Our burning of fossil fuels is, to an extent, a reversal of a process that happened millions of years ago. At one point, all this carbon was in the air. Over millions of years, life extracted it from the air before dying and getting buried. Geology took over from there, gradually converting the formerly living material into things like coal and oil. Since this process was relatively slow, it presumably didn't result in radical changes to the climate.

But a new study suggests that it came really, really close. Lots of the fossil fuels we currently use derive from the Carboniferous, a 60-million-year-long period where forests flourished across much of the Earth. While not sudden, activity during this time period did pull a lot of carbon out of the atmosphere, and so a researcher decided to look at some of the consequences. The results suggest that the Earth skirted the edge of a global freeze, forming glaciers in the mountains of the tropics.

Burying carbon

The Devonian is the period that predated the Carboniferous, and it's notable for the first appearance of both forests and tetrapods, the four-limbed vertebrates that include us. But forests really started getting going in the Carboniferous, leaving behind massive deposits of coal. At this time, the Earth also underwent an extinction event that saw the collapse of forests and the appearance of widespread glaciers.

So Georg Feulner of the Potsdam Institute for Climate Impact Research decided to see what was happening. To do so, he built a climate model specific for the Carboniferous.

Several things about the model were distinct from something that would simulate modern conditions. To begin with, Pangea, a single global continent, formed during the Carboniferous. This had a considerable impact on which areas of the planet could support forests. The Sun was also fainter during this time period, as it has brightened slowly throughout our planet's history.

But one key climate factor stayed the same: variations in the Earth's orbit influenced how much sunlight our planet received. These cycles currently influence the onset and exit from glacial periods, and there's evidence that they've been altering the climate deep into the planet's history. But their influence also depends in part on the configuration of the continents, so Feulner used his Pangea-style climate model to determine which orbital configurations were warmest and coldest.

Studies of the atmosphere's carbon dioxide levels during this period suggest they varied between 150 and 700 parts-per-million. So after identifying an orbital configuration that led to cooling, Feulner plugged in the low carbon dioxide value. The result was a global temperature of about 1.4 degrees Celsius. For contrast, last century's average global temperature was 15.8 degrees Celsius. A global temperature of 1.4 degrees Celsius was cold enough to put glaciers on mountain ranges in the tropics of Pangea.

On the edge

How close was this to a global glaciation, known as a "snowball Earth"? Feulner found that a snowball Earth would have occurred when carbon dioxide concentrations were at 35 parts-per-million, or about a third of the measured levels at the end of the Carboniferous. But that's an extremely conservative estimate. To begin, he didn't test any concentrations between 35 and 100 ppm, so it's possible the snowball would start at somewhat higher concentrations. The error bars on the 100 ppm value are also large, meaning that 35 ppm is actually within the range of the measurement error. Finally, it's likely that large volcanic eruptions, which would cause a sudden cooling, would occur during this period, possibly tipping the Earth into a snowball state.

In other words, it's a bit surprising that the Earth didn't end up as a snowball during this time—Feulner calls its escape from this fate "surprisingly narrow." And he suggests that the two factors that prevented a deep freeze are feedbacks that help keep carbon dioxide in the atmosphere: both plant growth and chemical weathering of rock, processes that pull carbon dioxide out of the air, would have slowed down during a severe cold period.

Even though the topic is a potential climate catastrophe, there's a certain appealing symmetry to the thought that the coal that's currently helping heat our planet up almost froze it solid as it formed.

PNAS, 2017. DOI: 10.1073/pnas.1712062114 (About DOIs).