Science & Tech
How peering into our ancient past could transform our understanding of contemporary climate change
In the late summer, when the Ottawa River water levels drop, the bedrock reveals hints of a primordial marine world. Etched in stone are odd circles, each over half a metre in diameter and joined together like a cluster of cells. These strange circles are faint imprints from a moment when rock-like creatures filled the waters, a time before humans and dinosaurs — and they hold clues to our modern atmosphere.
On a humid day near the end of summer, I walk down to the Ottawa River shoreline to see these circles, together with geologist Neil Carleton. After wandering through a small patch of forest, we step down onto the exposed river bottom, and there, embedded in the flat rock, we see the mysterious rings. Smiling, Carleton gestures towards them: “Voila!” he says. Then, quietly, more to himself than me: “Hot diggity!” They’re called stromatolites, and Carleton and I are gazing at their fossilized remains.
Stromatolites are layered formations of sand and single-celled organisms (called cyanobacteria) that grow towards the sun thanks to photosynthesis. Rare today, they were the dominant life form for about two billion years. Like plants, Carleton explains, they sucked in copious amounts of carbon dioxide and returned billows of oxygen. “Oxygen was the byproduct,” Carleton says enthusiastically. Stromatolites like these, over two billion years ago, helped create Earth’s oxygen-rich atmosphere, which later allowed for an explosion of complex life.
Through these 460-million-year-old stromatolite fossils carved into Ottawa’s riverbed, we’re reminded of a time when loads of oxygen was pumped into the skies — in contrast with today, as we pump loads of carbon dioxide. We’re injecting CO2 at a rapid rate, and it’s disrupting the equilibrium that supports life on Earth as we know it today — one that slowly formed over millions of years. Our atmosphere, consisting of nitrogen, oxygen, carbon dioxide and other molecules, exists in a fine balance. It’s kind of a Goldilocks climate for us, neither too hot nor too cold, and carbon dioxide is key to regulating this temperature. Tweak the delicate mixture too much, and too quickly, and it can cause major temperature shifts.
In fact, over the past three decades, paleoclimatologists — historians of our atmosphere — have discovered a more “recent” time, about 56 million years ago, when vast plumes of carbon dioxide gushed into the atmosphere and transformed life on Earth. They believe it’s the closest analogue to modern climate change that’s ever existed — and what they’ve uncovered should give us pause about our current trajectory.
About 56 million years ago, below the ocean surface, a powerful series of volcanic eruptions pierced through the bed of the North Atlantic Ocean, spewing out lava. It was so powerful that it raised part of the ocean floor, between Greenland and Europe. Beneath the waves, hot magma bled down through ocean sediments and cooked layers of buried organic matter. Over time, this volcanism and baking of organics vented incredible amounts of carbon dioxide and methane into the atmosphere, the equivalent of burning all the planet’s known fossil fuel reserves. Global temperatures increased at least five degrees Celsius, within range of some of today’s upper-end scenarios from the Intergovernmental Panel on Climate Change.
The event is called the Paleocene-Eocene thermal maximum, or PETM for short, named for the epochs it transitions between. If our planet’s timeline were a 100-metre sprint track, stromatolites would show up on the 22-metre mark. Looking way down the track, the PETM shows up with 12.5 centimetres left. For context, humans begin less than a centimetre from the finish line.
Climate scientists tell us if we don’t stop pumping CO2 into the sky, things are going to get a heck of a lot hotter. They offer a range of scenarios based on how much we get our act together, and they model what could happen if we don’t. According to the Intergovernmental Panel on Climate Change, global temperatures by 2100 could be between 1.5 and 5 C above pre-industrial averages. The
consensus today, if we stay on our current business-as-usual trajectory, is a world between 2.5 and 3 C warmer by the end of this century. But a lot could change, for better or worse, and even with these projections, uncertainty remains.
“The PETM is extremely valuable to us because we have a natural analogue” to modern climate change, says the University of Hawaii’s Richard Zeebe, one of the world’s leading experts on the time period. Otherwise, “we can speculate and we can run models, but we don’t know exactly what the future will look like.” While there were other warming periods in Earth’s history, nothing compares to the PETM’s CO2-induced warming over such a short time period. Some similarities are almost eerie, and we can learn from them. “This is an actual natural example that we can study in Earth’s past,” says Zeebe — from ocean chemistry to the fossilized stories written in stone. “We can look at the climate change that happened at this time — and we can look at the consequences.”
Paleoclimatologists first discovered the PETM in the ocean. Using large ships that looked like mobile offshore oil rigs, they dredged up deep sea sediment cores. To do this, a long pipe is plunged deep into the ocean, then into the seabed, and down into the ancient past; a tube of plastic in the pipe captures the soft sediments. Normally, the cores were a chalky white, but for the section coinciding with the PETM, the cores were red, indicating an abundance of red clay dust deposited from the shores of dry landmasses. The calcium carbonate shells of many tiny marine creatures, called foraminifera, disappeared in the PETM section of the cores, hinting at increased acidity. Indeed, further research (some of it done in Canada’s icy Beaufort Sea) confirmed the PETM oceans were packed with CO2 and very acidic.
But to find more evidence of the PETM, researchers left the oceans to learn more on land. There was something odd going on. Species obliterated. Increased acidity. Was there something we could learn to mitigate the damage of our own CO2- induced crisis? Perhaps paleoclimatologists would find more information perusing the geological library of land fossils.
In Wyoming’s Bighorn Basin, scientists found what they were looking for: a large red streak across the hills. It was a dusty red ribbon, seared into the mountains, that they hadn’t fully investigated — and it dated back to the PETM. But unlike the ocean cores, it was red for another reason. Harsh and volatile weather, mainly dry with intermittent heavy rains and storms, oxidized the soils and created the red hue. But to truly understand how the PETM affected life on Earth, scientists needed to find fossils: a glimpse of bygone life.
Three decades ago, one scientist decided to take up the challenge. As Scott Wing, curator of paleobotany at the Smithsonian National Museum of Natural History, tells me over Zoom from his home in Washington, D.C., wearing a pink Hawaiian shirt and with a fern plant in the background, he didn’t expect it would take him more than a decade.
Each summer for 12 years, his team scoured the barren hills of Wyoming’s Bighorn Basin, searching near the streak of red for signs of ancient life. In 2005, he had a eureka moment. While he was shovelling the dirt, several fossilized plants of exactly the right age emerged. “I started laughing, and then I started to cry because I was so happy,” he told a documentary filmmaker. By collecting a diversity of plant leaves, Wing and his team were able to plot their characteristics and determine the nature of the temperature changes 56 million years ago. The frequency of smaller leaves with smoother edges meant the climate was warmer and dryer.
A picture emerged from the burgeoning research: the Bighorn Basin was hot, dry and tropical, altered from the lusher epochs that bookended the period. And there was still a lot of life. Twisted bean trees grew up from the earth of the basin, with lizards and turtles inching by shoots of horsetails and cattails. Tiny horses (Sifrhippus) galloped by winding waterways, alongside a variety of other small hoofed mammals, like Meniscotherium and Diacodexis. Even early primates scampered through a scattered canopy. Farther north, in the Arctic, it was also hot but teeming with ancient crocodiles, and pre-primates with robust teeth that could survive in the Arctic winter darkness. The continents weren’t too different from where they are now, except India had yet to slam into Asia.
Critically, the research showed that temperatures became hotter and the oceans more acidic — and all this was caused by a major gush of CO2 into the oceans and atmosphere. But to really understand where we might be going, to see how climate change could affect us now, we need to understand how the PETM affected this tangled bank of life.
For one thing, some animals got smaller. PETM horses shrank down by a third to the size of house cats in the first 130,000 years of the PETM. Feline-like carnivores called creodonts also shrank, as did the hoofed and more herbivorous condylarths, among many others. Today, we’re already seeing similar trends, including shrinking Soay sheep in Scotland, red deer in Norway and California squirrels. But if creatures are getting smaller, what’s wrong with a bunch of cute miniature animals? According to one study, reduced body size can affect fertility, lifespan and the ability to handle periods of stress. In general, bigger animals tend to live longer. Their larger bodies store more energy, and dictate what they eat and also who they get eaten by. The animals of the PETM had more time to adapt, and more space to move to, whereas for our modern fauna, there are significant human barriers to moving north or south — and the swiftness of modern climate change may not give them enough time to adapt.
What’s more, the oceans during the PETM, especially the deep oceans, became a giant acidic soup. About half of all benthic foraminifera, those tiny marine creatures, went extinct. Other foraminifera, once abundant in the pre-PETM oceans, changed their morphology due to the acidic environment. And on the surface of the ocean, dinoflagellates (single-celled organisms with tiny, locomotive flagella) bloomed in coastal areas, kind of like the “red tide” algal blooms we see today, indicating major environmental stresses. In our current era, warming oceans are already decimating large swaths of coral reefs home to countless marine species, and acidification is weakening the shells of oysters and molluscs. Not only does this affect the growth and abundance of these creatures, but it also hurts the coastal communities that depend on them.
As temperatures warmed during the PETM, plants disappeared in huge numbers from the areas that became inhospitable, appearing in wetter and cooler environments more conducive to life. In the Bighorn Basin, Scott Wing saw a shift in the thousands of plant fossils he and his team uncovered. As temperatures rose, local plants vanished from the region only to be replaced by those normally found in hotter areas, like the Gulf Coast, close to 3,000 kilometres southeast.
As in the past, global migrations are now happening due to warmer climates: pine beetles shifting north and decimating boreal forests; ticks mov- ing poleward and spreading Lyme disease. In research from Science and Nature covering four thousand species of both plants and animals globally, roughly half were escaping hotter climates and sliding poleward to colder areas. Terrestrial creatures crept north about 16 kilometres a decade, and marine species moved four times faster. The impacts of contemporary warming on agriculture could be especially transformative. “The wheat belt is going to be in Canada,” Wing says, “and not in the U.S.” He added that in some arid environments, the heat could “make the Dust Bowl era look like child’s play.”
Estimates put the number of extinctions that occurred during the PETM at around 10 per cent of all species. That might sound low to some, considering the drastic changes the planetary system went through. Wing cautions me from drawing a lesson from this. In many ways, we are still in the relatively early days of PETM research, he says, with most studies limited to specific geographic areas like the Bighorn Basin. The field is still wide open for further research, literally and figuratively.
But Wing’s words of caution go beyond this, too. Other changes during the PETM have not yet manifested today — but Wing and other experts believe they could, and they would, be significant threats to our planet. Whether they happen or not largely depends on timing.
It took anywhere from around 5,000 to 10,000 years for temperatures to rise during the PETM, all the resulting effects. Geologically speaking, that’s barely the blink of an eye. Even then, many species still had time to adapt to the higher temperatures: to shrink in size, to migrate. Despite the relative speed of warming, the adaptation process continued.
But what occurred during the PETM in thousands of years is happening now in hundreds. We are warming our planet 10 times faster than during the PETM, and so any solace we take in the relatively low PETM extinction rates should break on the shoals of our more rapidly heating reality. Two degrees over several hundred years could cause much more damage than warming over several thousand. “I can’t emphasize how important that is,” Wing says.
Zeebe, too, seems equally troubled by the current rate of warming. “How the system will react is not exactly clear,” he says.
This uncertainty extends to another issue related to timing: if, how and when the dominoes fall. While underwater volcanic eruptions and the burning of organic matter sparked the PETM, many experts are split on whether this then triggered other phenomena — such as the release of methane pockets trapped below the ocean floor or of large amounts of carbon stored in peat deposits. Wing gravitates towards this second camp, where the volcanic activity during the PETM set off chain reactions in other greenhouse gas-emitting events, further accelerating the gush of CO2 and rapid heating. “My guess is that it was, in fact, triggering something else,” he says.
I once looked at these types of scenarios — like thawing permafrost releasing stores of pent-up methane — as doomsday outliers: scary as hell but unlikely to happen. But the science on these triggers, or tipping points, is now stronger. Like a teetering Jenga tower, climate change could spark the crash of other systems. For example, the West Antarctic Ice Sheet, which contains about six per cent of the world’s fresh water, is especially vulnerable to global warming-induced collapse, which could raise global sea levels by 3.3 metres. If that happens, it could displace more than 100 million people. Scientists believe the triggering temperature for the ice sheet collapse is between 1.5 and 2 C above pre-industrial levels — and we are already at 1.2 degrees of warming. What’s more, as one report from the Intergovernmental Panel on Climate Change put it, the collapse of this ice sheet would be “irreversible for decades to millennia.”
We were no longer even situated near the beginning of time. We were, in some ways, merely creatures living among the stars and just one part of a long planetary history.
The thawing permafrost scenario, which would release billions of tonnes of methane into the skies, is also likely already underway. And again, this sudden ejection of stored greenhouse gas would be, according to another report from the Intergovernmental Panel on Climate Change, “irreversible on time scales relevant to human societies.”
This reference to future timescales, well beyond our lifetimes, brings us to the third and final issue of time — and the hardest to wrap our minds around. When the PETM started, it took 5,000 to 10,000 years to stuff the atmosphere with vast plumes of carbon dioxide. Bloated with CO2, the atmosphere then took more than 150,000 years to return to pre-PETM levels. Once saturated in the atmosphere, it takes a long time to naturally sequester the molecule. “Most people are not aware of this,” says Zeebe. And for Wing, this was the biggest lesson he wanted people to draw from the PETM: the understanding that the impacts of our actions now will last millennia. To return carbon dioxide to pre-industrial levels — from 419 to 270 parts per million — could take as long as 10,000 years. “For human purposes,” Wing told me, “it might as well be permanent.” For a species that thinks in election cycles, this is hard to grapple with.
It’s for this reason, climate scientists warn, we must urgently lower emissions to prevent us from going over at most 2 C of warming. That’s non-negotiable, and we’ve already wasted precious time. But consider- ing that the CO2 already jammed into the atmosphere will be with us for a longer time — for thousands of years, even if we went to net-zero emissions today — many think we need to act with equal urgency to adapt. Protecting against the devastating droughts and rising seas, especially in countries lacking protective infrastructure, is increasingly critical. And so is, a growing chorus of experts say, the need for technologies like direct air capture: machines that suck carbon dioxide directly from the air and store it deep within the Earth or use it to create usable products like fuel. Ten years ago, most would have thought direct air capture was a “moral hazard,” taking attention away from things we can do now to lower emissions, like renewable power and electrification. But Wing believes time has already run out, and perceptions are shifting.
“We’ve gone from worrying about moral hazards,” Wing says, “to worrying about how the hell do we limit the amount of damage.”
Our atmosphere takes on roughly an additional two parts per million of CO2 every year. If we can get our acts together, thousands of years from now our descendents might, after cursing us for getting them into this mess, grudgingly thank us for the (relatively) early efforts to limit the fallout.
Pouring a cup of coffee in the morning, the steam swirling off the black, liquid surface, I rarely think about the long expanse of geologic time, despite inhabiting a world chock-full of reminders. I rarely let my mind wander to the Ottawa River, where Carleton and I saw the stromatolite fossils, those oxygen-producing powerhouses, etched in rocks like giant circular cells. Or to the Bighorn Basin, where PETM creatures lived in a dryer, hotter climate. Or even to the countless other fossils below my feet, these traces of Earth’s prehistoric past. Instead, I’m preoccupied with my daily to-do list and already anticipating my second coffee.
That’s why we have paleoclimatologists. While we think in days, weeks and years, they think in epochs. Through rocks, fossils and sediments, they study the slow march of time and the equally slow evolution of species and climates. Climate change, which is altering our biosphere in hundreds of years, is alarming to paleoclimatologists like Zeebe and Wing who normally observe these changes in thousands or millions of years.
Wing, who speaks with an impassioned yet methodical cadence, shares how his research of deep time gave him a new philosophic perspective. Near the end of our conversation, he reflected on the larger impact of scientific thought. How, since Galileo, humans had discovered their true insignificance: we were not the centre of the solar system. Because of Darwin, we were no longer even situated near the beginning of time. We were, in some ways, merely creatures living among the stars and just one part of a long planetary history.
But Wing has had to rethink that notion. “Now, all that is true,” he tells me, “but what our technological society is capable of doing is profound on a planetary scale. The effects will last for geologic time.” If paleoclimatologists were to study our current moment millions of years down the road, the fingerprints of our actions would show up everywhere. The chemicals leached into the ground, the layers of non-biodegradable garbage, the spike of countless species killed from climate change and industrialization. If the Paleocene-Eocene was a monumental event, so too is what many describe as the Anthropocene — a human-shaped epoch underway today. The point being: we may not be the centre of the universe, but we are not insignificant. “That is a pretty profound shift in how we think about ourselves.”
This story is from the May/June 2023 Issue
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