured the interactions of different chemical constituents in the upper ocean, then looked at how they responded to different levels of CO2 entering the system at different rates. The basic dynamic, MIT explains, is that dissolving carbon dioxide makes seawater more acidic, decreases the concentration of carbonate ions, and eventually dissolves the calcium carbonate shells of marine life, thereby making those dying organisms less likely to sink and slowing down their removal from the upper ocean. 

“It’s a positive feedback,” Rothman says. “More carbon dioxide leads to more carbon dioxide. The question from a mathematical point of view is, is such a feedback enough to render the system unstable?”

Not up to a certain point, the model showed. But “once the levels crossed a critical threshold, the carbon cycle reacted with a cascade of positive feedbacks that magnified the original trigger, causing the entire system to spike, in the form of severe ocean acidification,” MIT notes. Rothman placed the beginning of that cycle toward the end of the century.

“We already know that our CO2-emitting actions will have consequences for many millennia,” said Timothy Lenton, professor of climate change and Earth systems science at the University of Exeter. “This study suggests those consequences could be much more dramatic than previously expected. If we push the Earth system too far, then it takes over and determines its own response,” and “past that point there will be little we can do about it.”