This is Greenland, but I imagine it’s also what most of the planet looked like 300 million years ago. Image: NASA Earth Observatory

Coal, that ancient dead plant stuff responsible for electrifying the world and blackening the lungs of millions, is basically synonymous with global warming these days. But when all that plant corpse concentrate was first locked away in the ground, it took a lot of carbon out of the air. A new climate modeling study posits that when this happened about 300 million years ago, it wreaked havoc on Earth’s climate, pushing our planet to the brink of global glaciation.

Yes, you read that correctly—coal might have played a role in one of the most brutal cold snaps in Earth’s history. And that has some unsettling implications when considering our current epoch of coal-fueled global warming.

This story starts way back in the Neoproterozoic nearly a billion years ago, when Earth’s land surface was still mostly barren. At that time, the planet’s climate was prone to dramatic ups and downs, jumping from temperate to hell frozen over a handful of times during the aptly-named Cryogenian period 720 to 635 million years ago. Earth would have looked unimaginably different during the so-called “snowball” episodes of the Cryogenian, with glaciers extending all the way to the equator and ice covering some or all of the oceans.

Conventional wisdom holds that the Earth stopped snowballing at the end of the Cryogenian. But there’s growing evidence that at least one more recent chapter in Earth’s history was marked by eternal winter—the transition from the Carboniferous to the Permian period 300 million years ago. Sediment and fossil records from this time suggest that atmospheric CO2 levels took a nosedive, falling as low as 100 plus or minus 80 parts per million. (Our atmosphere is roughly 400 ppm CO2 today, with 120 ppm coming from human activity.)

To figure out what that the carbon crash meant for Earth’s appearance, Georg Feulner, a paleoclimatologist from the Potsdam Institute for Climate Impact Research in Germany, ran state of the art climate models, tuned to several different orbital configurations and with different levels of CO2 in the atmosphere. His models show that the threshold for global glaciation would have been about 40 ppm CO2—meaning Earth either snowballed at the dawn of the Permian, or narrowly avoided doing so.

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It’s hard to know for sure because of the uncertainty surrounding levels of CO2 in the atmosphere so many hundreds of millions of years ago. But Fuelner himself admits that the 40 ppm threshold might be conservative, because his models don’t take into account the cooling effects of volcanic eruptions, and other atmospheric processes.

A reasonable person might ask why we care if Earth looked like Hoth hundreds of millions of years ago. The reason has to do with the circumstances leading up to the CO2 crash: specifically, the death and burial of vast amounts of vegetation toward the end of the Carboniferous. That buried carbon would go on to form the majority of the coal reserves responsible for humanity’s industrial revolution— for electricity, iPads, Teslas, and transcontinental flights.

And, of course, all of that coal would go on to make a major contribution to the greenhouse gases currently pouring into our atmosphere and causing global temperatures to rise. “While not directly related to global warming, the findings show the climatic significance of the carbon stored in fossil fuels—and in particular coal,” Fuelner told Earther.

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Indeed, the same fossil plant material whose formation caused Earth’s temperature to plummet is now being burnt through thousands of times faster by power-hungry humans. It’s both impressive and terrifying to contemplate.

“In a minute fraction of the time it took [coal reserves] to form, what humans have been able to do is staggering,” Jeremy Hoffman, a climate and Earth scientist at the Science Museum of Virginia, told Earther. He added that while he doesn’t think the study has direct implications for our future, “it colors our understanding of the rapidity by which humans are changing the atmosphere.”

It also reminds us that changes to the greenhouse gas mix in our atmosphere can profoundly alter Earth’s habitability. If you start hearing climate skeptics argue that coal pushed the Earth into an ice age long ago, be sure to remind them of that.