Here’s where Earth stores its carbon

Asteroid impacts, giant lava outflows and now humans have released enormous amounts of carbon.

Human-driven carbon pollution is wreaking havoc on the global climate, from bleaching tropical corals to melting polar ice caps. But the amount of carbon in Earth’s oceans and atmosphere barely scratches the surface of the planet’s vast carbon reservoirs.

Over the last decade, researchers affiliated with the international Deep Carbon Observatory have taken inventory of where Earth keeps its carbon, and how carbon cycles throughout the planet. Although Earth’s carbon cycle has generally kept all but the tiniest bit of carbon stashed underground, asteroid impacts and massive volcanic eruptions have occasionally released catastrophic amounts of carbon into the atmosphere.

Investigating these historic upsets, outlined in a series of papers published in October in Elements, may lend insight into the consequences of rampant carbon pollution today.

About 43,500 billion metric tons of carbon is found aboveground — peanuts, compared with the 1.845 billion billion tons stockpiled in Earth’s mantle and crust. Estimates for the carbon content of Earth’s core are murky, but “core carbon is pretty locked up,” says Deep Carbon Observatory geologist Celina Suarez of the University of Arkansas in Fayetteville. Mantle carbon, on the other hand, continually escapes through volcanoes and mid-ocean ridges, and sinks back down with subducting tectonic plates. 

Where Earth’s carbon is found

The vast majority of Earth’s carbon is stored inside the planet, with a whopping 1.845 billion billion metric tons in the mantle and crust, and a meager 43,500 billion tons above the surface.

Typically, “what [carbon] comes out goes back in,” Suarez says. But analyses of carbon in rock from different times in Earth’s history have revealed events that severely upended Earth’s balanced carbon budget. Among these cataclysms was the Chicxulub asteroid strike thought to have wiped out the dinosaurs about 66 million years ago. The impact vaporized carbon-rich rock, releasing hundreds of billions of tons of carbon dioxide into the atmosphere (SN: 11/2/17).

Where carbon above Earth’s surface is found

All the carbon found aboveground, including in life-forms (the terrestrial biosphere), oceans and the atmosphere, tallies up to about 43,500 billion metric tons.

Other disasters include a handful of enormous magma eruptions called large igneous provinces, which each covered up to a million square kilometers. Such widespread lava flows, which could have released a few billion tons of carbon each year as they erupted, may have contributed to mass die-offs like the Permian-Triassic extinction event 252 million years ago (SN: 5/6/11).

Today, people flood the air with carbon at an even higher rate of about 10 billion tons per year. That’s around 100 times the current emissions of all of Earth’s volcanic regions, from volcanic eruptions as well as carbon passively leaking from soil, lakes and other sources, says Tobias Fischer, a Deep Carbon Observatory volcanologist and geochemist at the University of New Mexico in Albuquerque.

We’ve already seen far-reaching consequences of rampant human carbon emissions (SN: 9/25/19). But studying calamitous carbon releases throughout Earth’s history may help us anticipate how runaway carbon pollution plays out in the long run, Suarez says.

Carbon comparison

Earth’s history is punctuated by asteroid impacts, such as the dino-dooming Chicxulub impact, and catastrophic lava outflows that have released enormous amounts of carbon into the atmosphere, throwing the climate out of whack. Human-driven emissions are currently pumping the atmosphere full of carbon faster than past cataclysmic lava outflows called large igneous provinces, and far outpace modern Earth’s natural release of carbon from processes like volcanic activity.