After having witnessed the warmest summer on record, many are understandably wondering just how hot it could get? Perhaps there are some clues in Earth’s past. If we’re looking for a paleo-analog for the deadly heat we might face if we continue to warm our planet through fossil fuel burning and carbon pollution, the so-called “PETM” (Paleocene-Eocene Thermal Maximum) of 56 million years ago might do. The average temperature of the planet reached an infernally hot 90F—and large parts of the planet would have been unlivably hot for us, had we existed back then. In an excerpt from his illuminating new book “Our Fragile Moment: Lessons from Earth’s Past to Combat Climate Crisis“, Dr. Michael Mann looks to this episode in the distant past to show us what our future may bring.
What was the PETM actually like? Of course, we don’t have photos or documentary evidence, but a rather striking picture emerges from the fossil record of plants and animals. Mangroves and rainforests reached Arctic latitudes. Hippos, alligators, and palm trees graced Ellesmere Island, off the northwestern coast of Greenland, suggesting lush, balmy conditions near the North Pole. There’s evidence that some tropical ocean regions became so hot that they were abandoned by many organisms.
My former Penn State colleague and friend Timothy Bralower is one of the leading experts in the world when it comes to the PETM. He notes that temperatures were a balmy 68°F off the coast of Antarctica, where it is close to freezing today, and a scorching 97°F off the coast of West Africa, adding, “I’ve been swimming in Miami in August and it feels like a bathtub at 88°F, but 97°F is virtually uninhabitable!”
The Bighorn Basin in Wyoming today is home to the badlands, a dusty northern desert environment of scrub and sagebrush. During the late Paleocene, just prior to the PETM, it was subtropical forest, similar to northern Florida today, with swamps of bald cypress, palm trees, and crocodiles. During the PETM, mean annual temperatures appear to have reached 79°F, more similar to southern Florida. The swamps disappeared and rainfall turned more intermittent. It became hotter but also drier.
Was the drying part of a widespread trend? Probably not. Fossil pollen evidence suggests that tropical forests flourished and spread at this time. Climate model simulations of the PETM, using elevated CO2 levels consistent with the paleo data, suggest that western North America was likely one of the exceptions to the rule, one of the handful of continental mid-latitude regions that saw drying, primarily during summer, due to high surface pressure and the poleward migration of the jet stream. Many other regions—particularly in the tropics and subpolar latitudes—likely saw increased precipitation. Warmer air holds more moisture, so when conditions are favorable for rainfall—which they would have been over much of the planet—you get even more of it.
Over a large part of the planet, it would have been both very hot and very humid. That’s a bad combination. The old cliché is that “it’s not the heat, it’s the humidity,” but as anyone who has been to Las Vegas in August will tell you, that’s not true. It’s both. In fact, the best measure of susceptibility to heat stress combines temperature and humidity into a single variable. It is called the wet bulb temperature.
When I was a graduate student at Yale, I was a teaching assistant for the very popular undergraduate course “Oceans and Atmospheres” taught by my former Ph.D. committee member Ronald Smith. One of my favorite labs was the weather lab, where we would take the students up on the roof of the Kline Geology Laboratory and introduce them to a standard meteorological station, containing various types of weather instruments inside a little white shed elevated a few feet off the ground. Among the more interesting instruments was the sling psychrometer. It consisted of two thermometers attached side by side. One had a cloth wick covering on the bulb, which you saturate with water. There’s also a little rope attached to the instrument that allows you to “sling” it around in the air, speeding up the evaporation of the wet bulb thermometer, which cools off in response to the heat lost due to evaporation. It eventually reaches some new, lower equilibrium temperature.
The difference between the dry bulb (no cloth covering) and wet bulb thermometer readings is a measure of the relative humidity of the atmosphere on that day. And the wet bulb temperature measures the lowest temperature an object — which could for example be a human being like you or me — can reach through evaporative cooling at the prevailing temperature and humidity.
Our core body temperature is typically about 98.6° F. Mine runs somewhat lower, about 97°F. That makes me a veritable cold-blooded reptile compared to my wife and daughter, a source of endless battles over the setting of the thermostat. Skin temperature is typically lower by 4–9°F, depending on the level of physical activity, which helps transfer excess heat from the body’s core to the skin and then the surrounding air. Sweating helps keep the core temperature from rising, but it becomes increasingly ineffective as a cooling mechanism the more humid the air. A wet bulb reading of 86°F exceeds guidelines for safe physical activity, and a wet bulb temperature of 90°F, which feels as hot as a dry temperature of 131° F, is dangerous even without physical activity. Wet bulb temperatures of 95°F are comparable to a dry temperature of 160°F. At this point, your skin can no longer shed excess heat to the air. Even in the shade you will die in a matter of hours.
In most places today the wet bulb temperature never exceeds 86°F. The 95°F wet bulb survivability limit, however, has now been exceeded at least briefly in some locations in South Asia, the coastal Middle East, and coastal southwest North America. These are regions with close proximity to both very high sea temperatures and extreme summer heat — conditions jointly conducive to exceptionally high wet bulb temperatures. In The Ministry for the Future, science fiction writer Kim Stanley Robinson begins his engaging fictional account of our near-term climate crisis future with a heat wave in India where web bulb temperatures remain above 95°F for days on end, killing two million people. Life, alas, is beginning now to imitate art. During a historic early-season heat wave in spring 2022, temperatures in Chennai, India, reached 94°F with seventy-three percent relative humidity. That’s a wet bulb temperature of 86°F. And that was in early May.
With as little as 4.5°F additional global warming, something we could witness in a matter of decades in the absence of substantial climate action, this limit could be exceeded regularly in parts of South Asia and the Middle East that are home to as many as three billion people. Warming of 18°F could eventually be reached if we burn all estimated fossil fuel reserves (or if we burn a fair share of them and we happen to encounter some nasty destabilizing feedbacks). At that level of warming, much of the human population could at least occasionally be subject to this deadly heat limit. “A heat wave all year long” indeed.
Some of my own research involves looking at climate model projections to assess the potential for severe heat exposure in the United States. My collaborators and I recently examined simulations used by the Intergovernmental Panel on Climate Change (IPCC) to project future changes in heat stress accounting for both heat and humidity. We found that short- to medium-duration episodes of extreme heat stress are likely to increase more than three-fold across densely populated regions of the United States in the Northeast, Southeast, Midwest, and desert Southwest by the end of this century in a high carbon emission scenario. Other research I’ve conducted suggests that these very same model projections may be underestimating the true heat stress risk, as the models tend to underestimate the wavy summer jet stream conditions that are associated with the most persistent heat extremes. Adverse heat-related health impacts are already being experienced by outdoor workers in Las Vegas, Los Angeles, and Phoenix.
It is relevant, in this context, to imagine what conditions might have been like during the PETM when the global average temperature was an almost inconceivably hot 90°F. That’s 30°F warmer than today. Adding insult to injury, it was generally humid as well. What would it be like to suddenly be transported back in time to the PETM? There’s a good chance you could find yourself experiencing a daytime high temperature of 100°F and eighty-two percent relative humidity. That combines the heat of a sauna with the humidity of a steam room, a hybrid entity that humans don’t build in the real world for good reason—it’s deadly. Eighty-two percent relative humidity and 100°F amount to a wet bulb temperature of 95°F. If you found yourself subject to those conditions without access to refrigeration, air conditioning, or a cold pool to dive into, you’d soon die from exposure.
The conclusion is that Homo sapiens—at least in our current form, and without the luxury of modern technology—couldn’t have lived during the PETM, except possibly near the poles. Yet, many other mammals appear to have made out fine. There was a die-off of deep ocean biota—likely related to the anoxic conditions induced by deep ocean warming. But we didn’t see any mass extinction of mammals. Instead, we saw a combination of migration and evolution. Evolution works if you’ve got thousands of years to work with, which is a luxury we don’t have today, but which PETM life-forms did. The primary adaptation, other than migrating poleward in an effort to escape the heat, was dwarfing.
As a rule, the way to cool down is to get smaller. That’s what happened during the PETM—in an evolutionary sense. Larger representatives of a given mammal species couldn’t get rid of heat very well. They selectively died off and failed to reproduce. The smaller members of their species were more likely to survive and more likely to pass along their genetic traits. And so on. That’s how the process of dwarfing proceeds.
We see some dramatic examples during the PETM. Horses, which had only recently appeared on the scene, shrunk by thirty percent in size (and scaled back up seventy-six percent as the PETM came to an end). It would be tempting to conclude that the warming therefore wasn’t a big deal. They just adapted, after all. Much as some critics of climate action insist that we will just “adapt” to the impacts of climate change. But understand that any selective pressure so great as to shrink animals by thirty percent in over as little as 10,000 years, implies substantial mortality of those with maladaptive traits (namely, larger size). Think about that fact the next time you hear a climate contrarian insist we can simply “adapt” to climate change. It is true that our species can likely survive 9°F warming. It is also true that hundreds of millions of our fellow human beings will likely perish from it.
As always, there were winners and losers in the PETM. Our primate ancestors were winners. Even though the PETM would have been too hot for us, it did provide a selective advantage to our much smaller progenitors, the first primates. Ocean bottom–dwellers, on the other hand, were the losers. While surface forams (single cell organisms that live in the open ocean) in general made out okay, deep-sea benthic forams were devastated by acidification. Perhaps as much as fifty percent of all benthic foram species went extinct. In fact, deep-sea acidification was so extreme that sediment cores are relatively devoid of calcite shells, many of which were literally dissolved. And we think that lesser, but still significant, acidification took place in the upper ocean.
Ocean circulation changes, too, might have played an important role in the PETM. A clay mineral known as kaolinite is produced by silicate weathering and carried off in streams and rivers to the ocean. The fact that anomalous levels of kaolinite are found in PETM-dated ocean sediments suggests that the overall increase in rainfall during the PETM likely led to increased continental runoff, delivering large amounts of freshwater to the ocean. As we’ve seen before, a large input of freshwater to the ocean can disrupt the so-called ocean conveyor belt circulation. A combination of climate model simulations and carbon isotope data from forams shows that not only did this disruption occur, but that it led to the burial of warm, oxygen-depleted waters at the ocean bottom. The acidification, warming, and deoxygenation of the deep ocean would have constituted a triple whammy for deep-sea life.
The deep ocean warming, estimated to be as much as 5–7°F, could have destabilized seafloor methane hydrate, constituting a potential trigger for the large methane pulse that is argued to have contributed to the PETM warming. It might have generated other ocean changes as well. Although the extinction event seems to have been confined to the deep ocean, the PETM did cause some notable changes in the upper ocean. There were widespread blooms of dinoflagellates in coastal ocean regions. An ancient form of red tide algae, these blooms were likely favored by what is known as eutrophication: the increased continental runoff would have delivered increased nutrients such as nitrogen into the coastal regions, leading to large outbreaks of dinoflagellates. As with modern red tides, the blooms of dinoflagellates would soon run through their boom-and-bust cycle, dying, decomposing, and consuming ocean oxygen in the process, thereby threatening other sea life including fish populations. As today, warming ocean waters would have exacerbated these dangerous and deadly occurrences.
Unlike the current warming, we know that the PETM episode of abrupt warming was natural in origin. Or do we? One study suggested that there were likely two distinct pulses of carbon input into the system. The second was consistent with a massive input of methane released in response to warming. But the carbon source that triggered the initial warming? The study could not pin it down. So, let’s have a bit of fun for a minute. And while we’re doing that, we’ll keep in mind the fact that science often gains insight by ruling out what cannot be true and what cannot have happened. That is the spirit in which we will embrace the Silurian hypothesis. I’ll begin by telling a story from my childhood. Growing up as an American in the 1970s, I watched religiously a children’s TV series called Land of the Lost. It was an instant hit when it premiered in the fall of 1974. As an eight-year-old boy obsessed with dinosaurs and time and space travel, I felt that the show was custom made for me. I was hooked.
The program featured a family who found themselves trapped in a bizarre subterranean land inhabited by dinosaurs, Ewok-like ape people called Pakuni, and malevolent lizard people called Sleestak. The Sleestak descended from a once peaceful and advanced race of reptilian bipedal humanoids (called Altrusians), but degenerated over time into the primitive, barbaric individuals who inhabited the ruins of their once great civilization.
The storyline for the first season was written by science fiction writer David Gerrold who, among other things, wrote the famous “The Trouble with Tribbles” episode for the original Star Trek series.
Gavin Schmidt is a contemporary of mine who is currently director of the NASA GISS climate modeling laboratory, having taken over from former director James Hansen some years ago. When Schmidt was growing up in the 1970s across the pond in the United Kingdom, he, too, was watching science fiction programming, the BBC TV series Doctor Who to be specific. One episode featured lizard people, awakened by nuclear testing after 400 million years of hibernation. These intelligent, bipedal reptilians ruled over the dinosaurs, but were forced to hibernate deep within Earth’s crust to escape a global catastrophe. They are called the Silurians (since we’re only having fun here, we’ll overlook the fact that the Silurian period actually predated reptiles by a hundred million years and dinosaurs by nearly two hundred million years).
Early 1970s TV and film was full of tales of collapsed ancient lizard civilizations. Why? I have some thoughts. The early to mid-1970s was the apex of environmental dystopianism. It gave us films like Silent Running and Logan’s Run premised on scenarios of environmentally driven societal collapse. The 1973 film Soylent Green, starring Charlton Heston, was arguably ahead of its time. Premiering decades before widespread awareness of the climate crisis, it was premised on the devastating societal consequences of global warming. The story takes place, coincidentally, in the year 2022.
Then there’s the 1968 dystopian film Planet of the Apes, which once again starred Charlton Heston, in the role of an astronaut who has found himself stranded on a planet ruled by intelligent, ape-like hominids. Toward the end of the film, he realizes that he had time traveled when he happens upon the archeological remains of his own civilization. It had destroyed itself through nuclear annihilation. The ape-like hominids had evolved to fill the void that was left behind.
Perhaps there is something archetypal about the notion of an intelligent civilization gone extinct under enigmatic circumstances. Maybe tales of this sort trigger something deep down in our own primitive lizard brains, some instinctual sense of our tenuousness on this pale blue dot we call home. Possibly such tales resonated with the dystopian environmental ethos of the 1970s, as we began to understand the threat posed to our planetary home by worsening air and water pollution, disappearing forests and habitats, and the nuclear-fueled Cold War that was simmering. Conceivably you’re wondering what any of this has to do with the PETM.
What if an intelligent pre-human civilization like the Altrusians or Silurians existed tens of millions of years ago on Earth and extinguished themselves through catastrophic planetary warming, courtesy of an energy-greedy, fossil fuel–burning spree? Would we know it? This is the very thought experiment that was pursued by my friend and colleague Gavin Schmidt and his coauthor astrobiologist Adam Frank in a 2018 article titled, appropriately, “The Silurian Hypothesis.”
The project was a bit of an accident — as novel scientific pursuits, to be perfectly honest, often are. Adam Frank is a deeply inquisitive astrophysicist with a passion for addressing truly big questions, as I learned during a fascinating conversation over coffee on a chilly February day in 2019 while he was visiting the Happy Valley of State College, PA. He is also a leading advocate for the search for extraterrestrial intelligence (SETI), a continuation of the legacy of Carl Sagan, who cofounded the Planetary Society back in 1980 to advocate for the ongoing search for life in the cosmos.
Back in 1961, at the very first scientific SETI meeting, the astrophysicist Frank Drake formulated a mathematical expression for the number of communicative civilizations in our galaxy as a product of various terms: the rate at which stars are produced, the number of planets per star, the fraction of those planets that would be habitable for life, the fraction of those on which life actually arises, the fraction of those that produce intelligent civilization, and the fraction of those that develop radio communication. The final factor is the typical lifetime of such civilizations. Carl Sagan was one of the ten scientists present at that meeting. He believed that the last factor was likely to be the limiting one. In other words, the key question, in Sagan’s mind, was whether or not technological civilizations could avoid self-destruction. It is not unreasonable to speculate that these early musings on Sagan’s part might well have prepared him for the later role he would play in the 1980s in the debate over the nuclear arms race.
In 2017, Adam Frank paid a visit to Schmidt, a climate modeler. He was interested in the related astrobiological question of whether prospective industrial civilizations that arise on other planets might extinguish themselves through fossil fuel–driven warming. As I’ve learned from numerous conversations and collaborations over the years, Gavin Schmidt is an outside-the-box thinker. He’s also a devil’s advocate. So, he turned around and asked a stunned Frank a question of his own: “How do we know that a past civilization didn’t already do this on Earth?” And so we come to what’s called the Silurian hypothesis: how do we know that the rapid carbon spike behind the PETM warming, for example, wasn’t the extinction-causing act of some ancient fossil fuel–hungry civilization? What sort of evidence might a subsequent civilization like us hope to find fifty or sixty million years later?
Though scientists and novelists alike have speculated about such things for decades, Schmidt and Frank moved the ball down the field quite a bit by examining in detail the sort of geological and archeological evidence that would and would not be left behind by an intelligent, civilization-building species that drove itself to extinction through environmental destruction in the deep geological past. They note, for example, that the typical sorts of evidence we might imagine—continental-scale graves of human skeletons, collapsed edifices, cars and trucks, foundations of homes, etc.—simply wouldn’t remain. Geological weathering and erosion along with plate tectonics would have destroyed any artificial structures and objects older than about ten million years.
There would be no direct evidence, in the sense of archeological sites or preserved artifacts, of an industrial civilization that only existed for a few centuries, a veritable fleeting geological moment. A very small fraction of living things ever become fossilized. So, if some race of reptiles or early mammals in the late Paleocene developed a civilization that lasted even for 100,000 years, let alone a few centuries, it would be easy to miss in the fossil record.
What evidence might we expect to find? We might see sharp coincident spikes in oxygen and carbon isotopes in preserved sediments, indicative of a rapid rise in greenhouse gases and temperatures. But that’s precisely the sort of evidence we see with the PETM!
We might also expect to see a spike in nitrogen isotope ratios, indicative of the large-scale use of fertilizers, and increased anoxic zones in oceans, due to eutrophication, ocean acidification, and extinction of calcareous biota preserved in sediments. We could detect anomalous levels of lead, chromium, antimony, rhenium, and other mined metals in sediments. Interestingly, we do see these sorts of changes in the PETM and during other past episodes of rapid climate and environmental change due, for example, to increased erosion and continental runoff.
To be clear, Schmidt and Frank aren’t actually suggesting that sentient lizards caused a warming spike fifty-six million years ago. Turns out, there is a perfectly good (and alas, far more mundane) explanation for what happened. Occam’s razor, in the end, prevails. The authors concede that the hypothesis is almost certainly wrong. I asked Schmidt for the most compelling piece of counterevidence offered by his critics. His answer was: “Our experience with deep mining. These are metallic deposits that date back sometimes billions of years, and as far as I know, there is no evidence that they have been tapped previously.” Yet the hypothesis isn’t obviously wrong, either. It demands consideration and close examination.
Schmidt and Frank were merely posing the question of how future beings — including perhaps denizens of our planet millions of years hence — would know if a civilization like ours extinguished itself through environmental degradation and, specifically, a fossil fuel–driven abrupt warming event. The Silurian hypothesis was motivated by a deeper question that scientists like Adam Frank, David Grinspoon, Carl Sagan, and even the great physicist Enrico Fermi have long pondered: Is there other life out there? If so, why haven’t we heard from it? Some have speculated that intelligent civilizations, perhaps, tend to sow the seeds of their own destruction through environmental ruination and warfare. And it’s certainly worth asking: Is that our inclination? And if so, can we defy that impulse?
Excerpted from the book ‘Our Fragile Moment: Lessons from Earth’s Past to Combat Climate Crisis’ by Michael Mann. Copyright © 2023 by Michael Mann. Reprinted with permission.