Carbon dioxide emissions may trigger a reflex in the carbon cycle, with devastating consequences, an MIT study finds.
MIT researcher Daniel Rothman has found that when the rate at which carbon dioxide enters the oceans pushes past a certain threshold – whether as the result of a sudden burst or a slow, steady influx – the Earth may respond with a runaway cascade of chemical feedbacks, leading to extreme ocean acidification that dramatically amplifies the effects of the original trigger.
This global reflex causes huge changes in the amount of carbon contained in the Earth’s oceans, and geologists can see evidence of these changes in layers of sediments preserved over hundreds of millions of years, according to an MIT press release.
Over the last 540 million years, the ocean’s store of carbon changed abruptly, then recovered, dozens of times near the time of four of the five great mass extinctions in Earth’s history. But while there have been different triggers to these events, once they were set in motion, the rate at which carbon increased was essentially the same.
At the ‘precipice of extinction’
According to Rothman, our oceans are absorbing carbon much faster than the worst case in the geologic records that he reviewed, putting us as what he calls “the precipice of excitation”. And if it occurs, the resulting spike – as evidenced through ocean acidification, species die-offs, and more – is likely to be similar to past global catastrophes.
“Once we’re over the threshold, how we got there may not matter,” says Rothman, a professor of geophysics and co-director of the Lorenz Center in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “Once you get over it, you’re dealing with how the Earth works, and it goes on its own ride.”
A carbon feedback
In 2017, Rothman predicted that, by the end of this century, the planet is likely to reach a critical threshold, based on the rapid rate at which humans are adding carbon dioxide to the atmosphere. When we cross that threshold, we are likely to set in motion a freight train of consequences, potentially culminating in the Earth’s sixth mass extinction.
Rothman has since sought to better understand this prediction, and more generally, the way in which the carbon cycle responds once it’s pushed past a critical threshold. In his new study, he has developed a simple mathematical model to represent the carbon cycle in the Earth’s upper ocean and how it might behave when this threshold is crossed.
Scientists know that when carbon dioxide from the atmosphere dissolves in seawater, it not only makes the oceans more acidic, but it also decreases the concentration of carbonate ions. When the carbonate ion concentration falls below a threshold, shells made of calcium carbonate dissolve. Organisms that make them fare poorly in such harsh conditions.
Shells, in addition to protecting marine life, provide a “ballast effect,” weighing organisms down and enabling them to sink to the ocean floor along with detrital organic carbon, effectively removing carbon dioxide from the upper ocean. But in a world of increasing carbon dioxide, fewer calcifying organisms should mean less carbon dioxide is removed.
“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?”
‘An inexorable rise’
Rothman captured this positive feedback in his new model and found that no matter the rate at which he added carbon dioxide to an already stable system, the carbon cycle in the upper ocean remained stable. In response to modest perturbations, the carbon cycle would go temporarily out of whack and experience a brief period of mild ocean acidification, but it would always return to its original state rather than oscillating into a new equilibrium.
But when he introduced carbon dioxide at greater rates, he found that 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. The system did, eventually, return to equilibrium, after tens of thousands of years in today’s oceans – an indication that, despite a violent reaction, the carbon cycle will resume its steady state.
“When you go past a threshold, you get a free kick from the system responding by itself,” Rothman explains. “The system is on an inexorable rise.”
Two effects cancel each other out
Although carbon is entering the oceans today at an unprecedented rate, it is doing so over a geologically brief time. Rothman’s model predicts that the two effects cancel each other out: faster rates bring us closer to the threshold, but shorter durations move us away.
In other words, if today’s human-induced emissions cross the threshold and continue beyond it, as Rothman predicts they soon will, the consequences may be just as severe as what the Earth experienced during its previous mass extinctions.
“It’s difficult to know how things will end up given what’s happening today,” Rothman says. “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.”