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HomescienceExplained | Will the next supercontinent really drive mammals to extinction?

Explained | Will the next supercontinent really drive mammals to extinction?

By Robin George Andrews for Scientific American

There are dunes as far as the eye can see, parched and shimmering in the unforgiving sun. This gold and ochre sea of sand would seem infinite to any living creature wandering its vastness. But no one wanders this wasteland, and there is no water to drink. In this bereft, forsaken place—a place in which all landmasses have merged to form a single “supercontinent”—moisture and most animal life are merely a faded memory.

This isn’t Tatooine or Arrakis. This is Earth, a quarter-billion years from now.

Well, maybe.

There have been several supercontinental eras in the past; each were forged when tectonic plates—giant slabs of solid rock floating atop Earth’s mantle—thrust landmasses together like pieces of a jigsaw puzzle. A supercontinent’s assembly is a leisurely thing because tectonic plates annually shift their positions at roughly the same rate at which toenails grow. The most recent supercontinent, known as Pangaea, formed more than 300 million years ago and gradually disintegrated across the next hundred million years. And should researchers’ projection of the future be correct, the next one, dubbed Pangaea Ultima, will form around Earth’s equator 250 million years from now, with potentially ruinous effects. According to a recent study published in Nature Geoscience, Pangaea Ultima’s rise will likely lead to a precipitous fall in biodiversity caused by scorching surface temperatures that could render more than 90 percent of that future supercontinent uninhabitable for mammalian life.

The study sparked extensive press coverage when it appeared in late September. Most outlets astutely noted that humans, also being mammals, would have a hard time living on Pangaea Ultima, too.

But whether this would really be “the end” depends, of course, on who you ask.

Some scientists simply scoff at the idea of making forecasts 250 million years into the future. “This is completely unfalsifiable,” says Robert Stern, a plate tectonics expert at the University of Texas at Dallas. But others appreciate the study, seeing it as a helpful thought experiment and a provocative reminder that no world—Earth included—can traverse geological timescales unscathed. “There is nothing sacred about that time,” says Elena Shevliakova, a climate modeler at the National Oceanic and Atmospheric Administration, who did not participate in the new research. “Physics-wise, [Pangaea Ultima’s climate] is plausible. It’s nothing crazy.”

The study’s doomsday framing is certainly attention-grabbing. Several scientists, however, perceive this paper as a story less about the fate of mammals but rather of habitable worlds. “This is the kind of imaginative science people should be doing,” says John Damuth, an ecologist and evolutionary biologist at the University of California, Santa Barbara, who was not involved in the paper. “This tells you how planets work.”

The Ever-Changing World

Despite its dire conclusions, this study was born from whimsy. Alexander Farnsworth, a climate scientist at the University of Bristol and the paper’s lead author, has done plenty of serious climate modeling before, including the sort that investigates Earth’s deep past. But when he felt stir-crazy while quarantining during the early stage of the COVID pandemic, he decided “to do something a little bit different—a little bit fun.”

He had already applied climate models to Westeros (one of the continents from Game of Thrones) and Arrakis (the planet from Dune). Why not turn those same forecasting techniques to Earth’s notional next supercontinent—essentially a fictional world, from our ephemeral point of view—and see what happens? When his initial modeling revealed “a quite toasty time period,” Farnsworth decided to team up with colleagues to turn his “fun” into a proper paper.

This is what they found: The suturing together of Pangaea Ultima’s constituent parts would increase volcanic activity, powering a massive pulse of explosive, greenhouse gas–belching eruptions that raise global temperatures. An immense desert would occupy the supercontinent’s interior, while conditions at the coasts would resemble a lethally sweltering sauna. And the sun, having burned through more of its hydrogen fuel, would shine slightly brighter in the sky, exacerbating the planet’s thermal troubles.

But as the paper itself notes, such projections are riddled with uncertainties. Let’s go through them step-by-step as the hellscape of Pangaea Ultima emerges.

Earth’s plate tectonics ensure that the planet’s face is forever changing. Alas, this comes with a geological downside: most rocks that would otherwise serve as helpful tracers of ancient events are instead churned to molten oblivion beneath Earth’s ever-shifting crust. Consequently, no one can say with confidence how many supercontinents have come and gone in the planet’s history, and the gaps in our knowledge grow the further we look back in time.

Working out the past motions of tectonic plates is tricky enough. But ascertaining future arrangements is even harder. The debate over the timing, assembly and final form of primeval supercontinents means that “it is not clear how long it may take to form the next one,” says Dietmar Müller, a geophysicist and plate tectonics expert at the University of Sydney, who did not participate in the new study. And not all supercontinents necessarily include all continental landmasses—sizable stragglers can exist in isolation.

Pangaea Ultima is one all-continent possibility. It was first proposed (and alternately called Pangaea Proxima) in 1982 by geologist Christopher Scotese, who is also a co-author of the new study. A handful of competing tectonics models culminate in equatorial supercontinents such as Ultima, too. But one of the proposed supercontinents, Amasia, would sit at a far higher—and thereby cooler—latitude. “If the supercontinent that straddles the north pole happens, then mammals probably end up continuing to dominate,” says Jessica Whiteside, a geochemist and paleoclimatologist at the University of Southampton in England, who was not a part of the new research.

The Crematorium Climate

Let’s say that Pangaea Ultima is the chosen supercontinent that materializes a quarter-billion years from now. By that time, the sun will be shining 2.5 per cent brighter than today—a near certainty of astrophysics. But our local star won’t be the biggest thumb on the climate’s scales.

The supercontinent’s assembly would involve the collision of various tectonic plates—and some of those plates will tumble (or subduct) below others, sloughing off molten material that bubbles back up as a globe-warming surge of volcanism. “All this smashing around and reconfiguration would result in the release of greenhouse gases,” such as water vapor, carbon dioxide and methane, Whiteside says. Volcanic eruptions can induce cooling, too, by venting out sulfur dioxide that combines with water vapor to form sunlight-reflecting aerosols. But that cooling would be brief because aerosols are quickly washed out of the sky, leaving greenhouse warming to dominate the world.

The heat would be especially withering deep within Pangaea Ultima’s borders. Far removed from the global ocean’s moisture-carrying winds, the parched landscapes there would be on par with today’s driest and most desolate deserts, denuded of plants and precipitation alike. At the coasts, however, an abundance of atmospheric water would be a bigger problem. “You get more water vapor and more evaporation from the oceans nearby, and so you’re increasing that relative humidity,” Farnsworth says. And as anyone knows who has experienced a sticky summer afternoon, it’s not the heat that gets you—it’s the humidity.

The killer combination of a topographically flat and bone-dry interior could also stifle any river-borne transport of carbon-rich sediments otherwise destined to be sequestered in the ocean. This could in turn short-circuit the global process of rock weathering that subtly but powerfully sways Earth’s thermostat. But such effects depend in part on the specifics of mountain ranges and other topographic features that are essentially impossible to predict. “That’s actually quite hard to know or, I would say, very hard to know,” says Alex Whittaker, a landscape dynamics researcher at Imperial College London, who was not involved in the new study. “And we’re not very good at understanding the relationship between [rock] weathering and climate today, let alone 250 million years from now.”

What’s a “Mammal,” Anyway?

Let’s pessimistically assume that we are still in the worst timeline, and Pangaea Ultima is a truly infernal place. How would land mammals fare? Keep in mind that Earth’s mammalian forebears were already put through the planetary wringer at least once, somehow enduring the Chicxulub impactor that slammed into the planet 66 million years ago and sent the dinosaurs out with a bang.

That our ancient ancestors survived at all is partly thanks to their ability to regulate their internal body temperature, which mammals can do in multiple ways. “If you think about desert foxes or desert hares, they have huge ears, and they’re maximizing surface area to exchange heat with the environment. Hot blood is going up to these thinly insulated parts of the body,” where it escapes into the environment, says Andrea Rummel, a comparative physiologist at Rice University, who was not involved in the research.

“Sweating is also huge—probably the most important thing,” Rummel adds. “The animal doesn’t really have to work to do anything once it pushes the water out. But water is expensive, and you have to go get more of it when you run out of it,” which would be an issue in future hyper-arid deserts. Even worse, above certain thresholds for temperature and humidity, no method of thermoregulation can help animals cool down; the ambient environment simply won’t accept additional waste heat, and organisms will be poached to death.

There are, however, other ways to evade the surface furnace. “Today a lot of desert species are nocturnal to escape the intense heat of the day,” says Gemma Benevento, a macroevolutionary paleobiologist at the Senckenberg Biodiversity and Climate Research Center in Germany, who was not a part of the study. “Burrowing, or even moving entirely underground, could also help to shield organisms from rising temperatures.”

With all this in mind, if we could imagine a mammal wandering about the deserts of Pangaea Ultima, what would it be like? It would have to be “something with huge surface area to dump heat, something that could go underground to avoid heat, and [it would have] some kind of water capture method,” Rummel says. It would be like the big-eared desert mouse in Dune, then.

Would evolutionary biology across the span of 250 million years grant mammals brand-new thermoregulatory adaptations? Don’t hold your breath. “What if you raised the typical body temperature of a mammal?” Rummel asks. Could that push the top limit of temperature survivability even higher? “That’s the big question. And all the available research to date in mammals seems to indicate that higher thermal limits are slow to evolve, if they evolve at all,” she says. “It’s easier to move your cold tolerance, and it’s harder to move your heat tolerance.”

Overall, if Pangaea Ultima comes to pass, “it’s very hard to make a mammal that can withstand the kinds of environments they’re proposing would occur,” Damuth says. Even if far-future mammals did find ways to manage the heat, the house-of-cards-style collapse of food chains could still starve them to extinction.

But all this hinges on what the term “mammal” even means. “If we go back 250 million years, the lineage that gave rise to mammals was quite different anatomically and physiologically,” Benevento says. “It’s absolutely inevitable that over timescales of hundreds of millions of years—and even less—the mammals we know today will have long since gone extinct or evolved.”

Evolution can seem slow to us, but across geologic timescales, it’s lightning fast. This makes a quarter of a billion years simply too far into the future to forecast fates for mammals—or most any other kind of multicellular life, for that matter. “Who knows what we’ve got to look forward to over that period of time? In 250 million years, the entirety of dinosaurs evolved [and] then went extinct, more or less,” says Tori Herridge, a mammal paleontologist at the University of Sheffield in England, who did not take part in the research.

The formation of a supercontinent is not a fast and furious horror show like a cometary impact or, say, humanity’s rapid-fire gaming of the climate and annihilation of natural habitats. It gives animals time to adapt. “It’s completely different to the scenario that we’re talking about with climate change right now,” Herridge says. “I don’t think Pangaea Ultima is the biggest problem mammals have to face right now. Let’s see first if they make it through the next 100 years.”

The mammalian aspect of this study is so uncertain that it is essentially unknowable. But its inclusion makes the terrible potential of Pangaea Ultima all the more tangible to us luxuriously well-hydrated, thermoregulated, surface-dwelling primates—likely a key factor in the study’s widespread press coverage.

This supercontinental tragedy may never come to pass. But if it does, and even if the land is scorched, one may wonder: What about the oceans? This study only assessed the survivability of land mammals, so “the whales may be safe,” Damuth says. Perhaps the denizens of the seas shall inherit the Earth—if, that is, they survive humanity’s heavy hand.

But don’t be so sure: Farnsworth’s team isn’t done dreaming up ways to destroy the future Earth. “We want to do the marine side of it next,” he says, with a grin.

(Published 17 October 2023, 12:28 IST)

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