The Formation of Today’s Planet-Warming Coal Nearly Froze Earth Long Ago

The planet was nearly a snowball during the Carboniferous Period about 300 million years ago, when forests died and slowly transformed into coal.

Burning coal and other carbon-heavy fossil fuels is warming the Earth. But the formation of those fuels nearly left the planet covered in sheets of ice.

At least two earlier periods in the past billion years saw a global glaciation known as “Snowball Earth,” and the planet’s escape from a third was “surprisingly narrow” as planet-warming carbon dioxide levels plunged, according to a new study.

“Ironically, it’s the same carbon we now push back into the atmosphere when we burn coal,” Georg Feulner, a paleoclimatologist at Germany’s Potsdam Institute for Climate Impact Research, told Seeker. “Back then it brought us almost to global glaciation, and now it’s causing global warming.”

The bulk of today’s coal formed during what’s known as the Carboniferous Period, a 60-million-year span that ended about 300 million years ago. At that time, fossil records show swampy tropical forests covered the Earth’s two large continents. Carbon dioxide concentrations in that primeval atmosphere may have ranged as high as 1,000 parts per million — about triple what they were when scientists began regularly recording them in the 1950s.

Using reconstructions of continental drift and estimates of solar luminosity that he fed into a computer model, Feulner tried to recreate the climate in the late Carboniferous. Using data from fossil records, earlier studies estimated that CO2 levels have fallen to around 100 ppm. The massive continental forests absorbed most of the carbon — and when they died, “that dead wood was turned into coal in the Earth’s crust.”

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With less CO2 in the air, average global temperatures would have fallen. Average global temperatures would have ranged from 12 degrees C to less than 2 (54 to 36 degrees F). Rain turned to snow, and ice sheets built up on the southern continent. And the advancing ice accelerated the cooling as the ice reflected more of the sun’s energy back into space.

The Earth was likely spared from becoming a complete snowball only because the plunging temperature killed enough plants to keep CO2 levels to around 100 ppm, while the models predicted total glaciation would have occurred around 40 ppm, Feulner said.

Feulner’s findings were published Oct. 9 in the US research journal Proceedings of the National Academy of Sciences.

Coal still provides about 30 percentof the world’s power and more than 40 percent of its greenhouse gas emissions. CO2 concentrations have risen from roughly 275 parts per million at the start of the Industrial Revolution to more than 400 today. Feulner said the study highlights the importance of moving away from carbon-rich fossil fuels.

“The one thing we can learn is how climatically important those fossil fuel reserves are that are still in the ground,” Feulner said. “Over the Carboniferous, coal formation and other processes took about 900 ppm CO2 out of the atmosphere. If we burn a lot of coal, it will just add enormous amounts of CO2 to the atmosphere.”

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