When Earth Hits the Fan
Deep time, mass extinctions, resilience, and a history of climate change
Few things are as mind-blowing as deep time. Our planet was formed 4.5 billion years ago, and complex life has existed for about one tenth of that time, or 500 million years. Humans have been around for approximately 250,000 years, and it wasn’t until 20,000 years ago that our ancestors conquered every bit of habitable land in the globe. Writing, one of the most ancient human inventions, is modern technology by comparison: it has existed for only about 5,000 years, one nine-millionth of Earth’s history. In other words, if we compress the history of the planet into 24 hours, humanity’s recorded history spans all of one tenth of one second.
Struggling to wrap my head around such time scales fuels a certain obsession with deep time that has already inspired me to explore the long-term future. Perhaps because we are about to leave yet another calendar year behind, my most recent fixation has been with the very distant past, in particular the history of life and mass extinction events: the brief, high-intensity periods in geological time where an exceptionally high number of species became extinct in a relatively short period.
We now have a fairly good understanding of the causes and impacts of these catastrophic events. Most strikingly, the history of mass extinctions is, in one sense, the history of climate change.
Due to the interplay of factors like orbital oscillation, geologic activity, and the impact of life itself, the chemistry of the oceans and the atmosphere keeps changing, causing the climate to shift as it seeks an equilibrium. These shifts then unfold over millennia, triggering extreme climate states that inspire evocative labels such as snowball Earth, greenhouse Earth, or hothouse Earth.
Geologically speaking, and leaving anthropogenic climate change aside, today we live in a stable and relatively mild climate, an interglacial period sandwiched between ice ages that come and go in cycles. Depending on the path we forge in the coming decades, the next ice age could happen within dozens to hundreds of thousands of years.
In The Ends of the World, the book that inspired me to write this article, Peter Brannen gives a detailed account of the role of climate change in all five mass extinctions in Earth’s history. The common thread is the role that CO2 has played in drastic changes to the climate and ocean chemistry, and how the planet eventually reaches equilibrium — and invariably recovers.
Earth has a brilliant way of dealing with too much CO2. When carbon dioxide in the atmosphere goes way, way up from increased volcanic activity, the planet warms up from the greenhouse effect. But the catch is that in this warmer, stormier, high-CO2 world, CO2 is also drawn back down in the earth even faster. This is because the more acidic rain, warmer temperatures, and increased rainfall created by excess CO2 all work in concert to intensify rock weathering. This causes the planet to cool off more quickly when it gets too warm, by drawing down more CO2, which ends up as limestone in the ocean. When the planet finally cools, the process of rock weathering slows down as well, the drawdown of CO2 relents, and the planet returns to an equilibrium.
That is, until volcanos start throwing up CO2 in the atmosphere once again.
Next to tectonic activity and the Sun, life stands as the other key element able to alter the global climate. In the long history of our planet, it wasn’t until life showed up that things got really interesting.
Life originated 3.7 billion years ago, almost immediately once the planet cooled enough to sustain liquid surface water. Microbes ruled the planet for billions of years and evolved the ability to photosynthesize, altering the climate on a planetary scale by filling the atmosphere with oxygen. 500 million years ago, this Great Oxygenation Event wiped out a large number of microbial life forms for which oxygen was toxic. Conversely, it set the stage for the Cambrian Explosion, a massive diversification of complex life able to harness that oxygen and spread across the globe, giving rise to the reign of plants and animals over our blue marble. Along for the ride was life’s ability to unwittingly alter the planet’s chemistry forever.
Early mass extinctions
End-Ordovician, 443 million years ago
Soon after the Cambrian Explosion, the world of the Ordovician was very different from ours. Continents were uninhabited and likely barren, clustered in the southern hemisphere. Most land was submerged under shallow seas teeming with life, but they were seas without fish; the world’s waters were ruled by trilobites, bottom dwellers sporting a tough exoskeleton that enabled them to survive throughout several extinction events.
Volcanos fired for much of the 45 million years of the Ordovician, creating massive mountain ranges, some of which, like the Appalachians in North America, are still around today. When volcanos finally quieted down, these mountains were weathered down by rain, pulling CO2 from the air into the ocean. There, carbon was trapped within life itself, fueling blooms of plankton that sank to the bottom of the sea, trapping it underwater.
That drawdown of carbon triggered a great ice age that cooled the tropics and locked up much of the planet's water as ice, dramatically lowering sea levels, thus shrinking the sole habitat available to ancient organisms, causing 85% of species to perish.
Late Devonian, 358 million years ago
The Devonian period is known as the “age of fishes” due to a remarkable diversity of fish and proliferation of the first vertebrates. Meanwhile, on land, the first trees were evolving. With vast continents theirs for the taking, plants took over all the dry surfaces of the planet in what was essentially a tropical greenhouse climate. As the world’s first forests formed, plants had to suck carbon from the atmosphere to enable them to grow increasingly tall.
The evolution of deep-root systems gradually broke down the virgin continental landmass, intensifying rock weathering and the sequestration of atmospheric CO2, thus increasing the rate of nutrient runoff into the oceans. This led to an explosion of algae that, as it died and decomposed, starved the oceans of its oxygen and killed most of the marine life1. All that embedded carbon sank to the bottom of the ocean, causing a massive drop in atmospheric CO2 concentration — and originating of much of the oil & gas we use today.
With little CO2, Earth’s atmosphere wasn’t able to hold on to enough heat, and what followed was a series of ice ages that lasted for prolonged periods. The planet cooled so much that there is evidence of glaciers forming at sea level in the tropics. All that ice piling up on land would have led to lower sea levels, reducing habitat areas for marine life and possibly disrupting oceanic currents and nutrient distribution. The collapse of the food chain was gradual but steady, eventually leading to the extinction of 75% of species.
The big one: End-Permian, 252 million years ago
By the time that the Permian period was in full swing, less than 300 million years ago, the world had warmed. There were megamonsoons along the coasts of the supercontinent Pangaea. Meanwhile, the continent's heartlands were desiccated. Immense distances to the coast, combined with mountain ranges created by the collisions that formed Pangaea, gave rise to vast arid regions with harsh desert conditions that likely went without rain for hundreds of years. Conifers became the dominant land plant in the Permian, and reptiles thrived in both wet and dry conditions, along with the weird-looking synapsids, ancestor of all modern mammals.
The latter part of the Permian saw one of the largest volcanic events in Earth's history. The Siberian Traps, a vast region of volcanic rock, erupted continuously for millions of years, releasing massive amounts of carbon dioxide and methane. Note that the term “volcano” doesn’t quite convey the right idea for what was happening; the Siberian Traps were a fissure eruption, effectively a crack in Earth’s crust that formed a curtain of lava covering millions of square kilometers in molten rock. It dwarfed any volcanic explosion witnessed in human history by orders of magnitude.
Greenhouse gases from the Siberian Traps caused severe global warming by belching at least 10,000 gigatons of CO2 into the air — twice as much as we could ever burn up. Most estimates place the End-Permian atmospheric CO2 concentration at an astonishing 1,000 to 8,000 parts per million (ppm).
By contrast, we went from 280 ppm in pre-industrial times to 420 ppm today, which has already raised global temperatures by more than 1°C. We are probably headed to 600 ppm2, potentially resulting in a 3°C rise in temperature. The worst scenario scientists typically consider is one of unmitigated emissions that could get us near 1,000 ppm by the end of this century, a worst-case future that we have fortunately already avoided. The CO2 concentration seen in the End-Permian is thought to have increased the global mean temperature between 6°C and 10°C.
The increased CO2 levels also led to ocean acidification and hypoxia similar but more extreme than in the Late Devonian. Marine ecosystems were made worse by water temperatures as warm as a jacuzzi.
These inhospitable conditions triggered a decline in biodiversity, a collapse of the food chain, and a cascade of extinctions, with a whopping 96% of all species vanishing. This Great Dying stands as the single worst event in the last 500 million years of our planet’s history.
Late extinctions
End-Triassic, 201 million years ago
Immediately following the End-Permian, the Triassic was a tough time to be around. Seas starved of oxygen, steamy 40°C waters, vast arid regions, and temperatures of 60°C on land made it difficult for life to recover. Trees are absent from the fossil record for 10 million years, as are marine and terrestrial animals from the entire middle of the planet. This era has been likened to a post-apocalyptic greenhouse.
Then, halfway through the Triassic, it started to rain, and it didn’t stop for a million years. Brannen explains what ensued:
Dinosaurs appeared. Not long after, the first flowers blossomed. Crocodile ancestors followed, along with the first true mammals. The planetary deluge [has] been called “the greening of the Triassic Earth.”
The end of the period was like a replay of the End-Permian: massive volcanoes, skyrocketing of CO2, global warming, and ocean acidification. 80% of all species were wiped out, including coral reefs and the big reptiles that dominated the Triassic.
End-Cretaceous, 66 million years ago
Earth’s most recent mass extinction led to the demise of three quarters of all life, including the dinosaurs. Despite being the best known geologic event in pop culture, it was a freak accident unlike any of the other mass extinctions, as it was not caused by gradual climate change. Instead of unfolding over dozens of thousands of years like the other four events, the fate of those at the end of the Cretaceous was sealed once the impact of asteroid Chicxulub turned Earth into an apocalyptic hellscape in a matter of hours.
As fascinating as that event was, down to the horrifying details following the impact, it is out of scope for this article, except to point out that there is some evidence that severe climate change was already afoot at the time, which may have played a role in the extinction, with the asteroid acting as the coup-de-grâce.
The meaning of “event” in geologic terms
The term event used throughout this piece must be interpreted carefully. With the exception of impacts from killer asteroids, mass extinctions are sequences of incremental, protracted, gradual changes that take at minimum thousands, and sometimes millions of years to unfold. While dangerous changes to the climate during these events certainly had an evolutionary impact at the species level, those changes likely went completely unnoticed in the lifespan of any individual animal.
That is one very important difference between extreme climate change in our geological past and what we are living through today. The amount of CO2 in the atmosphere, the mean global temperature, and the state of the oceans have all been much worse at various points in our planet’s history, and will get much worse again, with or without our help.
But, again, here we’re talking about incredibly slow changes over long stretches of time. The current surge in atmospheric CO2 driven by human activities is wholly unprecedented in the history of our planet.
Here’s Brannen again:
One innovation in particular has turned us into a truly geological force: our global effort to take as much ancient carbon from the rock record as possible and ignite it all at once in the atmosphere. This is a superpower normally reserved for continental flood basalts.
Climate skeptics often dismissively state that “the climate has always changed.” That is in a sense true, of course. As we have seen, Earth’s climate has changed even more dramatically — over millions of years, is the part that’s usually left out.
This time it’s different. The speed and scale of the experiment we have put in motion, and are now attempting to correct for, is unlike anything our planet has seen before. Humans have the technology to survive a rapidly changing climate. Other species may not have enough time to adapt.
And yet, Earth will be just fine
While each mass extinction event had its own distinct causes and consequences, they all fundamentally reshaped life on Earth by transforming the climate. Beyond just a catastrophe, each event was a major driver of evolution. Each die-off purged the dominant groups once so finely tuned to their former climate, freeing up ecologic niches and creating opportunities for new species who could better adapt to the new planet they found themselves on. Earth moves on, and life finds a way.
Even coral reefs, ecosystems that have all but vanished during past extinction events, have managed to scrape through and spread again multiple times. Corals will likely disappear over the next century due to human-induced climate change. However, millions of years from now, they will thrive again in a new geologic age.
Dinosaurs got to evolve into the greatest dynasty in Earth’s history, ruling the planet for over 150 million years because the big reptiles that had been standing in their way could not survive the End-Triassic. Mammals, including humans, have an asteroid to thank for our place at the top of the food chain today.
And yet, mammals too are doomed, although not from present climate change — the impacts we’ll see over the next century will be a challenge for sure, but a minor one that humankind will certainly overcome. Call it a trial run. Then, in 250 million years, another supercontinent will form and the planet will be in for a blockbuster sequel to the End-Permian. Maybe by then humans will have taken to the stars, or at least have understood enough about mass extinctions to see one coming, and be ready for it.
Go deeper
Life on Our Planet on Netflix tells the history of life with stunning visuals. Plus, Morgan Freeman steals the show.
Dig into the End-Permian extinction:
Watch a minute-by-minute account of the disaster that followed the Chicxulub asteroid impact.
For background playing, all 4.5 billion years of Earth’s history condensed in one hour is boringly fascinating:
In the present day, this phenomenon is seen in several parts of the world (although in much smaller scale than the Devonian event) due to runoff from fertilizer overuse and sewage treatment plants. The most notable case is an annual occurrence in the Mississippi River Delta that gives rise to the Gulf of Mexico Dead Zone.
From the Intergovernmental Panel on Climate Change (IPCC) SSP2-4.5 scenario called “middle of the road”, which is looking increasingly likely. The more worrisome and not unlikely scenario SSP3-7.0 “regional rivalry / rocky road” could see CO2 concentration approach 900 ppm.
Great article Andre. I especially liked the counter to climate change skeptics that change this time is coming much more rapidly than ever before.