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The world keeps score on human-induced climate change with graphs that show the steady, relentless buildup of carbon dioxide in the Earth’s atmosphere, from 280 parts per million of carbon dioxide (CO2) in preindustrial times to almost 415 ppm today. The causes of this accumulation go far beyond a simple, one-way, upward flow. The carbon atoms floating in the air right now have been traversing the globe in all directions for hundreds of millions of years, ricocheting from atmosphere to sea and back, into the mineral, living, and human-built worlds—both above and below the Earth’s surface—and once again into the atmosphere.

 

With the global endowment of carbon atoms always on the move, the quantity of carbon dioxide in the atmosphere rises and falls gradually in natural rhythms over geologic time. But by pulling huge quantities of fossil fuels from deep in the Earth’s crust and burning them, industrial societies have tilted the carbon pinball machine, triggering extraordinarily fast global heating. And as stock markets and global GDP ping up and down, so too the world’s carbon atoms, for the two are related. Now affluent, energy-hungry economies are grasping for ways to keep burning fossil fuels, keep their GDP rising, and at the same time reduce the amount of carbon in the atmosphere. The goal seems straightforward, but the paths toward it are narrow and twisted.       

 

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On the Surface

 

One can begin tracking the Earth’s carbon cycle at any point in time and space. Let’s start with carbon moving from the atmosphere into the shallow underworld of the soil. That happens when a green plant absorbs CO2 from the atmosphere and uses it to produce carbohydrates and other compounds, a portion of which flow downward to help build the plant’s root system. Sooner or later the carbon stored in a root ends up in the soil, some of it when the root leaks sugars, some when other organisms feed on the roots, and some when root tissue dies and decomposes.

 

Most of the world’s non-agricultural soils are complex ecosystems built on a foundation of carbon coming from the roots of mostly perennial plants. These soils are commonly rich in soil organic matter, carbon-rich material that the belowground ecosystem has generated; think of the forest floor perhaps. Carbon from the organic matter is passed along from species to species, down the underground food chain. All along the way, a small but steady stream of CO2 is respired by soil organisms, making its way back into the atmosphere, ready to be absorbed again someday by plants through photosynthesis.

 

This cycle spun along nicely throughout most of the history of terrestrial life. But in the ten thousand years since humans adopted agriculture, on every landscape where farmers have tilled the soil and replaced the huge carbon deposits in soils from perennial root networks with modest deposits from annual crops, the process has gone haywire. With the cultivation of annual crops, soil organic matter is broken down by microorganisms much more quickly than when the soil is left undisturbed, sending waves of CO2 into the atmosphere in greater volumes than can be pumped back into the soil by plant roots. After a few decades of such disruption, soils are badly depleted of organic matter and nutrients.

 

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Deep Carbon Rising

 

Even as farming continued to deplete soil carbon, humanity managed to flourish for millennia by relying on energy that shone from above and the fertility of the underworld that extended down inches to yards underfoot. Then, starting in a big way two centuries ago, we reached into a far deeper underworld to extract plant- or algae-derived carbon compounds that had taken a detour from the surface world’s carbon cycle many millions of years ago. The coal deposits that kicked off the Industrial Revolution were tens to hundreds of feet below the soil surface. The oil and gas deposits that have supercharged industry in the twentieth and twenty-first centuries were deeper yet, from thousands of feet to several miles down.

 

This ancient carbon would rescue agriculture, at least temporarily, from its 10,000-year boom-and-bust cycle. Over millennia, tillage has badly depleted much of the world’s farm land, not only of carbon but also of another crucial element, nitrogen. In the early twentieth century, with demand for food growing and crop yields sinking, fossil carbon came to the rescue. Through combustion, hydrogen atoms from methane (CH4, which in that era was produced through “gasification” of coal; today, the source of methane is fossil gas) could be made to react with nitrogen (N2) from the air to produce ammonia (NH4), from which synthetic nitrogen fertilizers could be made. The leftover carbon, which had last seen daylight 200 to 300 million years before, wafted up into the atmosphere.

 

Carbon compounds from the deep underworld supercharged farming in other ways, as diesel-powered tractors took over from solar-powered draft animals in tillage and other field operations. These machines would carry forward the millennia-long depletion of carbon in the shallow underworld, and they would also pump exhaust fumes laden with fossil carbon into the atmosphere; as a result, petroleum, like methane, propped up crop production while at the same time degrading the soils and climate on which that production had always depended.

 

Off the farm, meanwhile, factories, vehicles, and power plants were burning ever-greater quantities of oil, gas, and coal in producing the material cornucopia that has come to overwhelm the world around us—to the point that the mass of all human-made objects and structures now exceeds that of all living global biomass. Meanwhile, humans and our livestock now make up 95 percent of the biomass of all terrestrial vertebrates, leaving wild mammals, birds, reptiles, and amphibians to account for only 5 percent.

 

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Pumping and Mining Can’t Be Undone

 

Given all that, it’s not surprising that today, there’s far too little carbon in the shallow underground, where it is badly needed, and far too much up in the air, where it threatens us all. That imbalance will be much more difficult to correct than it was to create. The vexing challenge is how to completely halt the pumping and mining of fossil carbon from the deep underworld while seeing to it that much more carbon moves into the shallow underworld.

 

Disruption of the carbon cycle starts with pumping and mining. Once fossil fuels have been extracted and burned, their carbon merges into and remains in the Earth’s carbon cycle, and it stays there in perpetuity. At any one time, a lot of the carbon in circulation is being held in the tissues of living plants, where it’s prevented from causing trouble; however, a carbon atom in a leaf or a seed or even a tree trunk is not locked away permanently like the carbon in a coal seam. It will, eventually but inevitably, return to the atmosphere.

 

The same is true of carbon that’s stored in the shallow underworld. Soil organic matter is made up of two broad components. One is quickly broken down in the soil food web, releasing its carbon back into the atmosphere within the course of a season or year, while the other may stay intact for years or decades; neither cache of carbon is permanent. The only way to keep carbon locked away for good is to keep our collective hands off the fuels of the deep underworld, for evermore.

 

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Hacking the Carbon Cycle

 

The human-built world we now see around us, both the good and the bad, would never have existed without the fossil-carbon bonanza of the past century. It follows that a decision to leave oil, gas, and coal permanently in the deep underworld would dramatically transform our material reality. This world has worked out very well for the people and institutions that hold the decision-making power in our society. They don’t want to give it up, so they’re looking to academic and industry experts to figure out innovative means of gaming the carbon cycle, of shuttling carbon among its various abodes in a way that can keep the combustion-powered bonanza rolling along without broiling the Earth.

 

Scientists and engineers have come up with some sophisticated but speculative ideas for bringing fossil fuels up from the deep underworld, burning them, capturing a proportionate amount of atmospheric carbon dioxide, and burying that in the deep underworld. Once it’s nabbed and entombed, some geologists assure us, that carbon will be stored away as securely as native fossil-fuel reserves are.

 

The most basic of these carbon-juggling strategies, a quick round trip, would extract carbon dioxide from a gas- or coal-fired power plant’s exhaust before it leaves the smokestack, liquefy the carbon dioxide, and inject it under high pressure deep into old oil and gas wells. But the process requires so much energy and expense that its only real-world use has been for injecting captured CO2 into near-depleted oil wells to push out the last few precious barrels. In other words, fossil carbon that has been prevented from going into the atmosphere is used to bring up other fossil carbon that will be spewed by cars and trucks into the atmosphere. (The captured carbon dioxide could also be used as an ingredient for manufacturing plastics, concrete, biofuels, and other products, but virtually all of the carbon stored in those products also will make its way back into the atmosphere, sooner or a little later.)

 

In a second strategy, gas and coal power plants would be allowed to dispatch carbon straight into the atmosphere as they always have, while facilities located elsewhere would compensate by sucking air directly from the atmosphere and separating out the CO2. The CO2 would then be liquefied and injected into the deep underworld. It sounds simple enough, but this route is even more costly and energy-hungry than smokestack capture and storage. Carbon dioxide makes up only 0.04 percent of the Earth’s atmosphere. To sort through thin air to extract that one molecule out of every 2,500 requires eye-popping expenditures of energy. Additional energy is needed for the liquefication and injection of the CO2. One could run the carbon-capture plants on renewable energy, but far better would be simply to shutter the fossil-fueled plants and use renewable energy to run society.

 

Rube-Goldberguesqe plans for stashing carbon would not only entail shifting a lot of it vertically between the sky and the underworld; they also would require development of a huge new industry to transport carbon horizontally across the Earth’s surface. Many of the geologic formations that are suitable for storing CO2 deep in the Earth lie at great distance from the sites where the gas would be captured. Therefore, some experts estimate, a full-scale carbon capture and storage effort in the U.S. alone would require as much as two and a half million miles of CO2 pipeline, approaching the current total length of all oil and gas pipelines in the country today. That would play havoc with land rights and ecosystems. The pipelines could also be a menace to the health of anyone living in their vicinity. Just ask the residents of Sartartia, Mississippi, forty-nine of whom ended up in the hospital emergency room with symptoms including disorientation, convulsions, headache, nausea, shortness of breath, memory problems, and unconsciousness one night in 2020 when a nearby CO2 pipeline burst.

 

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Too Good to be True

 

One of the chief users of such a CO2 pipeline network would be a much-buzzed-about but largely hypothetical process called “bioenergy with carbon capture and storage.” It’s advertised as the ultimate carbon hack, a means of generating electricity without pulling any fossil carbon out of the deep underworld and without emitting carbon from any source into the atmosphere. To start its incredible journey, carbon would be absorbed from the air in the usual way, through photosynthesis by forests, grasslands, or farmed crops. The wood or other plant tissue thus produced would be pelletized and burned to generate electricity. The resulting CO2 emissions would be captured from the smokestack, and, as with other capture schemes, it would be liquefied and injected into the deep underworld.

 

The climate establishment, including the Intergovernmental Panel on Climate Change, has latched onto this process as a way of making carbon emissions run in reverse. Instead of sending carbon from the deep underground into the atmosphere, they say, companies will generate energy while moving carbon from the atmosphere into the depths. It sounds too good to be true. And, of course, it is.

 

The process’s many steps—producing and harvesting biomass crops, hauling the biomass to the processing factory, grinding and pelletizing, hauling pellets to the power plant, removing carbon dioxide from the exhaust, liquefying the carbon dioxide, hauling the liquid to the injection field, and squirting it into the earth under high pressure—would eat up staggering quantities of energy. When that’s subtracted, the resulting net energy would be far too little to make the enterprise worthwhile. And if fossil fuel (or biomass) is being burned to power the production, transport, and processing of the biomass, a large share of the carbon relocated from the atmosphere to the underworld would be replaced by fossil carbon going the other way.

 

The worst impacts from this attempted hack of the carbon cycle would be felt in the shallow underworld of the soil. To pull less than one-third of human-generated CO2 out of the atmosphere would require that bioenergy crops be grown on as much land as is already used to produce the world’s food, feed, and fiber. These vast acreages of biomass crops, with cycles of clear-cutting and replanting, would further deplete the Earth’s soils of organic matter and nutrients. Huge quantities of carbon would leave the shallow underworld and re-enter the atmosphere at a time when we desperately need carbon to be moving in the opposite direction. The Earth could lose half of its native forests, grasslands, and savannahs. More biodiversity could be wiped out than would be lost with a global temperature rise of two and a half degrees Celsius above pre-industrial times—the very scale of disaster that these schemes are intended to prevent.

 

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We Ask Too Much of Plants

 

Bioenergy with carbon capture and storage is based on an assumption that the plant kingdom is an infinitely adjustable carbon-processing machine that can produce ample quantities of carbon-based products like food and fiber while also supplying carbon-based fuels to power plants and also stuffing unlimited quantities of additional carbon into the shallow underworld. But the world’s farmland is already being pushed to the limit to keep production of annual crops going, most grasslands have been run down by overgrazing, and replacing healthy forests with plantations of fast-growing, low-quality tree species would cause a spiral of ecological degradation.

 

Perennial grain crops such as Kernza® and other species under development at The Land Institute  have the potential to score a hat trick, producing both food and bioenergy while at the same time improving soil health. But cropping systems are not factories, and no biological carbon-processing system can be cranked up high enough to support the extravagant speed and scale of the industrial world’s current food and energy production. Asking Earth’s ecosystems to continue providing carbon compounds to feed, clothe, and house humanity while supplying enough additional carbon to restore the health of the world’s soils is in itself a breathtakingly tall order. To demand that they also meet the global economy’s energy demand would be an exercise in turnip-bleeding.

 

If affluent societies could become much less materially voracious and learn to function on much less energy, we could end our exploitation of the deep underworld and at the same time restore the shallow underworld of the soil to full health. Fanciful plans for “fixing” the climate could be ditched, and that good old-fashioned carbon cycle could repair itself and begin once more to hum along undisturbed.

 

Perhaps we ask too much of plants. Perhaps we can instead act more like plants by subsisting on energy from the sun and nutrients from the shallow underworld—and leaving carbon of the deep underworld entombed for all time.