This was the Great Oxygenation Event. The result was nothing short of complete mayhem. The oxygen reacted with atmospheric methane, forming carbon dioxide and water. This additional carbon dioxide was used by photosynthetic organisms to produce more oxygen. This coincided with the creation of new continental crust that underwent chemical weathering, pulling even more carbon dioxide from the air. Lastly, the sun may have entered a dim phase, shining less warming rays at the planet. Taken together, these events worked like ripping the blankets off in the middle of the night.
Most life forms that didn’t die from freezing were killed by oxygen poisoning. Those that survived adapted to tolerate or even make use of free atmospheric oxygen. For two hundred million years, this ice age, dubbed the Huronian, held the world in its icy grip. Ice-covered rock doesn’t absorb carbon dioxide, eventually causing greenhouse gasses to build up and started melting the ice sheets. After the Huronian Ice Age ended, the Earth was swelteringly hot a billion years with little or no ice even in the coldest places. The warm seas host an ever-increasing diversity of life.
More than once in history, our planet was covered in ice. It is yet to be known if the Earth was entirely frozen solid, or covered in slush instead. - Image by Julio Lacerda
The Cryogenian Ice Age
After a billion years of constant heat, around 850 million years ago, a gradual change started to occur. The sun may have entered another cool period and plate tectonics had shifted the continents around, placing them near the equator. Equatorial moisture and heat increased the rate of chemical weathering, pulling carbon dioxide out of the air.
130 million years later, the Cryogenian Ice Age was in full swing. When it got cold enough, ice sheets started forming at the poles. Bright ice reflects sunlight back into space, keeping the Earth from absorbing that heat. The bigger the ice sheets, the less heat was absorbed and the colder it got. Paleoclimatologists are divided on whether this feedback loop continued until the ice sheets reached the equator, but one thing is sure — Earth had never been this cold before or since.
The presence of the ice sheets prevented chemical weathering and allowed carbon dioxide to build up, but the cooling effect of the white ice sheets together with the dim sun kept the world locked in ice for two hundred million years. During this time, the world only warmed up for a relatively brief respite of 10 million years before succumbing to the ice once again.
Only when oxygen-producing life forms were greatly reduced in number, the atmospheric oxygen levels dropped. This allowed volcanic methane to build up alongside carbon dioxide, the sun regained its usual vigor. Eventually, the ice melted again. The Cryogenian era — the coldest period of Earth’s history — ended 630 million years ago.
Barring a brief period known as the Andean-Saharan glaciation, the world was once again hot and ice-free for 270 million years. During this time, life positively flourished. Most crucially for our story, plants developed lignin, the tough material that wood is made of, and started to grow tall. Entirely new ecosystems developed in the shade of trees.
The Karoo Ice Age
In the meantime, the supercontinent Pangaea assembled around the South Pole. This disrupted the warm-water currents in the oceans, which had a cooling effect. Great forests covered the continent, but no living thing had developed the ability to decompose wood. Dead trees fell and new trees grew on top, creating deep piles of wood that would become coal beds. This process drew carbon dioxide from the atmosphere that further cooled the planet. Carbon dioxide levels reached an all-time low. Eventually, around 360 million years ago, the world was drawn back into an ice age — the Karoo. Dragonflies the size of pigeons, millipedes longer than a grown up man’s height, and scorpions the size of a large cat; these were the creatures of the age.
Eventually, however, the ice age grew too cold even for the trees. When the forests stopped growing and ice once again covered bare rock, carbon dioxide levels stabilized. The high oxygen levels made everything highly flammable, and forest fires were common, returning some carbon dioxide into the air. Finally, termites developed a symbiotic relationship with gut bacteria that allowed them to digest lignin, marking the end of an age where forests could deposit unlimited amounts of carbon dioxide. 260 million years ago, after 100 million years of ice, the ice sheets receded once again.
At the end of the Permian Period, 70% of land-dwelling species vanished. The herbivorous Lystrosaurus somehow survived the ordeal and dominated the landscape, making up as much as 90% of the known vertebrate fauna after the extinction. - Image by Julio Lacerda
The Mesozoic Era
A mere eight million years later, 252 million years ago, disaster would strike in the form of a Siberian supervolcano. A continent was covered time and again with lava oozing from fissures, forming the enormous geological formation known as the Siberian Traps. Lava burned the coal deposits from the previous era, and the volcano itself spewed carbon dioxide and methane in record amounts. Larges swathes of the ocean became deoxygenated, and sulfate-reducing deep-ocean bacteria bloomed and spewed out poisonous gases. 90% of all life on Earth died.
This time, however, the climate did not shift. An influx of greenhouse gases doesn’t have a lasting effect if the world is already hot. Life returned with vigor and quickly developed into dinosaurs and conifers. For nearly two hundred million years, the environment stayed relatively warmer when dinosaurs roamed the Earth.
The Cenozoic Era
65 million years ago, of course, a great asteroid struck the coast of what is now Mexico. The impact threw dust into the upper atmosphere, blocking the sun and plunging the world into a decades-long cold period. When the dust cleared, the only dinosaurs that survived were the feathered theropods we call birds.
Mammals greatly diversified. While before the Cenozoic the largest mammals couldn’t surpass 15kg in weight, a few million years after the asteroid impact beasts like Uintatherium could reach two tons, about the same size as a rhino. Through this rapid evolution, the first members of modern families appeared, like rodents, ungulates and early elephant relatives.
About 49 million years ago, the world was 8°C hotter than it is now. The carbon dioxide content of the atmosphere ensured that there was no ice anywhere on Earth, and the world was as far from an ice age as possible. The Arctic Sea at the north pole was cool and land-locked. Fresh river water, warm from the sun, formed a layer above the cold, salty sea water. A freshwater fern known as Azolla had evolved to thrive in these conditions and formed a floating green mat on the surface of the Arctic Sea.
For tens of millions of years, Azolla fronds grew in the Arctic, wilted, and died. Dead fronds dropped into the dark, cold depths, where they formed the carbon-rich shales that eventually became Arctic oil deposits. In time, 80% of the atmospheric carbon dioxide content was deposited in the form dead Azolla plants at the bottom of the Arctic ocean. The change was gradual. The outcome, inevitable.
The last "Ice Age" was the colder phase in a cooling event that is still underway. This phase saw the beginning of mankind's existence, alongside megafauna such as the wooly rhino. - Image by Julio Lacerda
The Quaternary Ice Age
With carbon dioxide levels low, the Earth’s surface began losing heat. Eventually, ice caps formed at the poles. This killed off the Azolla ferns that caused the situation, but by then it was too late — the white icecaps reflected sunlight, causing additional cooling. This feedback loop continued until the Quaternary Glaciation, the current geological period, began 2.58 million years ago.
We are currently in a phase of the Quaternary known as the Holocene, an interglacial period that started 11,000 years ago. Interglacial periods come very regularly and are caused by periodic changes in the Earth’s orbit. These regular patterns are known as Milankovitch cycles.
One thing that becomes clear in a chronology like this is that every ice age has a unique beginning. Every time, a new set of circumstances combined to push the world out of balance and into an era of cold temperatures and polar ice caps. What’s interesting is that, regardless of how the ice age started, the end is invariably due to a buildup of greenhouse gases. With white ice reflecting solar heat back into space, only an atmosphere high in greenhouse gases can retain enough heat to make a difference.
While the entire existence of the human race has taken place in an ice age, the Quaternary Glaciation, this ice age is quite young. Ordinarily, one would expect the Holocene interglacial to last another few thousand years before the ice sheets return to crush and scrape the high latitudes of our globe, and then the ice age would continue for millions of years until conditions change enough for the normal greenhouse Earth to resume naturally.
Of course, these are not ordinary times. For the first time ever, life on Earth is capable of putting greenhouse gases into the atmosphere instead of taking it away. Whereas the Azolla fern and the Carboniferous forests transformed vast amounts of carbon dioxide into sediments with coal and oil, humanity can do the reverse. Previously, only volcanoes and the occasional wildfire did this.
Needless to say, humanity works quickly. It took Azolla over 40 million years to reduce carbon dioxide levels from hothouse to ice age, and the Carboniferous forests took at least as long. We’ve only been busy for two hundred years (that’s 0.0005% as long) and we’ve already reached the carbon dioxide levels present when dinosaurs roamed the Earth.
The outcome itself is not a bad thing. By all accounts, life flourishes when the Earth is hot, and a little heat is vastly preferable to kilometer-thick ice sheets.
There have been two major events in Earth’s history where CO2 levels rose very suddenly instead of gradually. One caused by a large asteroid impact, and another as consequence of the Permian-Triassic extinction event, when 90% of living things at the time perished. That puts into perspective the severity of drastic changes in our planet’s atmosphere and ecosystems.
While global warming - and for that matter, global cooling - are part of a naturally variable planet, these environmental changes usually happen in the span of millions of years. They allow many organisms to gradually adapt. When not enough time is given to Earth life, mass extinctions occur - and even though some hardy animals and plants always manage to survive at the end, ecological havoc is unavoidable.
That’s why we humans, being part of Earth and dependent on its equilibrium to thrive, need to be conscious of our own power. By not responsibly taking care of the atmosphere and the environment, we risk causing a mass extinction as severe as the many apocalypses our planet has been through. And though life will certainly survive in some way, ourselves and most of the fauna and flora we take for granted may not.