U.S. Geological Survey

There's a lot more carbon stored inside the Earth, where it benignly remains for millions of years, than there is overloading the atmosphere. But because of the discrepancy in immediate human impact, all we ever hear about are the deadly consequences of the stuff above ground as it accumulates in carbon dioxide molecules and amplifies climate change. With this concentration spiraling higher and higher, it’s becoming increasingly clear that mitigating carbon dioxide will not be enough to stave off the impacts of global warming. We’re going to have to get some of the stuff out of the air—push, pull, shove, shoot, bury, or however we can get it done.

A growing body of research sheds light on some of these subterranean options and how they could one day—possibly in the near future—help save humanity from the worst of the self-inflicted environmental consequences associated with a warming planet. To understand the science behind these approaches it’s important to grasp the process underlying them: the deep carbon cycle. And to understand the deep carbon cycle, it helps to remember that “life as we know it depends on essentially carbon-based molecules to make everything,” as George E. Harlow, a geologist at the American Museum of Natural History in Manhattan and an expert on geological processes, told me.

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Life came to depend on carbon, Harlow explained, because of the molecule’s extreme chemical diversity: It can bond and form compounds with almost every element on the periodic table. Carbon helps form coal. It helps form diamonds. It helps form me and you and all of our pets. And, of course, it’s one of two elements—the other being oxygen—that make up carbon dioxide.

The difference between the deep carbon cycle and the atmospheric carbon cycle—in which plants photosynthesize carbon dioxide that was released into the atmosphere by animal respiration, decaying organic matter, and the burning of fossil fuels in the last couple hundred years—is a bit like the difference between Harlow’s office and the rest of the natural history museum. The museum itself, teeming with visitors and hyperactive energy, is all atmosphere, fast-moving and changing. Meanwhile, behind a few walls there’s Harlow and other other staff, slowly churning away without getting much attention, but responsible for most of the big changes taking place, such as new exhibits and discoveries.

Understanding How Carbon Comes and Goes

Carbon and other elements are pulled down deep into the planet through subduction zones, the part of plate tectonics where two plates collide and one bends and slides underneath the other. As part of this undertaking, all the organic “crud” that has compiled on the ocean floors is pulled under the sea.

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But all carbon is not lost. In fact, most of it eventually makes its way back to the Earth’s surface when it’s blasted from volcanoes. So while filth-spewing volcanoes are often associated with releasing ashy particles that block the sun and cool the planet, they also emit huge doses of carbon dioxide that warms the planet in the long run.

The tectonic plates were mapped in the second half of the 20th century.
wikipedia

While most of the carbon dioxide returning from the depths of the planet re-emerges through volcanoes, an unknown quantity works its way back up through the hydrological cycle—the movement of water around the planet—and along the fault lines. According to one recent study, this “tectonic gassing” of carbon dioxide across the seams of the planet, which has gone mostly unmeasured, may be more significant than was previously assumed—although still minuscule when compared to human-driven fossil fuel emissions.

Tobias Fischer, a geochemist at the University of New Mexico working to better quantify emissions of carbon dioxide from the Earth’s interior and contributor to the study, told me that they were “quite surprised” to find carbon dioxide emissions seeping through soil that was located far away from volcanoes and other earthly springs. He said this finding in turn led them to notice that whenever they crossed a fault line, they would see a spike in carbon dioxide emissions.

When they extrapolated the data they gathered at the East African Rift, the world’s largest active continental rift, they found that all these fault lines could emit about 70 megatons of carbon dioxide every year. This is significant in terms of the entire natural carbon cycle in which the Earth releases around 540 megatons of carbon dioxide per year. It is not significant when compared to human emissions, which of late have reached about 40 gigatons a year. So Earth is releasing somewhere around 1.5%-2% of the carbon dioxide per year that humankind is generating.

Fischer said it’s hard to determine this ratio precisely because “the atmospheric background keeps going up and up,” meaning human emissions increasingly dwarf those of the natural world, including volcanic emissions.

“Now there is just so much anthropogenic carbon dioxide that the volcanic eruptions don’t make a difference,” said Fischer.

Mount Sinabung in Indonesia erupting in 2015.

In Too Deep

In Harlow’s opinion, the situation is bad enough above ground that “we should be trying everything” to get carbon dioxide out of the atmosphere, whether than means storing it underground, in plants, or some other form of carbon sink.

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“We should not necessarily be going into production, but we should be investigating all of the options,” he said. “We should be looking a various nuclear options, hiding carbon dioxide, pumping carbon dioxide underground along with water, getting rid of some carbon dioxide.”

Harlow suggested the oil industry could pitch in more, saying they “already make big claims about sequestration because in order to get oil out of the ground they have to pump something back in.”

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When fossil fuel companies extract fossil fuels, they create large, empty underground repositories that could be ideal for storing carbon dioxide. However, to date, while lots of time and money has been invested in making it attractive to companies to do just this, high construction costs and fluctuating energy prices have proven inhibiting.

Furthermore, what Harlow was referring to is something known as Enhanced Oil Recovery (EOR), a technique that allows increased recovery of oil in depleted or tough to access oil fields. In the case of carbon dioxide, the gas is flooded into oil fields through injection wells to enhance flow. While this approach has been used by oil companies for years, only in the last decade or so has CCS been seriously considered.

According to the U.S. Department of Energy, "carbon dioxide flooding has the potential to not only increase the yield of depleted or high viscosity fields but also to sequester carbon dioxide that would normally be released to the atmosphere."

Carbon sequestration is possible in many places, including the ground, plants, and the ocean.
Credit: D. Curtis.

Even if this sequestered carbon dioxide doesn’t enter the deep carbon cycle and remain underground for millions of years, it should still do the trick.

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“All it has to do is stay there for 100,000 years, not forever,” said Harlow. “Just long enough that we fix what we’ve done wrong.”

If it sneaks out after that, the natural system should be able to take care of it. Vast forests of photosynthesizing trees, for example, which humans continue to clearcut at rapid rates, act as natural carbon sinks (assuming they still exist in 100,000 years).

The oceans absorb a lot of carbon dioxide in the short term, but this leads to ocean acidification, sometimes referred to as climate change’s “evil twin” due to the nasty side effects on marine life. And with enough acidification, the ocean will become saturated and lose its ability to take in more carbon dioxide.

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Fischer said what would need to happen in order for the ocean to absorb more carbon dioxide would be for the surface water to migrate down into the deep ocean and allow for new water to rise to the surface, where it can absorb gases. This is part of the carbon cycle, but does not appear to occur rapidly enough to slow human-driven climate change.

“There aren’t many options for taking carbon dioxide out of the atmosphere,” said Fischer. “If we want to reduce the amount of carbon dioxide in the atmosphere, that’s what we have to do.”

Meaning we can no longer rely on nature to clean up our mess.

Not So Rock Solid

Harlow said that most of the carbon on the Earth’s surface can be found in carbonate minerals found in rocks such as limestone and dolomite, calling this “nature’s form of sequestration.”

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He suggested that potentially “running nature on fast-forward” could be a way of sequestering carbon. Certain natural springs have been observed to transport carbon molecules underground via weatherization and the accumulation of acidic rain, and it might be possible to artificially speed up this process.

For instance, in Oman a portion of the Earth’s mantle has jutted up through the crust. Researchers at Columbia have determined that these rocks react rapidly with the carbon dioxide abundant in air and groundwater, but which is unavailable in the deep earth. These carbon-starved rocks remove carbon dioxide from the air at rates that could prove helpful in reducing atmospheric concentrations in the near future.

Dolomite rock.
wikipedia

Peter Kelemen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory and leader of this research, told me that “rocks below the crust, from the mantle, are far from equilibrium with the atmosphere and the oceans, and thus they react very rapidly to take up carbon dioxide and form solid carbonate minerals.”

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According to Kelemen, while using mantle rock to take up carbon dioxide has been proposed before, taking advantage of spontaneous natural reactions could save money and energy.

“In the simplest of our proposed methods, one would drill holes to circulate seawater through mantle rocks near the seafloor, extracting dissolved carbon dioxide, and returning carbon-depleted water to the surface where it will draw down carbon dioxide from the atmosphere,” he said. “Circulation of water could be driven mostly by thermal convection, taking advantage of the fact that rocks get hotter with depth in the Earth.”

Also of great significance is the sheer quantity of mantle rocks that have been brought to the Earth’s surface by plate tectonics. According to Kelemen, the size of this carbon storage reserve is big enough to take up billions of tons of carbon dioxide for centuries. He estimates that the method could remove about one billion tons of carbon dioxide from the atmosphere every year if the equivalent number of boreholes were drilled as there are oil and gas production wells worldwide right now. To put that in perspective, he said that the burning of oil produces about 14 billion tons of carbon dioxide per year. Again, the massive scale of human impact on the planet, and the planet’s carbon cycle, becomes apparent.

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Kelemen said that while the natural uptake of carbon dioxide by mantle rocks doesn’t propose any significant environmental hazards, scaling it up “is likely to cause local deformation and disruption of groundwater systems.”

For this reason, he thinks it best to start with pilot experiments using boreholes to access mantle rocks beneath the shallow sea floor.

Harlow also cautioned against going full bore into any of these geo-engineering solutions to climate change. As for the carbon dioxide-sucking rock solution, he succinctly stated that “we’d better hope it doesn’t run away, because then we’re dead.”

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It's hard to imagine an atmosphere depleted of carbon dioxide, but then again, 200 years ago no one imagined we'd be flooded by levels not seen in millions of years.