Daniel Rothman works on the top floor of the building that houses the Massachusetts Institute of Technology (MIT) Department of Earth, Atmospheric and Planetary Sciences, a big concrete domino that overlooks the Charles River in Cambridge, Massachusetts. Rothman is a mathematician interested in the behaviour of complex systems, and in the Earth he has found a worthy subject. Specifically, Rothman studies the behaviour of the planet’s carbon cycle deep in the Earth’s past, especially in those rare times it was pushed over a threshold and spun out of control, regaining its equilibrium only after hundreds of thousands of years. Seeing as it’s all carbon-based life here on Earth, these extreme disruptions to the carbon cycle express themselves as, and are better known as, “mass extinctions”.
Worryingly, in the past few decades geologists have discovered that many, if not most, of the mass extinctions of Earth history – including the very worst ever by far – were caused not by asteroids as they had expected, but by continent-spanning volcanic eruptions that injected catastrophic amounts of CO2 into the air and oceans.
Put enough CO2 into the system all at once, and push the life-sustaining carbon cycle far enough out of equilibrium, and it might escape into a sort of planetary failure mode, where processes intrinsic to the Earth itself take over, acting as positive feedback to release dramatically more carbon into the system. This subsequent release of carbon would send the planet off on a devastating 100-millennia excursion before regaining its composure. And it wouldn’t matter if CO2 were higher or lower than it is today, or whether the Earth was warmer or cooler as a result. It’s the rate of change in CO2 that gets you to Armageddon.
This is because the carbon cycle is happy to accommodate the steady stream of CO2 that issues from volcanoes over millions of years, as it moves between the air and oceans, gets recycled by the biosphere, and ultimately turns back into geology. In fact, this is the carbon cycle. But short-circuit this planetary process by overloading it with a truly huge slug of CO2 in a geologically brief timespan, beyond what the Earth can accommodate, and it may be possible to set off a runaway response that proves far more devastating than whatever catastrophe set off the whole episode in the first place. There could be a threshold that separates your run-of-the-mill warming episodes in Earth history – episodes that life nevertheless absorbs with good humour – from those that spiral uncontrollably toward mass extinction.
While it has been more than 60m years since the planet surpassed such a threshold, by Rothman’s calculation we are about to set the planet on just such an ancient and ominous trajectory, one that may take millennia to eventually arrive at the destination of mass extinction, but that may be all but inevitable once we have pushed off from shore.
It turns out that there are only a few known ways, demonstrated in the entire geologic history of the Earth, to liberate gigatons of carbon from the planet’s crust into the atmosphere. There are your once-every-50m-years-or-so spasms of large igneous province volcanism, on the one hand, and industrial capitalism, which, as far as we know, has only happened once, on the other.
Mass extinctions aren’t just very bad things. They are not civilisation-halting pandemics, like Covid-19, that kill far less than 1% of a single species of primate. Mass extinctions are not what happen when the world loses a quarter of its vegetation and a third of North America is sterilised, as happened only 20,000 years ago when mile-thick ice sheets ploughed over Canada. They are not Yellowstone super eruptions, three of which have detonated in a little over the past 2m years – each of which would have devastated modern agriculture and industrial civilisation, but none of which had any effect on global biodiversity. These are part of the bargain of living on Earth. Life wouldn’t have made it this far if it were vulnerable to the sorts of routine indigestion that are part of the workaday operation of a volcanic planet.
But while ours is a sturdy planet, resilient to all manner of unthinkable insults to which it is regularly subjected, once every 50-100m years, something truly very, very bad happens. These are the major mass extinctions when conditions on Earth’s surface conspire to become so vile everywhere that they exceed the adaptive capacity of almost all complex life.
Five such times in the history of animal life this devastation has reached (and in one case far exceeded) the somewhat arbitrary cutoff of wiping out 75% of species on Earth, and so garnered the status of “major mass extinction”. These are known in the paleontology community as the big five (though dozens of other minor mass extinctions of varying severity appear in the fossil record as well). The most recent of the big five struck 66m years ago, a global catastrophe sufficient to end the age of gigantic dinosaurs.
It left behind a 110-mile crater, one discovered in 1978 under Mexico’s Yucatán Peninsula by geophysicists working for the Mexican state oil company Pemex. The size and shape of the crater implied that a six-mile-wide asteroid instantaneously put a 20-mile-deep hole in the ground, followed, three minutes later, by an (extremely temporary) 10-mile-high mountain range of exploding molten granite – 76% of animal species were taken down in the maelstrom.
By comparison, the devastation wreaked by humans on the rest of the living world is relatively mild, perhaps clocking in less than 10%. Well, at least for the time being. According to an influential 2011 Nature study by palaeobiologist Anthony Barnosky, if we keep it up at our current rate of extinctions, we could jump from our (still horrifying) ranks of a minor mass extinction into the sixth major mass extinction anywhere from three centuries to 11,330 years from now, indistinguishable to future geologists from an asteroid strike. Even worse, there could lurk tipping points along the way, in which the world’s remaining species fall away almost all at once, like the nodes of a power grid failing in concert during a network collapse.
Given how catastrophic the impact of humans on the biosphere has been already, it’s chilling to think that the crescendo of our mass extinction might still lie in front of us.
In our planet’s history, one stretch of time stands as uniquely instructive – uniquely hapless, volatile and deadly – when it comes to CO2 overdoses. Three-hundred-million years ago, the planet repeatedly lost control of its carbon cycle and suffered 90m years of mass extinctions, including two of the biggest global catastrophes of all time – both CO2-driven nightmares. In one case, it nearly died. It was felled, in the words of the palaeontologist Paul Wignall, by “a climate of unparalleled malevolence”. At the very end of the Permian period (252m years ago), enough lava erupted out of Siberia and intruded into the crust that it could have covered the lower 48 US states a kilometre deep.
A kilometre deep.
The rocks left behind by these ancient lava flows are known as the Siberian Traps. Today, the Traps produce spectacular river gorges and plateaux of black rock in the middle of Russia’s boreal nowhere. The eruptions that produced them, and that once covered Siberia in 2m square miles of steaming basalt, are in a rare class of behemoths called Large Igneous Provinces (Lips).
Lips are by far the most dangerous thing in the Earth’s history, with a track record far more catastrophic than asteroids. These once-an-epoch, planet-killing volcanoes are of a different species entirely than your garden-variety Tambora or Mount Rainier or Krakatau, or even Yellowstone. Imagine if Hawaii was created not over tens of millions of years and scattered across the Pacific, but in brief pulses in less than 1 million years, and all in one area (and sometimes emerging through the centres of continents). Lips are the Earth’s way of rudely reminding us that our thin rocky surface, and the gossamer glaze of green goo that coats it, sits atop a roiling, utterly indifferent planetary drama. It’s one in which titanic currents of rock draw down entire ocean plates to the centre of the world to be destroyed and reborn. When this process suffers a hiccup, Lips gush out of the crust like tectonic indigestion, leaving gigantic swaths of the Earth buried in volcanic rock. Depending on the pace and size of these eruptions, if they’re big enough and fast enough, they can destroy the world.
At the end of the Permian, in the greatest mass extinction of all time, these eruptions would have featured terrifying explosions, no doubt inducing brief volcanic winters and acid rain. There was also widespread mercury poisoning, and toxic fluorine and chlorine gas, which would have been familiar to suffocating soldiers in the first world war trenches. Most importantly – and most unfortunately, for life – billowing out of the Earth in the biggest catastrophe in history was a planet-deranging amount of carbon dioxide.
Curiously, as the Siberian lava has been dated ever more precisely, it turns out that it wasn’t until 300,000 years into the eruptions – and after two-thirds of this lava had already erupted, flooding the northern reaches of Pangaea in steaming rock miles thick – that this worst mass extinction of all time actually began. This is strange. These volcanoes would have been pumping out all the usual nightmare stuff this entire time, putting industrial polluters to shame – and doing so for hundreds of millennia before the mass extinction began. There would have been uncountable, unthinkably violent eruptions, and noxious storms of acid rain. But the biosphere is tough. And as bad as it was, turning a third of Russia into a volcanic hellscape, it doesn’t explain why, after all those countless centuries of misery, life suddenly winked out en masse, even at the bottom of the ocean, on the other side of the planet.
What was the mechanism for the mass extinction? “You can rule the lavas out,” says Seth Burgess, a geologist at the US Geological Survey. But something about these Siberian volcanoes must have dramatically changed after 300,000 years, when the world quickly disintegrated. So what was it?
The planet started burning fossil fuels.
The result was a flux of carbon into the system so massive that it overwhelmed the planet’s ability to regulate itself and pushed the world out of equilibrium.
All on their own, volcanoes emit lots of CO2: as much as 40% of the gas from a venting volcano can be carbon dioxide. But after Siberia had been smouldering at the surface for countless generations, something far more menacing began to cook below. Colossal 1,000ft-thick sheets of magma, stymied in their ascent to the surface, instead started spreading sideways into the rock far underground, like incandescent rhizomes, baking through the underworld. This is when everything went to hell.
These massive magma roots were burning through an old layer cake of Russian rock eight miles thick. The quarter-billion-year pile of strata had accumulated in the vast Tunguska basin: the remnants of bygone salt flats and sandstones, but more catastrophically, carbon-rich limestone and natural gas deposits from ancient seas, and coals from ages past. The magma cooked through all these fossil fuels and the carbon-rich rock underground on contact, and detonated spectacular gas explosions that shattered the rock far above, erupting at the surface as half-mile craters that spewed carbon dioxide and methane into the air by the gigaton.
After hundreds of thousands of years of familiar surface eruptions, the volcanoes had suddenly started burning through the subterranean world on a massive scale and began acting like enormous coal-fired power plants, natural gas plants and cement factories. “The burning of coal,” one scientist writes of the end-Permian extinction, “would have represented an uncontrolled and catastrophic release of energy from Earth’s planetary fuel cell.” The Siberian Traps suddenly started to emit far too much CO2, and far too quickly for the surface world to accommodate it.
Here’s a plausible sequence of events at the end of the Permian. First, and most simply: the excess CO2 trapped more energy from the sun on the surface of our planet – a simple physical process that was worked out by physicists more than 150 years ago. And so the world helplessly warmed – models and proxies both point to about 10C of warming over thousands of years – pushing animal and plant physiology alike to their limits. It’s also a simple physical fact about our world that for every degree it warms, the atmosphere can hold about 7% more water, so, as the temperature climbed and the water cycle accelerated, storms began to take on a menacing, drowning intensity. As the ocean warms as well, it holds less oxygen.
Unfortunately, living in hot water is hard work, so the luckless animals in it required more oxygen to live, not less. Thus, as the ocean got hotter and more stagnant, the creatures in it began to fall away, and the seas began to empty. Making matters worse, the carbon dioxide in the air diffused into these gasping seas as carbonic acid (H2CO3). The entire global ocean became more acidic as a result, and the water was robbed of the chalky carbonate dissolved in it, and which animals used to build their shells. In these souring seas, the creatures became brittle and sickly, or even failed to form shells in the first place.
As this sea life was decimated, the global marine food web began to teeter and collapse. Meanwhile, the ecosystem on land was being destroyed by wildfire (themselves spewing even more CO2 into the air) and lashed by violent storms. Terrestrial wreckage washed into the ocean, blasting the coastal seas with decaying vegetation and minerals weathered out of the land, such as phosphorus, that acted as plant food, fuelling massive algae blooms offshore. The oceans, already wanting for oxygen from the heat, now began to suffocate in earnest as algae blooms died and decomposed.
As the CO2 continued to issue from the Siberian Traps in massive and unrelenting belches, the planet became hotter still, and the oceans didn’t have a chance in hell. CO2 was now pushing the planet outside the limits of complex life. And just as these lifeless, anoxic, hot seas began to spread, a spectre from the Earth’s ancient past was renewed on this dying planet.
Unlike most life on Earth with which we’re familiar, primitive anaerobic bacteria, having evolved aeons ago on an all-but-breathless world, don’t need oxygen to burn their food. For some, sulphate will do the trick. And on this rotting, suffocating world, this microbial life became ominously ascendant, breathing out hydrogen sulphide (H2S) as exhaust. Unfortunately, hydrogen sulphide is mercilessly toxic, instantly killing humans (and creatures like us), as it sometimes does today in manure pits, or around oil pads like those in Texas’s Permian basin. And so this dark cloud of primeval life spread insidiously through the deep and even into the shallows. The world was now very, very hot, very stormy, almost totally denuded of vegetation, with acidifying, anoxic oceans that belched unsparingly poisonous gas from an ancient microbial metabolism that killed anything that came near it.
On the other side of the planet from the eruptions, once-forested polar South Africa became so denuded of life that rivers that once happily curved and twisted – their banks anchored by living plant roots – now rushed straight over the scoured landscape in braided, sprawling arroyos. Unearthly hot and dry seasons razed the forests with fire, then alternated with apocalyptic superstorms that washed it all away. The animals that had stocked the now-vanished forests for millions of years vanished as well. In the rocks, fungal spores strangely appear in the fossil record all over the world, heralding the collapse of the biosphere. Even insects, whose sheer numbers usually cushion them against mass death, struggled to hold on.
While the heat devastated life at the poles, the Earth’s searing midsection had become plainly unearthly. As CO2 sent global temperatures soaring, the ocean in the tropics became as hot as “very hot soup”, perhaps sufficiently hot, even, to power outlandish 500mph “hypercanes” that would have laid waste to the coasts. In the continental interiors, the temperature would have leaped even further off the charts. In the planet’s most miserable hour, much of its surface came to resemble less Earth as we know it than the feed from a lander probe on some hopeless and barren exoplanetary outpost. Earth, in its darkest hour, was losing its Earthiness. In fact, the postapocalyptic ocean was so vacant that carbonate reefs all over the world came to be built again in the recovery not by animals such as the archaic corals and lamp shells that were driven extinct, but by calcified mounds of bacterial slime.
Everywhere. Even a short hike from my apartment in Boulder, Colorado, brings me face-to-face with this stromatolite rock from the end of the world, left behind by foul microbial mats. In the Colorado Front Range, where Earth history has been lifted out of the ground, tilted sideways and ornamented with ponderosa pine, one encounters this hummocky red rock laid down, layer by layer, by microbes in a deathly sea 252m years ago. It is wedged between more prosaic sandstones from the Carboniferous before it, and the dinosaur-trampled beach sands of the Mesozoic after it, hogbacks of which loom like a backstop behind Denver – the geology of happier times. But the implications of this brief wedge of bacterial rock, and a global ocean momentarily dominated by mounds of calcifying slime, are truly frightening.
Before long, almost every living thing on the planet was dead. The interiors of the continents were silent except for hot, howling winds that swept over the wastes – a dry desolation that alternated with punishing, unearthly storms that smelled like death. The oceans, whose open seas once flashed iridescent with shoals of bobbing spirals and tentacles, and whose nearshore reefs were once dappled fire-engine red to ultraviolet by life, were now putrid, asphyxiating, empty and covered in slime. Every gear of the grand, intricately interlocking biogeochemical machinery of this planet became jammed, decoupled or spun hopelessly out of control. Complex life, as a subset of this global geochemical churn, unravelled as well. All from adding too much CO2. If there is a geologic precedent for what industrial civilisation has been up to in the past few centuries, it is something like the volcanoes of the end-Permian mass extinction.
Now let’s pull back from the brink. However similar to this era our modern experiment on the planet might first appear, it’s worth acknowledging, even stressing, that the end-Permian climate catastrophe was truly, surpassingly bad. And on a scale unlikely ever to be matched by humans. Upper estimates for how much carbon dioxide the fossil-fuel-burning Siberian Traps erupted, ranging up to 120,000 gigatons, defy belief. Even lower estimates, of say 30,000 gigatons, constitute volumes of CO2 so completely ridiculous that matching it would require humans to not only burn all the fossil fuel reserves in the world, but then keep putting ever more carbon into the atmosphere for thousands of years. Perhaps by burning limestone for fun on an industrial scale for generations, even as the biosphere disintegrates. As it is, industrial civilisation could theoretically generate about 18,000 gigatons of CO2 if the entire world pulled together on a nihilistic, multicentennial, international effort to burn all the accessible fossil fuels on Earth.
But while the sheer volume of CO2 generated by the Siberian Traps dwarfs our present and future output, that total was achieved over tens of millennia. What is alarming, and why it’s worth talking about the Siberian Traps in the same breath as industrial civilisation, is that even in comparison with those ancient continent-spanning eruptions, what we’re doing now seems to be unique. It turns out that the focused, highly technological effort to find, extract and burn as much of the world’s fossil-fuel reservoir as is economically feasible, as fast as possible, has been extremely prodigious at getting carbon out of the crust – even compared to the biggest Lips in Earth history. In fact, the best estimate is that we’re emitting carbon perhaps 10 times faster than even the mindless, undirected Siberian volcanoes that brought about the worst mass extinction ever.
This matters because it’s all about the rate. There’s almost no amount of carbon you can pump into the atmosphere that, given enough time, Earth couldn’t buffer itself against. Volcanic CO2 is supposed to enter the system. Without it, none of this works: the climate wouldn’t be habitable, life would run out of raw material, and oxygen would run out. But everything in moderation. To maintain its homeostasis, the planet continuously scrubs CO2 from the atmosphere and oceans so that it doesn’t build up and cook the planet. But this process is very slow on a human timescale. It buries this CO2 in coals, oil and gas deposits, and, most importantly, ocean sediments that turn to carbonate rock over millions of years. When more modest-sized eruptions inject a massive slug of CO2 to the atmosphere, threatening to overwhelm this process, the Earth has several emergency handbrakes.
The oceans absorb the excess carbon dioxide, becoming more acidic, but in their millennial overturn they bring these more acidic surface waters to the seafloor on the downdraft of the planet’s great ocean currents. There they dissolve the seafloor’s carbonate sediments – the massive carpeting of tiny seashells at the bottom of the ocean, laid down by life over millions of years – and buffer the seas in the exact same way that a Tums settles an upset, acidic stomach. This is the first line of defence in the carbon cycle, and it works to restore ocean chemistry over thousands of years. Eventually, these forces work to restore the carbon cycle and coax the Earth back from the edge. On a world without humans or especially catastrophic Lips, these feedbacks usually suffice to rescue the planet. The excess CO2 is removed and transmuted to rock; the temperature eventually falls; and the pH of the ocean is restored over hundreds of millennia.
So it’s not just the amount of CO2 that enters the system that matters, it’s also the flux. Put a lot in over a very long time and the planet can manage. But put more than a lot in over a brief enough period of time and you can short-circuit the biosphere.
Unfortunately, the rate at which humans are now injecting CO2 into the oceans and atmosphere today far surpasses the planet’s ability to keep pace. We are now at the initial stages of a system failure. If we keep at it for much longer, we might see what actual failure really means.
If you want to overwhelm the system in a shorter time frame and shove the carbon cycle dangerously out of equilibrium, you need a much more intense infusion of CO2 into the oceans and atmosphere – faster than biology or weathering can save you. The modern global industrial effort to find, retrieve and burn as much ancient carbon buried in the Earth’s crust as possible in a matter of mere centuries might be up to the task.
Adapted from The Story of CO2 Is the Story of Everything: A Planetary Experiment, published by Allen Lane on 26 August. To support the Guardian, order a copy from Guardian bookshop. Delivery charges may apply
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