As subduction zones burned through the carbon-rich crust at the edges of the Pangaea supercontinent, any rock weathering that might have buried carbon dioxide now sputtered in the vast interior. Rainforests dried out and carbon dioxide levels ticked steadily upward. The planet stumbled toward apocalypse. On a world fully stocked with reptiles, dazzling reefs, ammonoids, sharks, massive amphibians, trilobites, giant sea scorpions and forgotten, lopped-off branches of our family tree, Siberia turned inside out. The landmass burbled over a million square miles of lava, injecting thousands of gigatons of carbon dioxide into the air and jackknifing the temperature by 10 degrees Celsius over thousands of years, killing off most animal life on Earth.
In the dreadful wake of the greatest die-off in history, the end-Permian mass extinction, the biosphere struggled for millions of years with high carbon dioxide levels during the torrid height of Pangaea. Having lost its tropical forests — a huge carbon sink — in the mass death, it stayed hot for millions of years, then weathered occasional mass extinctions during further bouts of CO2-spewing volcanism. But as the supercontinent finally splintered apart around 200 million years ago and the near-endless age of dinosaurs found its groove, the planet recovered to conditions more hospitable to animal life.
Newly evolved chalky plankton fed a steady conveyor belt of carbon gently snowing through oceans to the seafloor, where it was delivered to deep sea trenches, cooked and then released through the throats of volcanoes at the surface again as carbon dioxide. This helped keep this Mesozoic planet perpetually warm, culminating some 90 million years ago in the steamy, jungly dinosaur world of our public imagination. When this world was catastrophically interrupted by a hunk of space trash some 30 million years later, in the cinematic end-Cretaceous mass extinction, the mammals would inherit not only the Earth but the climate of the dinosaurs as well.
Our Mammalian Age
The climatic impact of the asteroid strike that doomed the dinosaurs, while global and catastrophic, was relatively short-lived. In these early days of our age, the age of mammals, some 50 million years ago, the planet remained hot; carbon dioxide levels topped 1,000 ppm. The alligators of Arctic Canada lay motionless in the swamps to wait out months of polar night, as dawn redwoods filtered the starlight. But then, carbon dioxide began to steadily fall from these sweltering highs. As the atmospheric concentration of carbon dioxide dropped below about 750 ppm around 33 million years ago, Antarctica swelled with a smaller version of the ice sheet we now take for granted.
Earth shuddered with yet another wave of extinctions in this initial chill. About 30 million years later, as carbon dioxide continued its slow ebb to below 300 ppm, the planet finally plummeted into another series of spectacular ice ages — when sea level fell hundreds of feet lower than it is today — punctuated by brief, warmer reprieves such as our own (some 11,000 years in progress), paced by seemingly trivial changes in sunlight reaching northern latitude summers.
Over the past 2.6 million years, this metronome between the icy, volatile, dry, dusty, low-CO2 world of the Pleistocene and the briefer, warmer periods that have repeatedly interrupted it (like the past few thousand years) shaped our evolution. That brings us to now, this infinitesimal moment of geological history — one of the most out of control in the history of animal life.
Poised at the end of a humdrum interglacial, in an otherwise chilly corner of Earth history, industrial civilization is emitting carbon dioxide at a clip 10 times faster than the apocalyptic volcanoes of the end-Permian mass extinction. In only a few decades, we have reproduced an antique level of carbon dioxide in the atmosphere unseen on Earth for millions of years, predating the evolution of our genus, Homo — from a time when camels roamed Arctic forests and sea level was 70 feet higher. Atmospheric carbon dioxide is back up to 420 ppm and rising.
The Earth system is extraordinarily far from equilibrium, and being pushed more violently so by the year. Where does that leave us?
“The message from the geological past is that this can go very badly wrong,” said Mills, noting that we don’t have an analogue for the global chemistry experiment we’re currently running.
“The end-Permian is just an insane event, but it’s a very long period of huge amounts of CO2 degassing,” he said. “So we have analogues in terms of the magnitude of temperature change. But we don’t have anything on this timescale. We understand how these mechanisms work when they’re slowed down, but actually understanding how it’s going to play out in timescales like decades is really, really difficult.”
“We’ve never hit the Earth system this hard, this fast before,” Stockey said.
For Tierney, the lesson of Earth’s animal history is that we live on a far more extreme planet than the relatively sheltered millennia-long human historical memory would lead us to believe. We invite a demonstration of this planetary volatility at our peril.
“Right now we’ve warmed just over 1 degree [Celsius],” Tierney said. “It’s pretty small, given that the range we’re finding in the Phanerozoic [the eon of animal life] is between, like, 11 degrees and 36 degrees [average temperature].” Today, the global average surface temperature is about 15 degrees Celsius.
“But even with that tiny bit of global temperature change,” she continued, “we already see all these major changes in climate — drought, bigger floods, bigger hurricanes, bigger fires. It just shows you how dynamic the Earth system is. It doesn’t take much of a temperature change to create a really different world.”
As we pull out to the geological scale again, and run the clock forward this time, we find that the human chemistry experiment, like all previous paroxysms of carbon dioxide, will be dealt with in good time. Over tens of millennia, no matter how much we put into the atmosphere, the rocks will weather, the carbon dioxide will be transmuted to rock, the planet will slowly cool. And perhaps hundreds of millennia from now, the violent human imposition on the climate will be relegated to the realm of Earth history, too.
Since Mills’ modeling can blindly reproduce the climates of the ancient past, he’s recently toyed with putting it into reverse to see what awaits our ever-changing planet in the far, far future. Picking up the pace a bit more, one at which we now see the continents perceptibly move across the face of the Earth, untold biomes and regimes of animals come and go, evolve and fossilize, until all the continents are reunited again some 250 million years in the future. As the sun has grown subtly brighter over this span, if this supercontinent struggled with high carbon dioxide levels like its Pangaean forebear did, then this would be an inimically hot world to animals, except for lone refuges fringing its polar far reaches.
And if another end-Permian-style bout of volcanism were to strike this “Pangaea Ultima,” then it could be the death blow for complex life. If not, then a spin or two around the galaxy later, and the sun will have grown bright enough, and the water cycle amplified enough as a result, that ramped up rock weathering will draw carbon dioxide so low that even grasses can’t hold on. Photosynthesis will stop, and that will be that.
While that might mean the end of our story, as we pan out to the galactic scale once more, perhaps other planets on other spiral arms will then find themselves similarly in full bloom for a while, miraculously managing their carbon for a rich, improbable, glorious season of life.
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