The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions

by Peter Brannen

But there’s a risk of stretching the analogy too far: Earth has been many different planets over its lifetime, and though in some salient and worrying ways our modern planet and its future prospects echo some of the most frightening chapters in its history, in many other ways our modern biocrises represent a one-off—a unique disruption in the history of life. And thankfully, we still have time. Though we’ve proven to be a destructive species, we have not produced anything even close to the levels of wanton destruction and carnage seen in previous planetary cataclysms. These are absolute worst-case scenarios. The epitaph for humanity does not yet have to include the tragic indictment of having engineered the sixth major mass extinction in earth history. In a world sometimes short on it, this is good news.

A sentiment exists—particularly among nonscientists—that the idea of humans seriously disrupting the planet on a geological scale is mere anthropocentric hubris. But this sentiment misunderstands the history of life. In the geological past, seemingly small innovations have reorganized the planet’s chemistry, hurling it into drastic phase changes. Surely humans might be as significant as the filter-feeding animals of the Cambrian Explosion.

Surprisingly, our ice age—which once hosted woolly mammoths and saber-toothed cats—isn’t over; it’s just on recess. Throughout the ice age of the past few million years, there have been dozens of so-called interglacials—brief windows of warmth, lasting only a few thousand years, when it gets warmer, the ice rapidly melts and retreats to the poles (where it is today), and the seas rise by hundreds of feet. We’re currently in one of these brief respites from the cold, but interglacials don’t usually last very long. This is all caused by the periodic wobble of the planet in space and the rhythmic changes to its orbit, which nudges the planet in and out of the sunlight, alternately locking much of the Northern Hemisphere in ice and then thawing it over and over again, like a geological metronome.

Hacking chunks off of these reefs and bringing them back to the lab—where they subjected them to a bit of geochemical wizardry*—Finnegan’s team discovered an abrupt drop of about 5 degrees Celsius in the tropical ocean at the end of the Ordovician. Five degrees might not sound like a mass extinction, but the rocks say otherwise. “The general consensus in the field is that the Ordovician mass extinction is closely tied to climate change,” he said.

The greenhouse effect was first described in the 1820s by French physicist Joseph Fourier, who noted correctly that the planet would be uninhabitably cold if it weren’t for Earth’s blanket of insulating gases. In 1859, Irish physicist John Tyndall discovered carbon dioxide to be one such greenhouse gas, and in 1896 Swedish scientist Svante Arrhenius predicted that doubling CO2 in the atmosphere would warm the planet by about 4 degrees Celsius, a prediction that’s roughly in line with those of our most powerful modern supercomputers. Needless to say, discussion of this basic science by actors with baldly political motivations can be depressing beyond words.

The accidental orientation of the continents has a profound influence on life. A few million years ago, when the planet descended into its current ice age after its long slow decline from the greenhouse climate of the dinosaurs, the world was configured in a very peculiar way: namely, the way it’s configured now, with long north-south coastlines that extend from the tropics nearly to the poles. For example, the turnip-like South America and its North American thought bubble stand upright and span almost every latitude. This arrangement has been a lucky one for the animals trying to navigate a finicky climate that, in recent history, keeps plunging in and out of ice ages. When the cold comes, or when the warmth returns, most of them have simply been able to march up and down the continents to stay comfortable.

Ward, inspired by the Alvarez Asteroid Impact Hypothesis, sought to make a name for himself here in these ominous layers between the reigns of the fallen gorgonopsids and the surviving Lystrosaurus. He was after the debris from a catastrophic asteroid collision that could explain the devastation. He hunted for a layer of iridium, bits of fallout ejecta—anything to explain the sudden death of the biosphere. But he couldn’t find it. What Ward and others found instead at the end of the Permian was a wild swing in the carbon cycle.

Like the catastrophic Chinese volcanism earlier in the Permian, the so-called Siberian Traps were an altogether different style of eruption than that with which we’re familiar—and occurred on a scale that beggars the imagination. Unlike today’s postcard-ready stratovolcanoes in places like Mount Fuji, Vesuvius, or Mount Rainier (or the ones that continually exploded throughout the Ordovician), the Siberian Trap eruptions are what’s known as “continental flood basalts.” And they are what they sound like: burbling floods of lava that cover whole continents, stacking up miles thick in frighteningly short periods of time (geologically). They are the single most destructive force in the history of animal life. Luckily they don’t happen very often. At the end of the Permian, Siberia briefly turned inside out as the Traps covered Russia in more than 2 million square miles (5 million square kilometers) of lava.

Burning every last oily drop and anthracite chunk of fossil fuel on earth would release roughly 5,000 gigatons of carbon to the atmosphere. If we do so, the planet will become unrecognizable, with huge swaths rendered uninhabitably hot for mammals like us (to say nothing of the more than 200 feet of sea level rise that would drown much of civilization). But as exceptional as humans are, estimates of the carbon released in the End-Permian mass extinction range from an utterly catastrophic 10,000 gigatons of carbon—twice as much as we could ever burn—up to a mind-meltingly unfathomable 48,000 gigatons. As a result, temperature estimates for the End-Permian mass extinction and its aftermath strain belief.

Carbon or CO2?

The 1989 Montreal Protocol to phase out ozone-destroying chlorofluorocarbons (including End-Permian gases like methyl bromide) is widely acknowledged to be the most successful environmental international agreement ever. But failure was never really an option. NASA simulations of the planet under a business-as-usual emissions scenario for these chemicals showed the ozone layer almost disappearing from the planet entirely by the 2060s, an unimaginable situation that would have doubled UV radiation at the planet’s surface and spawned a global wave of lethal mutations and cancers.

“Adding CO2 increases weathering for two reasons,” Payne said. “One is that it makes rainwater more acidic. But the thing that a lot of geochemists think might actually be more important is that it just warms the planet, which creates more evaporation and more runoff, and the more water you pump through the system the more you can drive chemical weathering.” But rock weathering takes time.

Along with extremely high heat, devastating ocean acidification, and ozone destruction, other proposed Permian killers include: intense forest-killing acid rain from volcanic sulfur dioxide, brief blasts of cold from sun-blocking sulfur aerosols, agonizing respiratory death from the noxious slew of gases billowing out of the volcanoes (gases that would not have been unfamiliar on a World War I battlefield), direct carbon dioxide poisoning, and mercury toxicity. With so many potential killers running amok, Doug Erwin has humorously dubbed the glut of suspects at the end of the Permian the “Murder on the Orient Express” theory of mass extinction. “Only Dante could truly do this world justice,” he writes. “Yeah, so there’s certainly no shortage of killers,” said Kump. Add to this whodunit two more suspects—the horrifying specters of ocean anoxia and anoxia’s poisonous bedfellow, hydrogen sulfide, which can be produced by bacteria only in the absence of oxygen. If you’ve ever smelled rotten eggs, you know what hydrogen sulfide is. At only one part per million, it already starts to suffuse the air with the unmistakable miasma of rank shit. At 700 to 1,000 parts per million, you die instantly.

As the planet slowly teeters, the amount of sunlight reaching different parts of the planet changes. For locales near the tropics, the effect can be switching from a monsoon climate to a drier one. As a result, lakes get deeper and shallower in roughly 20,000-year intervals, over and over and over again. The rocks when the lakes are shallow—red mudstone, with animal footprints and tree roots—are very different from when the lakes are deep—black, thinly laminated, with exquisitely preserved fish fossils. “The lake sediments are like a rain gauge that’s color-coded,” Olsen said. Sedimentary rocks laid down in these rift valley lakes of the End-Triassic are a veritable seersucker of red and black, testifying to the planet’s regular wobble. Olsen determined that the first, most devastating wave of extinction happened within just one of these layers, perhaps in fewer than 20,000 years—a geological instant.

Fossil plant life attests to the skyrocketing CO2. Plants breathe in carbon dioxide through tiny pores on their leaf surfaces. But there’s a trade-off for having too many pores and breathing easier: it’s also easier to dry out and die. This is why plants keep pores to a minimum: just enough to breathe, but no more than necessary. In times of high carbon dioxide, they’re able to get by with fewer pores as they sip from the CO2-rich air. In 200-million-year-old fossil plants, University College Dublin paleobotanist Jennifer McElwain found that the number of pores on the ancient leaves plummeted over the End-Triassic mass extinction to accommodate what must have been a deluge of volcanic carbon dioxide. As with the End-Permian, there’s also a huge shift in the carbon isotope record over the mass extinction, similarly pointing to a massive influx of carbon to the atmosphere.

Unlike the typical Hollywood CGI depictions of asteroid impacts, where an extraterrestrial charcoal briquette gently smolders across the sky, in the Yucatán it would have been a pleasant day one second and the world was already over by the next. As the asteroid collided with the earth, in the sky above it where there should have been air, the rock had punched a hole of outer space vacuum in the atmosphere. As the heavens rushed in to close this hole, enormous volumes of earth were expelled into orbit and beyond—all within a second or two of impact.

“So the first thing is the radiation from the fireball, which is so hot, it’s mostly optical, but then you get the ejecta arriving.” A phenomenal amount of earth was excavated from the crater. This was the ejecta, so named for its literal ejection into orbit. Loosed from the surly bonds of earth for a moment, the rock followed intercontinental ballistic trajectories to the far reaches of the globe. When it returned, it burned up in the atmosphere in a worldwide blizzard of meteorites. This is one of the mechanisms that provides the asteroid theory its globally lethal thrust. “It covered the earth within about an hour,” said Melosh. “As it started falling, the sky would have turned red, and it would have gotten oppressively hot. And then it would have gotten hotter, and hotter, and hotter.” Melosh and his colleagues calculated that the incoming rock would have subjected the surface of the earth to 10 kilowatts per square meter of energy.

When the asteroid struck the sulfate-rich carbonate bank of the Yucatán, it injected enough sunshine-blocking aerosols into the stratosphere, it’s thought, to dim the surface of the planet for months.

British geologist Anthony Hallam (with a somewhat unseemly triumphalism) cites this record of precolonial ecological ruin to “dispel once and for all the romantic idea of the superior ecological wisdom of non-western and pre-colonial societies. The notion of the noble savage living in harmony with Nature should be dispatched to the realm of mythology where it belongs. Human beings have never lived in harmony with nature.” That the human project since its birth, and human flourishing in general, seems to have played out at the expense of the rest of the natural world is one of the stark and unsettling discoveries of science. This destructive human shadow has grown in the past few centuries, and the list of extinctions in the very near past is tragic and well known:

But the carbon cycle is not the only earth system getting short-circuited thanks to human ingenuity. We’re also living through the largest disruption to the earth’s nitrogen cycle in 2.5 billion years.

It stands to reason that, until very recently, all vertebrate life on the planet was wildlife. But astoundingly, today wildlife accounts for only 3 percent of earth’s land mammals; human beings, our livestock, and our pets take up the remaining 97 percent of the biomass. This Frankenstein biosphere is due both to the explosion of industrial agriculture and to a hollowing out of wildlife itself, which has decreased in abundance by as much as 50 percent since 1970. This cull is from both direct hunting and global-scale habitat destruction: almost half of the earth’s land has been converted to farmland.

Now for the crazy part—and the part that should shed some light on how bad the Big Five mass extinctions actually were. Despite this record of devastation, and despite the casual gloom of the many science journalists and conservation nonprofits peddling the reality of a current sixth mass extinction on par with the first five, humanity has not yet come anywhere even remotely close to death tolls of the major mass extinctions of the past half-billion years . . . yet.

What is going on today is extremely unusual. We’re hunting and destroying animals at unfathomable rates, but if humanity were to disappear tomorrow, the planet might quickly recover. If we stopped dumping carbon into the atmosphere and ocean, in a few thousand years it would come out of the system as limestone. But we’re not likely to stop anytime soon, and alas, our plunder can’t go on forever without unleashing geologically significant devastation. In 2011, UC Berkeley paleontologist Anthony Barnosky and his colleagues published the paper “Has the Earth’s Sixth Mass Extinction Already Arrived?” Judging by the coverage of the paper in the popular press, the answer seems to be an unequivocal yes. But in fact the paper predicted that the planet would reach Big Five levels of extinction only after hundreds to several thousands of years of continued and unabated environmental destruction. This outcome would still be instantaneous from a geological point of view, but from a human perspective the sixth mass extinction is still, thankfully, a little ways off.

Huber said, “If you ask any schoolchild, ‘What were mammals doing in the age of the dinosaurs?’ they’d say they were living underground and coming out at night. Why? Well, heat stress is a very simple explanation. Interestingly, birds have a higher set point temperature—ours is 37 degrees Celsius, birds’ is more like 41. So I actually think that’s a very deep evolutionary relic right there. Because that wet-bulb temperature was probably maxing out around 41 degrees Celsius in the Cretaceous, not 37.”

The fate of the world, then, becomes an easily calculable cost-benefit analysis, one amenable to smug op-eds by economists. The corn belt will shift north by so-and-so degrees latitude, the GDP of certain countries will respond in kind, and it’s all very orderly and predictable. Unfortunately, this is not how the world has tended to behave in the geological past.