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50 million years ago, there were no glaciers anywhere on Earth.
The planet was too hot for them.
This is the way our Earth usually is. It’s not normally a world of ice.
The Sun burns too close. Too much of its light, its radiation, surges through this part of the solar system.
It takes a lot for Earth to cool down so much that it is safe for ice. It’s unlikely, so it’s rare.
But we live in an ice age. A brief chapter in Earth’s history when glaciers stretch across the poles.
Our ice age will endure for a while longer. For the rest of your life, and the lives of your children, and their children.
The world is getting hotter. The ice is melting.
Someday, our pollution could cause Earth to revert to its normal state. Glaciers would melt – all of them. The oceans would rise, and our cities would drown.
But for now, in this moment, the Earth has rarely been as cold.
A good thing, for we are children of the ice.
Welcome back to the first season of the Climate Chronicles.
In our last episode, we saw how humans have rearranged the puzzle pieces of our planet, and we asked whether a similar scrambling had happened before.
We learned how scientists identify these puzzle reshufflings in the distant past, and we explored a fun but farfetched possibility: that intelligent, industrialized creatures triggered one of the most abrupt climate changes in Earth’s history, some 56 million years before our species showed up.
In this episode, we’ll consider a much longer and, for us, more significant climate change that unfolded across some 45 million years. I’ll tell you why scientists think it happened, and I’ll show you how it eventually triggered something truly strange in Earth’s history: an ice age to which we might owe our very existence.
I’ll also raise another possibility – one that’s even more interesting, and much more plausible, than the Silurian Hypothesis.
Could it be that a world that is habitable is inherently a world that can remake itself? A world that is uniquely capable of altering its own climate?
If so, the Earth doesn’t just record its past – it makes that past on a grand scale. And before humans showed up, it made itself into a world of ice.
That’s why global warming is so dangerous today.
It’s not that it threatens the Earth. It’s that it imperils the ice our world has created over 45 million years. And most of Earth’s living things – us included – have evolved to depend on the ice.
It remains to be seen whether we could live with much less of it.
As I mentioned in our last episode, an ice age is often confused with the great glaciations of the Pleistocene epoch. Remember, that’s the epoch that began about two and a half million years ago, and ended around 11,700 years ago.
We’ll consider those glaciations in our next episode.
What I want to emphasize now is that the term “ice age” really refers to a time when it’s cold enough for glaciers to cover a lot of land. When glaciers get really big – when they spread across at least 50,000 square kilometers, or a little more than the size of Switzerland – we call them ice sheets.
Today, ice sheets cover nearly 15 million square kilometers across Antarctica and Greenland. At their thickest they are far taller than any skyscraper, and they hold about 99% of the world’s freshwater.
Over 8 trillion tons of ice have melted from these ice sheets over the past two decades. That’s about the weight of 32 million skyscrapers… enough to fill 40 New York Cities.
But there’s so much ice that the biggest ice sheet, covering eastern Antarctica, would take thousands of years to melt even if we keep burning coal, oil, and natural gas at our current pace.
It’s very unusual to live in an ice age. For about 90% of Earth’s history, there were no ice sheets anywhere on our planet.
Climatologists contrast this common, “greenhouse” state of the world with the rare icehouse in which we currently live.
It’s possible to get a bit more specific.
A recent study distinguished between “coldhouse” conditions, in which the Earth’s average global temperature is between 11 to 18 °C, “coolhouse” conditions, when average temperatures range between 18 and 22 °C, transitional states, where temperatures are between 22 to 25 °C, warmhouse conditions, where they’re between 25 and 28 °C, and finally hothouse conditions, where average temperatures are somewhere between 28 and 35 °C. Earth has repeatedly switched between these houses.
The Earth’s average global temperature is around 16 °C today, which puts it in the upper range of the coldhouse category. This planet rarely gets chillier.
But if global warming continues at its current pace, we may begin to tip the Earth into a coolhouse by the end of this century. Our planet will still be cold, relative to its usual average temperature, but it will also be hotter than it has been for millions of years.
Actually, you can think of global warming as a planetary time machine. With every degree we warm our planet, we recover a little more of the climate of the past. That’s a little simplistic, because the Earth has warmed and cooled for billions of years.
But beginning around 50 million years ago, it began to get chillier. And that’s why our warming of the planet threatens to restore a climate that predates humanity and the ecosystems on which we depend.
A climate that may favor another kind of life entirely.
Earth’s average global temperature was around 25 °C on the eve of the biggest collision since the end of the dinosaurs.
The surface or “crust” of our planet is divided into pieces, called tectonic plates, that drift on a vast zone called the mantle, where rock is so hot that it behaves a little like toothpaste.
About 45 million years ago, the tectonic plate that carries India, cruising north at a speed of around 20 centimeters per year, smashed like a wayward oil tanker into the much bigger Eurasian plate.
The Indian Plate pushed under the Eurasian Plate, its crust slowly disappearing into the mantle, while the Eurasian Plate bent upwards, forming the Himalayan Mountains.
The newly formed mountains blocked moist air blowing in from the Indian Ocean. As the air rose against the mountains it cooled down, condensing into clouds and then falling as heavy rain on the southern slopes of the Himalayas.
Now, rainwater mingles with carbon dioxide from the atmosphere, making it ever so slightly acidic. It also breaks down rocks, dissolving their minerals into its acidic water.
Splashing down the Himalayas, rainwater now picked up calcium and other minerals that had been trapped beneath Earth’s crust.
These minerals formed new chemicals by reacting with the carbon dioxide that the rain had pulled from the air. The chemicals, with their attached carbon dioxide atoms, eventually washed into the ocean.
This process is called “weathering,” and it’s why the amount of carbon dioxide in the atmosphere began to fall as the Himalayas started to rise.
High levels of carbon dioxide had been primarily responsible for the Earth’s high temperature, because of course carbon dioxide is a greenhouse gas. The less of it there was, the colder Earth got.
As these massive changes unfolded a little north of the equator, the Antarctic Plate was moving south. Soon, it began to break up the remnants of a supercontinent named Gondwana.
By about 55 million years ago, Antarctica had fully separated from what is now Australia. By about 40 million years ago, it started to break free from South America.
As Antarctica grew more isolated and drifted towards the south pole, the circulation of ocean water in the southern hemisphere rearranged itself. A cold current began to flow around Antarctica, isolating it from warm water.
By about 34 million years ago, the concentration of carbon dioxide in Earth’s atmosphere had fallen from about 1,000 PPM to around 600 PPM. By comparison, it’s at roughly 420 PPM as I’m recording this episode.
With carbon dioxide levels and in turn global temperatures slowly falling, snow that fell in the highlands of East Antarctica during the winter no longer completely melted in the summer. It piled up, year by year, getting higher and heaver and denser until it turned into glaciers that quickly grew into ice sheets.
This was the start of what’s called the Cenozoic Ice Age – an ice age we’re still in today.
The birth and growth of Antarctic ice sheets unfolded through self-reinforcing processes, or what climatologists call positive feedbacks.
We’ve all encountered such feedbacks before. If you’ve heard someone talk through a loudspeaker, for example, you’ve probably experienced a positive feedback.
Noise from a loudspeaker can come back through the microphone, which increases the noise from the speaker, which then goes back into the microphone, and so on.
That’s how a whisper someone speaks into a microphone can become an ear-splitting shriek.
The most remarkable and troubling thing about Earth’s climate is that it’s full of positive feedbacks.
Take the glaciation of Antarctica. Expanding ice sheets covered land, and eventually water.
Both land and water are usually dark. When something’s dark, it absorbs a lot of solar radiation, and that makes it hot. It’s why it’s usually not a good idea to wear black in the summer.
Ice, of course, is very bright, meaning that it reflects a lot of solar radiation. So does the white shirt that’ll keep you cool in the summer. It doesn’t heat up easily.
This is why the more ice there was, the colder Antarctica got, and the colder Antarctica got, the more ice there was. This is called the ice-albedo feedback, with albedo meaning the proportion of radiation reflected by a surface. It’s a classic example of a positive feedback.
Today, it’s working in reverse. The warmer the Arctic and Antarctic are, the less ice there is, the more dark water and land is exposed, and the hotter the poles become.
Millions of years ago, the expansion of Antarctic ice sheets and then sea ice created a greater contrast between the temperature of the equator and that of the south pole. This temperature contrast powered winds that, by about 11 million years ago, strengthened the cold current that had formed around Antarctica.
Here was another positive feedback. The supercharged current, known as the Antarctic Circumpolar Current, further isolated Antarctica from warm water. The continent’s temperature continued to fall, and its ice sheets kept growing.
But the consequences were much more profound than that. The stronger current pulled up frigid water from the deep ocean, lowering global sea surface temperatures. It also strengthened the global circulation of water through the world’s oceans.
You can see the same effect in a bathtub. If you swirl your hand, making a whirlpool, the rest of the water in the tub starts to move.
The stronger flow of water through the world’s oceans now pulled up more nutrients from the seabed, and this “upwelling,” as it’s called, sustained the growth of more plankton.
Here was yet another positive feedback. Plankton absorbs carbon dioxide. When plankton dies, it drifts to the ocean floor, and its carbon dioxide is trapped in ocean sediments. What’s more, cold water can absorb carbon dioxide more easily than warm water.
This is how positive feedbacks can work together to increase the magnitude and duration of climate change on Earth.
As Antarctica iced over, the concentration of carbon dioxide in Earth’s atmosphere kept falling, and the positive feedbacks responsible for the fall kept getting stronger.
Then, just under three million years ago, the relatively small Coco and Nazca tectonic plates collided with and sunk under the Caribbean plate, sparking volcanic eruptions that eventually helped create the isthmus of Panama.
This new isthmus connected North and South America, severing a watery link between the Atlantic and Pacific Oceans.
Warm, salty water now flowed northward more efficiently across the Atlantic Ocean. It travelled in a huge watery pump known as the Atlantic Meridional Overturning Circulation, or AMOC.
By now, the long decline of atmospheric carbon dioxide concentrations had also led to the emergence of ice sheets in the Arctic, the far north. Ironically, the arrival of more warm, salty water in the northern Atlantic fueled their growth.
Warm water led to more evaporation, and more precipitation in the North Atlantic. Heavy snowfall nourished the ice sheets, strengthening the ice-albedo feedback in the northern hemisphere.
By about 2.6 million years ago, at the start of our current geological period, the Quaternary, and epoch, the Pleistocene, a greenhouse Earth had transformed itself into an icehouse.
In fact, if we ignore abrupt but short-lived climatic shocks, like the one caused by the asteroid that helped wipe out the dinosaurs, then the Earth at the start of the Quaternary was colder than it had been for 260 million years!
Climatologists have a term for the factors that cause changes in Earth’s climate. They call them “forcings,” which is one of those words that’s both awkward to use in a sentence and perfectly clear.
It’s worth reflecting on two paradoxical things about the forcings that pushed our Earth into a totally new, bitterly cold state.
First, think of how much had to happen exactly as it did for the Earth to cool down as much as it did. It’s easy to see why ice ages are rare in our planet’s history.
But second, think of how much was set in motion by the first big change: the mega-collision between the Indian and Eurasian Plates.
That is the wonder and the horror of positive feedback. If you give Earth’s climate a big enough nudge, the dominoes can start to fall, one after another until a new planet is born.
That’s exactly what we’re doing as you read these words.
Now, the genes in living things are always changing, always mutating. That’s partly because random errors pop up when DNA copies, or “replicates,” itself. It’s like a copier smudging the occasional letter.
Every now and then, a random mutation results in a new trait that helps an organism survive in its environment. Organisms with these traits tend to have more offspring, and over time, the traits can spread through a population, becoming part of a species.
This process is called “natural selection.” It’s the scientific term for how individuals with traits better suited to their environment are more likely to survive and reproduce. Over many generations, if enough changes accumulate, natural selection can even lead to entirely new species.
It’s a common misconception that evolution is about constant improvement, leading inevitably to world-conquering, intelligent life. This misunderstanding has led some to think evolution was guided by a purpose, such as creating humans.
In reality, evolution works through random mutations in DNA, with natural selection favoring traits that happen to work well in a particular environment. Over time, this process helps living things adapt to their surroundings—but it’s not directed or goal-oriented.
All the same, when surroundings change, natural selection will allow many living things to accommodate themselves to new environmental conditions. Earth’s web of life – its biosphere – turns out to be remarkably flexible to environmental change. It has to be, of course, or it would no longer be around.
That doesn’t mean it’s impervious to harm. There have been five catastrophic mass extinctions in Earth’s history, when three quarters of all species went extinct, and it appears that humanity may be triggering the sixth.
But although there were many extinctions in the Pleistocene, the biosphere didn’t suffer nearly as much damage as it did in, for example, during the fifth mass extinction, when an asteroid wiped out the dinosaurs. On the whole, life on our planet transformed itself to cope with the multi-million-year cooling trend that began with the rise of the Himalayas.
Some microbes, for example, developed enzymes and membranes that were specialized for the cold, allowing them to thrive in frozen soil. Others banded together to form biofilms: weblike structures that, among other things, helped them retain heat and moisture.
Mosses and lichens evolved to use chemicals that work like antifreeze. Many plants accelerated their growth cycles, allowing them to reproduce in short summers, while others evolved the ability to remain dormant for long stretches of wintry weather.
Animals adopted more and more extreme hibernation behaviors. Large mammals evolved thick layers of fat and fur, or smaller, shorter extremities that slowed heat loss.
Some animals developed white fur to blend in with snow and ice, while others learned to escape deadly winters by migrating every year to warmer climates. Big herbivores specialized to feed on the grasses that covered expanding tundra environments.
Across much of the world, organisms had to adapt not only to cooling, but also to drying. You see, an icehouse Earth is a planet of deserts and dust.
Part of the reason is that expanding ice sheets led to much, much lower sea levels. This is because the water in ice sheets had evaporated from the oceans. While it was trapped in the ice, it couldn’t return to the oceans. So, the bigger the ice sheets, the lower the oceans.
There was less evaporation on an icehouse Earth, and the chillier atmosphere could hold less water. It’s a reversal from today’s trends, when more evaporation pumps more water into our increasingly hot atmosphere, which then falls in torrential rains.
As the climate cooled millions of years ago, the cold, stable air that lingered over ice sheets also rerouted prevailing winds, blocking the precipitation they carried from the oceans. Retreating forests and expanding tundra, meanwhile, led to less moisture on the ground and in the atmosphere.
An icehouse world was a planet with huge temperature differences between the equator and poles. These temperature differences played a key role in strengthening the flow of air across the Earth, or what scientists call atmospheric circulation.
Powerful winds lifted vast quantities of dust from growing deserts, especially around the barren edges of ice sheets.
So, over the course of about 40 million years, Earth had turned into a frigid and a dry, dusty world: a planet totally different from the one it had been before India collided with Eurasia.
Some creatures adapted to this dried-up Earth in truly unique ways.
Around seven million years ago, for example, in regions across Africa where lush forests were drying into open savannah, primates known as hominids began to evolve into a new family of animals.
Animals in this new family, confusingly referred to as hominiNS, adapted to their drying home by developing the muscles, the bones, and the behaviors that allowed them to walk on two legs.
This bipedalism was a logical adaptation to a drying environment. There are big advantages to walking on two legs in the savannah.
To acquire food and water, animals had to travel longer distances across flat, open terrain. Walking on two legs was slower, but much more energy efficient than walking on four.
What’s more, bipedal animals were less exposed to the Sun, a useful thing in dried-up, open environments that could be hot even in an icehouse Earth. Taller animals could also feel cooling breezes that were a little above the surface. To understand the advantage, just try lying down on a warm day and see how much hotter you feel.
Maybe bipedal hominins could more easily reach low-hanging fruit, and more easily notice predators from afar. As you can imagine, it’s impossible to know all the causes behind the evolution of bipedalism in hominins, much less to rank them by order of importance. But we can be quite certain that climate change played a key role.
Bipedalism had one more advantage – an advantage that arguably overwhelmed the others. It allowed hominins to more easily make and use tools.
Now, it took millions of years before hominins were more than occasional tool users. At first, individual hominins only used bits of stone to pummel and cut. Tool use wasn’t really universal or learned behavior that was passed on between generations. Think of how otters can use rocks to break open shellfish: that’s an example of occasional tool use.
But by around two million years ago, hominins had undergone a series of anatomical changes, especially to their shoulders, that allowed them to throw hard and accurately. Now they could strike animals from a distance, and that helped them hunt more efficiently. Hominin diets began to incorporate more protein and fats.
Between two and one and a half million years ago, hominin bodies also evolved in other ways. Hominins lost some of their body fur and added sweat glands. They developed long legs, narrow waists, and arched feet to provide extra bounce. They evolved neck ligaments to keep big heads steady, shoulders that let arms swing, and knees and ankles that absorbed shock. All adaptations for walking and running long distances on two legs.
By around 1.5 million years ago, hominins had transformed from a little prey species, swinging through the trees, to a big predator, roving across open land. They were omnivorous, eating roots, fruits, or seeds. But on two legs, they could also pursue any animal through the dry Savannah until it overheated and died.
Now that they could cover more ground and hunt efficiently, hominin communities could be bigger. Larger social groups meant more socializing, and socializing rewarded intelligence. More meat gave fuel for bigger and better brains, which in turn permitted more social coordination, and better hunting.
Tool use had now become habitual among hominins, meaning that it was a widespread, learned behavior. Using flint and other sharp stones, hominin communities learned how to craft and refine simple hand axes. Although they did not really depend on these tools for their survival, the use of hand axes had become part of their behavior and culture.
It required a good brain to use tools, and individuals with better memory, spatial reasoning, and motor coordination could use tools more effectively. They were more likely to reproduce, to transmit their genes to the next generation.
Passing along the knowledge of how to use tools also required teaching, learning, and culture, and so it further encouraged social communication and collaboration. All these intellectual tools demanded better and bigger brains, which in turn encouraged more and more diverse physical tool use.
So, you see, evolution can be a little like climate change. It can take self-reinforcing processes – it can take feedbacks – for both an organism and a planet to transform itself. And often it begins with one big nudge.
These feedbacks can be connected. You may have already guessed that our species, Homo sapiens, was among the last hominin species to evolve. Our emergence was a consequence of the climatic feedbacks that fueled the cooling of our planet. And, in a roundabout way, so is the global warming we’ve caused.
Forcing and feedback. These are the mechanisms by which our Earth can change into a different planet. It’s how our climate can go from a superheated greenhouse to a frigid icehouse. It’s why the web of life remakes itself.
Over the past 45 million years or so, the most powerful engine of climate change has been tectonic. It’s the pushing and pulling of the enormous plates that constitute Earth’s crust.
This tectonic movement seems to make Earth unique in the solar system. It may also be critical for the emergence and survival of life as we know it.
Plate tectonics recycles essential nutrients, such as phosphorous and potassium, in the world’s soil and ocean. It creates diverse habitats, and the gasses it releases may be responsible for our atmosphere.
Over very long timeframes, it seems to limit how much Earth’s climate can change by continually releasing and entrapping carbon dioxide.
The tectonic engine therefore causes Earth’s puzzle pieces to shuffle themselves, but it forces them into the same recurring patterns that are all conducive to life.
Of course, different arrangements stimulate the emergence of different kinds of life. And the coldhouse or icehouse arrangement happened to spur the evolution of big-brained primates, like us.
On Earth, and perhaps on all planets hospitable to life, the emergence of intelligence seems to have been a response to climate change.
But it may be an evolutionary dead end. We are setting in motion feedbacks, vicious cycles that, in overheating our world, are destroying the climate that created us.
We are children of the ice. But we are beginning to kill our parent.
We know we are. But that isn’t stopping us.
And if we continue, parent and child may die together.
For Teachers and Students
Review Questions:
- What are forcings and feedbacks?
- Why did the world begin to cool down, beginning about 45 million years ago?
- What are hominids? How about hominins?
- Did climate change influence hominid and hominin evolution? If so, how?
Key Publications:
Judd, Emily J., Jessica E. Tierney, Daniel J. Lunt, Isabel P. Montañez, Brian T. Huber, Scott L. Wing, and Paul J. Valdes. “A 485-million-year history of Earth’s surface temperature.” Science 385:6715 (2024).
McNeill, John R. The Webs of Humankind: A World History, Second Edition. New York: Norton, 2024.
Venditti, Chris, Joanna Baker, and Robert A. Barton. “Co-evolutionary dynamics of mammalian brain and body size.” Nature Ecology & Evolution 8, no. 8 (2024): 1534-1542.
Woodward, James. The Ice Age: A Very Short Introduction Oxford: Oxford University Press, 2014
Video and Audio Credits:
Scotese, C.R., 2019. Plate Tectonics, Paleogeography, and Ice Ages, YouTube video.
Audio Tools: AIVA, Runway.
Video Tools: Runway, Sora.
Funding provided by Georgetown University’s Earth Commons.
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