Breaching a 'carbon threshold' could lead to mass extinction

Credit: CC0 Public Domain

In the brain, when neurons fire off electrical signals to their neighbors, this happens through an "all-or-none" response. The signal only happens once conditions in the cell breach a certain threshold.

Now an MIT researcher has observed a similar phenomenon in a completely different system: Earth's cycle.

Daniel Rothman, professor of geophysics and co-director of the Lorenz Center in MIT's Department of Earth, Atmospheric and Planetary Sciences, has found that when the rate at which carbon 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.

Rothman looked through these geologic records and observed that over the last 540 million years, the ocean's store of carbon changed abruptly, then recovered, dozens of times in a fashion similar to the abrupt nature of a neuron spike. This "excitation" of the carbon cycle occurred most dramatically near the time of four of the five great mass extinctions in Earth's history.

Scientists have attributed various triggers to these events, and they have assumed that the changes in ocean carbon that followed were proportional to the initial trigger—for instance, the smaller the trigger, the smaller the environmental fallout.

But Rothman says that's not the case. It didn't matter what initially caused the events; for roughly half the disruptions in his database, once they were set in motion, the rate at which carbon increased was essentially the same. Their characteristic rate is likely a property of the carbon cycle itself—not the triggers, because different triggers would operate at different rates.

What does this all have to do with our modern-day climate? Today's oceans are absorbing carbon about an order of magnitude faster than the worst case in the geologic record—the end-Permian extinction. But humans have 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. Might the modern increase of carbon be too brief to excite a major disruption?

According to Rothman, today we are "at 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, who is publishing his results this week in the Proceedings of the National Academy of 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 made a dire prediction: 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 the new paper, he has developed a 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 ," 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, which comprises two differential equations that describe interactions between the various chemical constituents in the upper ocean. He then observed how the model responded as he pumped additional carbon dioxide into the system, at different rates and amounts.

He 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.

When he introduced 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 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 will resume its steady state.

This pattern matches the geological record, Rothman found. The characteristic rate exhibited by half his database results from excitations above, but near, the threshold. Environmental disruptions associated with mass extinction are outliers—they represent excitations well beyond the threshold. At least three of those cases may be related to sustained massive volcanism.

"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. This is what excitability is, and how a neuron works too."

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: Faster rates bring us closer to the threshold, but shorter durations move us away. Insofar as the threshold is concerned, the modern world is in roughly the same place it was during longer periods of massive volcanism.

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."

"We already know that our CO2-emitting actions will have consequences for many millennia," says 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—past that point there will be little we can do about it."

Explore further

By 2100, oceans may hold enough carbon to launch sixth mass extermination of species, mathematics predicts (Update)

More information: Characteristic disruptions of an excitable carbon cycle, Proceedings of the National Academy of Sciences (2019).

This story is republished courtesy of MIT News (, a popular site that covers news about MIT research, innovation and teaching.

Citation: Breaching a 'carbon threshold' could lead to mass extinction (2019, July 8) retrieved 17 October 2019 from
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User comments

Jul 08, 2019
so, 600,000,000 year ago when all the carbon in the crust was in the atmosphere, did the oceans go all hoopty?

Jul 08, 2019
What a bunch of of crap. Man's emissions equal 3% of the earths total emissions. In other words over a 33 year period man will only add 1 year of CO2 emissions. The author is claiming that over the next 90 years 3 extra years of emissions will cause a global catastrophe.

Jul 08, 2019
What a load of crap. Small government is much more important.

Jul 08, 2019
"We already know that our CO2-emitting actions will have consequences for many millennia,"

What a load of crap from psuedo-scientists.

Jul 08, 2019
Absolutely nothing will stop the AGW Cult's mass bullshite.

Jul 08, 2019
pseudo-scientists you say?
shoveling nothing but crap you claim?

yet they are the ones with the degrees
the educations
the funding public & corporate
the investors & entrepreneurs
the research facilities
the peer-reviewing colleagues

& what do you debierbots have to show for your useless existence?

& can you honestly claim, without threats, that anyone would publcvly admit to associating with you?

you deniertwats fear that Real Scientists are being proven right with the assistance of tens of thousands of fellow scientists, technicians, engineers, researchers in a thousand fields

you hate that it has become an absolute certainty you will be proven absolutely wrong

to be jeered at by the future

for every ignorant decision
& act of bad judgement
that for all of you
leaves your miserable existence
as monuments to failure

Jul 08, 2019
I'm glad we have so many people who know so much more than the experts in their fields.

Jul 08, 2019
What a load of coprolite. Everything gets recycled when it hits critical mass. A given volume of any gas will trigger a volume of another gas to mix with it and reduce its effects. Nitrogen in the Earth's atmosphere is and was a fixative for the roots of plants which made such things as peanuts possible. The CO2 in the atmosphere is necessary for the health of plants to help them to grow and give off Oxygen. Not being a botanist I am uncertain of the method exactly. But the more plants are allowed to grow, the less CO2 in the atmosphere. CO2 is food for these oxygenators. just stop cutting down the Earth's forests unnecessarily.

Thanks to willy for allowing me to borrow her word. Still trying to understand what she is going on about.

Jul 08, 2019
Almost forgot to mention that, politically, the best solution to resolve the AGW problem and mass extinction is to vote for the "social-progressive-liberals-leftists so that they will be better able to take over government to abolish Capitalism, Free Enterprise and Free Markets. We all know that "social justice" works best by disallowing 'property ownership'. So make sure to vote for the Leftists in future elections. When the new government(s) tax your income heavily and reduce you to a pauper, you can hold your head up with pride, for YOU have helped the homeless, the illegal immigrants and the willfully unemployed to be given most of YOUR capital that you worked for and you will never see it again. And keep them in power, for when you die, they will be sure to turn you into Soylent Green.

Jul 08, 2019
Life on Earth has always been more varied, numbered when temperatures are higher. 400 million years ago for example. Cold kills more people than heat has ever killed.

Jul 08, 2019
Knowing how ignorance and fear work together, the majority of the comments were 100% predictable.
I read them anyway, you never know, it could have been a rare coincidence of folks with functional brains that commented.
Alas, it was not so.
"The truth will set you free" but also requires facing harsh failures on occasion.

Jul 09, 2019
Conjecture , nothing more. Earth has been warming ever since last Ice Age 10,000 + years ago, with or without human input.
We should be more concern with plans how to survive upcoming change, instead of predicting predictable ! Will vast Siberia, Greenland or northern Canada grow wheat or Bananas? :-)

Jul 09, 2019
"For climate change, there are many scientific organizations that study the climate. These alphabet soup of organizations include NASA, NOAA, JMA, WMO, NSIDC, IPCC, UK Met Office, and others. Click on the names for links to their climate-related sites. There are also climate research organizations associated with universities. These are all legitimate scientific sources.

If you have to dismiss all of these scientific organizations to reach your opinion, then you are by definition denying the science. If you have to believe that all of these organizations, and all of the climate scientists around the world, and all of the hundred thousand published research papers, and physics, are all somehow part of a global, multigenerational conspiracy to defraud the people, then you are, again, a denier by definition. 

Jul 09, 2019
James if academic research was not so heavily funded by government grants their research would not be so biased. The government organizations are even worse. This is all happening with one objective in mind and that is the increase of central authority and the removal of individual freedoms. Academia is 100% behind this power grab and hopes to end capitalism and the free market.

Jul 09, 2019
Academia is constantly harping about income inequality and the plight of the poor while raising tuitions at a rate MUCH higher than the rate of inflation. It is responsible for the trillions of student loan debt while not even teaching the students useful skills that would enable the graduates to become employed and pay their HUGE debts back. Their latest plan is a return to indentured servitude where part of your income is paid directly back to them.

Jul 09, 2019
I hope people understand the sarcasm in my comment above...

Jul 09, 2019
The sad part is that the cultural and economic collapse of the West is a much bigger threat to the earth than a few hundred ppm increase of CO2.

The well planned and executed invasion of Europe and the US is proceeding along unimpeded and soon the financial collapse will cause the riots needed to insure the creation of the new One World Government that the .1% so dearly want.

Jul 09, 2019
MR166 I'm afraid your assertion gives the impression of a paranoid schizophrenic.

Given that fundamental science, the science that underpins all applied science, typically takes decades to yield an application, if it ever does, how many companies do you think are willing to fund it? Take cosmology for example. Can you give us some examples of companies that could take over from the NSF or NASA?

Given that it's other scientists that decide which grants get funded, why would scientists primarily want to end capitalism? You really think we all want to live in the Soviet Union? This is a very nutty assertion.

Jul 09, 2019
I hope people understand the sarcasm in my comment above...

On the internet, it's impossible to tell the difference between sarcasm and insanity without a sarc tag. That's where Poe's Law comes from. Just look at MR166's posts or any of the posts from chem trails guy.

Jul 09, 2019
"Take cosmology for example. Can you give us some examples of companies that could take over from the NSF or NASA?"

NASA was a great organization when it was concentrating on space. Then James Hansen turned the once fine agency into a climate whore.

Jul 09, 2019
"For climate change, there are many [... ] legitimate scientific sources. [...] If you have to dismiss all of these scientific organizations to reach your opinion, then you are by definition denying the science.

Deniers of Science arise! Heretics! Blasphemers! The Malthusian Death Cult has spoken. Keep up your wicked ways and the Earth shall burn!

(Nevermind that temperatures have been much hotter than the projected 2-3 degrees C that's predicted. Nevermind that we are in an Ice Age that is 1 million years old, and that life on Earth did not end when it was much hotter than now, or that it has been hotter in the recent geological past. Nevermind that the Death Cult has been consistently shrilly wrong for 200 years. Nevermind that the Death Cult's "climate science" is not science at all: legitimate science is proved by experiment and is falsifiable by experiment; "climate science" is not falsifiable, it is only statistics-based models.)

Jul 09, 2019
MR166 you haven't answered my questions.

Jul 09, 2019
Ummmm, NASA isn't concentrating on space?

Where do you people get this bullshit?

Jul 09, 2019
To tell you the truth I do not know how to fund science in a way as to eliminate the politics . I do know however that scientists that put the universities funding in jeopardy are quickly ostracized or fired.

Jul 09, 2019
I also know that peer review and publication is a good old boys club and has ceased to be a true checks and balances.

Jul 09, 2019
whenever you read the crapulous ravings of the denierbots?
keep in mind that they are puppet quislings of the putin-saudi fascist axis
every comment of theirs is agitprop on the behalf of a global betrayal of Human values & the American People

Jul 09, 2019
Astonishing isn't it.
The AGW Cult's Chicken Shites, believe CO2 will result in a mass extinction, yet they continue to burn fossil fuel like there is no tomorrow.
Keep braying at the heretics, you jackasses. You'll save the world.

Jul 09, 2019
Well MR166 I'm glad you admit that you have no alternative suggestion than the current system.

What evidence do you have that peer review is just an old boys club?

You do realize that every scientist competes with their peers - all around the world?

In my experience the problem is just the opposite. Too often what seem to be excessive criticisms are made and a lot of effort has to go into either arguing with the editor or doing additional experiments. Often though this improves the paper although it takes longer.

Most people outside science don't understand how central competition is to the whole process and how this means that any kind of global conspiracy or old boys club is out of the question. It is central to the process by which self-correction occurs. It's in one's best interest to call out anything that seems spurious in anyone's paper. You gain and they lose. But if you're wrong, just the opposite. It always gets sorted out eventually.

Jul 09, 2019
just like with ozone, killer bees, etc

Jul 10, 2019
Considering Volcanoes emit 250M tons CO2/year, and Humans emit 35B tons CO2/year, it's likely that tipping point will be reached after 100-150 years of human emissions. Since we have been emitting for about 80+ years at scale, we may have another 50 years (+-30 years) to go before tipping point.

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