The long
read
‘A
climate of unparalleled malevolence’: are we on our way to the sixth major mass
extinction?
Churning
quantities of carbon dioxide into the atmosphere at the rate we are going could
lead the planet to another Great Dying
By Peter
Brannen
Tue 19
Aug 2025 05.01 BST
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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