Coal is associated with being partly responsible for the warming of the Earth. But 300 million years ago, the formation of the combustible rock had the exact opposite effect, nearly causing the entire planet to be covered in ice.
A study published in the journal Proceedings of the National Academy of Sciences outlines this ancient phenomenon for the first time.
When trees in vast forests died during the Carboniferous and Permian periods – stretching from around 350 to 250 million years ago – the carbon dioxide (CO<sub>2) they absorbed from the atmosphere while they were living was buried. The plants' debris, over time, formed most of the coal that is burnt as fuel today.
The effect of all this CO<sub>2 being taken from the atmosphere and absorbed into the soil meant that the Earth cooled to such an extent that it only narrowly escaped what scientists call a 'snowball state' – where the entire planet's surface is completely frozen, an event that is thought to have occurred at least once.
"It is quite an irony that forming the coal that today is a major factor for dangerous global warming once almost lead to global glaciation," said Georg Feulner, author of the study from the Potsdam Institute for Climate Impact Research.
"However, this illustrates the enormous dimension of the coal issue. The amount of CO2 stored in Earth's coal reserves was once big enough to push our climate out of balance. When released by burning the coal, the CO<sub>2 is again destabilizing the Earth system."
The researchers used computer simulations to model the climate in Earth's deep past, revealing the significant influence of CO<sub>2 concentrations. The estimates from these models show that CO<sub>2 concentrations fluctuated widely but at some points sank to about 100 parts per million of all gases in the atmosphere, and probably even lower. Global glaciation occurs below 40 parts per million.
To put that into perspective, today carbon dioxide levels in the atmosphere have reached more than 400 parts per million, contributing to global warming.
"We should definitely keep CO2 levels in the atmosphere below 450 parts per million to keep our climate stable, and ideally much lower than that. Raising the amount of greenhouse gases beyond that limit means pushing ourselves out of the safe operating space of Earth," said Feulner.
"Earth's past teaches us that periods of rapid warming were often associated with mass extinction events. This shows that a stable climate is something to appreciate and protect."