by Usha Alexander
[This is the second in a series of essays, On Climate Truth and Fiction, in which I raise questions about environmental distress, the human experience, and storytelling. All the articles in this series can be read here.]
When I was a kid, I used to wonder about the possibility that the planet could slip back into an ice age. I grew up in the Rocky Mountain region of the northwestern USA, where winters lasted half the year and summers were brief and blustery. I hated being cold all the time. Aware that ice ages result from some sort of natural cycles, I worried what might happen if the planet should head that way again. I tried to imagine how we would construct cities and farms, how we would travel between countries or even build roads, if huge glaciers grew down from the Arctic Circle and smothered our little mountain town.
So I was surprised to learn, much later, that we actually do live in an ice age. In historical memory, we’ve been enjoying a warmish, rather pleasant phase of this ice age, to be sure—an interglacial phase, called the Holocene, that’s persisted for about ten thousand years. But interglacial phases, like our present one, have only been brief respites, as the ice age has cycled between glacial and interglacial phases over the past two million years. Past interglacials never lasted very long and, left to its own geological devices, all signs suggested that this one would end too, to be followed by a much longer glacial phase—the stuff of my nightmares.
In fact, a 2016 study indicates that the Holocene interglacial should already be ending. As it was, the weather of the fourteenth through mid-nineteenth centuries was already given to frequent fits and bouts of cold weather extremes. The precise causes for the drop in temperatures are complex, and though it didn’t get cold enough to become a real glacial age, the period is now playfully referred to as the Little Ice Age. During that time, centuries of sporadic, crop-crippling cold led to erratic harvests, reducing grain yields and causing periodic famine, helping to topple Chinese dynasties and destabilize the feudal order of the European Middle Ages. Persistent hunger and poor nutrition amplified the depredations of plague and other diseases across Europe. In England, the Thames regularly froze over. In North America, even the Rio Grande froze up more than once. Peoples across southern North America suffered frequent droughts and malnutrition, especially in the southwest, which remained persistently dry for a span of decades. Some agricultural peoples were forced to return to lives of nomadism. Across the northern latitudes, glaciers extended themselves. The Norse Greenland colony, which had been thriving for centuries, collapsed and was abandoned by around 1500 CE. Early European colonists in North America struggled to survive frequent winters of bitter cold and failed harvests.
What if the glaciers had continued growing after the Little Ice Age, rather than having shrunk? I might have been born into a civilization that had developed according to a different planetary reality: one in which northern Eurasia and Canada remained the sole province of nomadic families living by the kinds of technologies and techniques practiced today only by the inhabitants of extreme polar climes, such as the Inuits; one in which large scale agriculture was perhaps feasible only in the tropics and subtropics, limiting the size of urban agglomerations and the total human population. African, southern Asian, and Central American cultures might have dominated the settled world. All of modern history would have taken a different, unknowable path.
But instead, atmospheric carbon dioxide had already begun to rise, reaching about 280 ppm by the middle of the 19th century. It crept up due to the sweeping changes humans were already making to their environments, destroying forests, spreading agriculture, and vastly altering terrestrial and marine ecosystems by driving hundreds of species to critical endangerment or extinction. Such land-use changes had allowed atmospheric carbon dioxide to accumulate enough to forestall the planet’s descent toward the next glaciation. And today greenhouse gas concentrations have risen so much that they have likely forestalled the following glacial cycle beyond the next, altering the trajectory of the climate for at least a hundred-thousand years and ultimately sending us on a path equally uncharted and unknown to us as the glacial alternative that could be happening.
Of course, the planet is indifferent to this change; it has no preference for the ice age. On long geological timescales, temperature graphs show that Earth’s climate has been astonishingly unstable, and not typically icy. Sometimes fiery, sometimes torrid, sometimes sultry, for most of its four-and-a-half billion years, Earth has actually inhabited more of a “hothouse” state, much warmer than today and without the persistent polar ice caps and mountain glaciers so familiar to us. Even Antarctica has known its growing seasons. By contrast, the duration of “icehouse” states, when the world has been colder, have totaled only a few hundred million years, with the previous major ice age having ended hundreds of millions of years before our present one began.
But though Earth has spent far less time with ice overall, our species arrived in the midst of this ice age. During the past two-and-a-half million years as our genus, Homo, evolved, Earth has generally been much colder and much more heavily covered by ice than it is today. And though many of us habitually think of ourselves as separate from nature, we are in very essential ways the products of this ice age.
Fire
The past six hundred million years have been mostly a span of relentless heat, during which plants and then animals first climbed up and colonized Earth’s great, empty landmasses. Extreme heat was the backdrop for the rise and fall of the dinosaurs, and equally the setting for the subsequent ascendance of mammals. The heat reached its greatest extremes some fifty million years ago, with carbon-dioxide levels nearing 2,000 ppm (versus ~414 ppm today) around the time when our tiny, early primate ancestors were just starting to spread and diversify through the world’s forest canopies. Those early primates arose in the heat, adapted for the heat; but Earth continued to change, and the climatic conditions that gave rise to Homo sapiens would be very different. Global temperatures were slowly declining. Yet the weather still remained generally torrid across the planet, between thirty and forty million years ago, when the first snows started to persist in the mountains of Antarctica as permanent patches of ice.
Antarctica was only fully glaciated six million years ago, followed three million years later by the Arctic, as global temperatures continued to fall. During this time, the African continent was also growing drier. Wildfires were burning away forests, leaving grasslands in their wake, forcing the local fauna and flora to migrate or adapt to the changing habitats. Among these African fauna were our earliest bipedal ancestors, the Australopithecines, learning to roam the expanding savannah, with fewer and fewer stands of trees to climb to find safety or edible fruits and foliage.
Eventually, Australopithecus would entirely abandon a life in the trees. In her search for new foods, she discovered rich packets of nourishment living underground: tubers, corms, and rhizomes. She also discovered something amazing about fire: it left behind a bounty of charred animals and plants, ready food for the gathering, wondrously (if accidentally) cooked. Over several thousands of millennia, some of these resourceful primates of the African savannah were irreversibly changed by their new diet and way of life until, by about two million years ago, they would be recognizable as a new species: Homo erectus, increasingly adept at exploiting this new landscape, its new forms of nutrition, and fire.
Fire is a versatile tool, useful for everything from scaring away predators to foraging to keeping warm as the planet continued to get colder. We don’t know exactly when Erectus first controlled it in a hearth or intentionally cooked her food (the earliest confirmed hearth dates from a million years ago), but learning to cook greatly expanded her available food sources by rendering otherwise inedible plants more palatable and less toxic as well as increasing their bioavailable nutrition. Achieving greater nutrition with less work, and associated social behaviors around cooking, may have driven the change toward a range of more humanlike traits, including the reduced body-size sexual dimorphism (larger females), larger brains, more gracile teeth and faces, and smaller guts, which we first see in H. erectus, making them our earliest, recognizably human-like ancestors. With fire in her toolkit, Erectus ventured out of Africa into the cold, colonizing much of Eurasia, from Iberia to China to Java.
Ice
And the world grew colder still. Over the next million years, ice swelled into mountains across the polar-adjacent landscapes. Glaciers took hold of the planet, sometimes reaching their fingers down into today’s temperate zones, sucking up the ocean by tens of meters and stealing the rainfall from across Eurasia and North America, freeze-drying vast expanses of un-glaciated land into icy tundra. By this time, the world had fallen into its cyclic pattern of glacial and interglacial phases. The glacial phases were long and deep, lasting tens of thousands of years and fluctuating between states of extreme cold and… even colder; the interglacials were shorter and warmer, more like today, when the glaciers massively shrunk back and sea levels rose. But even within these major, multi-millennia cycles of cold, colder, and warm temperatures, the climate never stabilized, continuously fluctuating by lesser extremes over centuries.
As Erectus and her descendants in Africa survived these climatic shifts, they were growing bigger brained, their toolkits more varied and refined. Somewhere around three-hundred-thousand years ago, a branch of her descendants attained a human form we recognize as fully anatomically modern: morphologically indistinguishable from those of us alive today. These earliest humans ranged across Africa. Perhaps they were following the changing availability patterns of the wild foods they foraged as the planet fluctuated through its glacial phases. Or maybe they went in search of cooler summer nights during a hot interglacial period.
The last interglacial, dubbed the Eemian, occurred around 130,000 years ago, beginning with a temperature spike even warmer than today. That peak of the Eemian was the warmest climate modern humans have thus far ever lived through, nor did the spike last long before returning to temperatures somewhat cooler than today’s. And when the Eemian ended, around 115,000 years ago, it was followed by another deep glacial age, during which global temperatures remained extremely low for almost a hundred thousand years, sometimes averaging five degrees colder than today.
Surprisingly, it was during this frigid time, following the Eemian, that the ancestors of all the non-African peoples of today first crossed into Eurasia, by way of the Arabian Peninsula. Veering shy of the wintry mid-latitudes, they kept to the southern coastline of Asia, wandering eastward all the way to Australia by at least 65,000 years ago. Only tens of thousands of years later would some groups brave a westward migration into the chill of southern Europe. And much later still, some hardy folk made headway northward into the howling cold, through eastern Asia and into the Americas. Over hundreds of generations, they learned how to thrive on the tundra. Imagine! Having to invent proto-boots and parkas and mittens as you go—or die! Even with modern materials, I barely withstood the Idaho winters.
It was less than 12,000 years ago that the glaciers finally massively shrank back again, ushering in our current interglacial. The Holocene has graced us with unusual stability; the global average temperature hadn’t fluctuated by much more than a degree Celsius over the last ten-thousand years (including the Little Ice Age), until the last half century. There has never been a similar period of persistently stable, warm and wet climate since long before humans walked the Earth, not even since our early hominin ancestor, Erectus, first began to master fire, some two million years ago.
This temperate stability would enable another major transition in the human experience, as our ancestors finally stopped wandering and settled into villages, leading ultimately to experiments with plant and animal domestication, agriculture, and the rise of cities—all of which eventually altered Earth’s environment and climate enough to delay the onset of the next glacial phase. We could say that our modern civilization itself is a gift of the Holocene’s unprecedented, warm stability, which may even have been extended due to the earliest effects of anthropogenic alterations to the environment and climate, entirely unbeknownst to us at that time.
Gas
But ironically, in the decades since we’ve learned of it and clearly measured it, climate change has rapidly gotten ahead of us. It has thus far eluded, and may continue to elude, our ability to halt its relentless march. Since awakening to our plight, our goal has been to stabilize Earth’s climate in some state approximating the Holocene interglacial of the past ten thousand years; this cannot be achieved without limiting the accumulation of greenhouse gases in the atmosphere. But a new study suggests the concentration is now rising faster and is already higher than at any time in the past seven million years—and possibly the past 23 million years. That was during the Miocene epoch, long before the age of Erectus, or even bipedal Australopithecus. It was a time when the glaciers gracing the mountainsides of Antarctica were still but a backdrop to its grassy meadows and forested plains. The rapid rise of atmospheric carbon dioxide in the modern industrial age to levels not seen since the Miocene doesn’t bode well for the prospect of maintaining our Holocene interglacial climate regime.
Within my lifetime the global average temperature may reach the hottest condition humankind has ever survived—130 thousand years ago—and then exceed it. As the greenhouse gasses climb, rather than merely delaying the next two glacial cycles, there is a real chance that they might completely knock the planet beyond any interglacial state, entirely out of the present icehouse, sending it back toward a hothouse state, once again denuded of ice. Whether or not this planetary transition occurs, how much and how fast, depends upon what we do and other unpredictable risks of our circumstances.
We began as a tropical species, having made our homes in the Old World tropics and subtropics far longer than in any other region of the planet. Yet within a handful of generations, extreme heat may provoke a depopulation of the tropics as never before seen in the human story. It’s difficult to know or even clearly imagine how this disruption might manifest or be managed.
What would this mean for us as a species? As nations and communities? As a global civilization? Though we must do everything still within our powers to prevent the Earth from reaching the worst extremes of global heating, we must also recognize that, no matter what we do now, the world will inevitably get hotter—minimally, by another half a degree Celsius, as per the most optimistic goal of the Paris Climate Accords, and very likely by much more than that, as that goal rapidly slips beyond our reach—especially now that a new study suggests the climate is more sensitive to small changes in atmospheric carbon dioxide than had previously been supposed. The climate is already changing more rapidly than ever before in the history of our species. We may not be able to predict precisely how fast or how hot the world will become, but we know that heat will change the nature of the planet and the life upon it. This will pose grave challenges for humans and other creatures and require us to discover new ways to live.
[Part 3, Of Wanderers and Nomads]
Images
1. Mammoths, equids, and lions in southern Europe, Pleistocene, by Mauricio Antón. © 2008 Public Library of Science. Creative Commons.
2. Global average temperatures over the past 2000 years, by Ed Hawkins. 2020 Creative Commons.
3. Global average temperatures over the past 540 million years, by Glen Fergus. 2015 Creative Commons.
4. Reconstruction of A. afarensis, by Tiia Monto. 2016 Creative Commons.
5. Reconstruction of H. Erectus, image by Hay Krannen. 2019 Creative Commons.
6. Global deep sea (not surface) temperatures throughout the Cenozoic, by James Hansen, et al. 2013 Creative Commons.
7. Global temperature rise from 1880–2019, by NASA, 2019.