Johannes Lehmann, a professor of agricultural science at Cornell, is one of the world's foremost experts on biochar. He has calculated that if biochar were added to 10 percent of global cropland, it would store 29 billion tons of carbon dioxide equivalent—an amount roughly equal to humanity's annual greenhouse gas emissions. This approach would take advantage of a physical reality often overlooked in climate policy discussions: the capacity of the Earth's plants and soils to serve as a climate "sink," absorbing carbon that otherwise would be released into the atmosphere and accelerate global warming. Oceans have been the most important sink to date, but their absorption of CO2 is acidifying the sea—threatening the marine food chain—and raising water temperatures, which is causing sea levels to rise (because warm water expands). Meanwhile, the Earth's plants and soils already hold three times as much carbon as the atmosphere does, and scientists believe that they could hold a great deal more without upsetting the balance of natural systems.

Using photosynthesis and agriculture to extract carbon should not be confused with other methods that sound similar, such as "carbon capture and sequestration." CCS, as experts call it, is a technology that would capture carbon dioxide released when a power plant burned coal (or, in theory, other fossil fuels) to generate electricity. A filter would collect the CO2 before it exited the smokestack; the CO2 would then be transformed into a solid and stored underground. CCS assumes that coal burning would continue; the CCS technology would simply cancel out most of the CO2 emissions this coal burning would produce—and that's assuming the technology will actually work. So far, no nation on Earth has managed to operate a commercially viable CCS plant, despite an estimated $25 billion in subsidies.

By contrast, biochar and other photosynthesis-based methods of carbon extraction take advantage of natural processes that already help to regulate planetary health. "What we're really doing is bio-mimicry of fire," says Dr. David Shearer, CEO of Full Circle Biochar, the company that designed and built the kiln Lehmann uses at Cornell. According to Shearer:

"Historically it was fire that helped drive the carbon cycle on Earth, burning plants and trees and returning their embedded carbon to the soil in the form of charcoal. Contemporary societies have greatly restricted the use of fire. Producing biochar is a way to begin restoring the proper balance by catalyzing soil regeneration through the addition of biochar to soils."

Unlike CCS, biochar does not assume continued burning of fossil fuel. Rather, its feed stocks are waste materials that normal agricultural and forestry production methods leave behind in great quantities: tree trimmings, crop stalks, manure and the like—all of which need to be disposed of in any case and which now often end up in landfills, where their decay releases greenhouse gases into the atmosphere.

As biochar attracts more scientific and commercial attention, it has also acquired proponents and detractors. George Monbiot, a columnist for the Guardian, blasted the entire idea by seizing on one advocate's proposal to obtain biochar from vast tree plantations. Monbiot was correct that relying on plantations to produce biochar could cause poor farmers to be kicked off their land and food prices to rise as land was diverted to biochar. But Monbiot unfairly tarred all biochar supporters with the same brush, as he later admitted. In fact, Lehmann has always clearly stated that he did not favor the plantation approach. Joining Lehmann in this position is James Hansen, the NASA scientist who put climate change on the public agenda with his 1988 testimony to the US Senate that human activities were raising global temperatures. Hansen has endorsed biochar, along with expanded growing of trees, as vital tools for drawing down atmospheric CO2 levels to 350 ppm, the amount he believes is needed to stabilize Earth's climate.

Others remain skeptical that soil carbon sequestration could remove enough CO2 from the atmosphere to make a difference, and they point to a paucity of peer-reviewed studies validating the linkage. Lehmann, however, has tested biochar's carbon storage potential and other characteristics in field research in Kenya, Colombia, and the Amazon, as well as at the agricultural research station Cornell operates in New York state. At Cornell, he is producing biochar in a kiln whose shiny metal pipes and funnels make it look more like part of an electric power station than a cutting-edge agricultural device.

Notwithstanding my brave personal foray into compost testing at Cornell, Lehmann told me he does not plan to rely on the university's compost supplies to produce biochar. There are more ecologically efficient uses for that compost heap, he explains. Rather, Lehmann will use post-harvest cornstalks from other Cornell agricultural research plots. He adds that the kiln will also "generate liquid fuel from the gases that are produced while making biochar."

Such simultaneous fuel production is but one of the co-benefits of producing biochar. Studies by Lehmann and others have documented that adding biochar to soil also increases soil's fertility and ability to retain water, which in turn encourages greater crop yields. Adding biochar to soil therefore is also a form of climate change adaptation: Increasing a given piece of land's ability to absorb and retain water will make the land more resilient in the face of flooding as well as drought, both of which are projected to become more frequent and severe as climate change accelerates in the years ahead.

There is no one-size-fits-all technology for extracting carbon and sequestering it in soil, mainly because local circumstances, both social and physical, differ around the world. And despite his enthusiasm for biochar, Lehmann is the first to emphasize that it is neither a silver bullet nor the only feasible way of extracting carbon dioxide from the atmosphere. "There are and have to be several if not many approaches to sequestering [i.e., storing] carbon," he told me.

Other proven methods, he said, include growing trees—both in forests and mixed among field crops—and changing to less invasive tillage systems. Instead of industrial agriculture's practice of removing crop residues and plowing soil before planting, which releases large amounts of carbon into the atmosphere, "no-till" cropping leaves residues in place and inserts seeds into the ground with a small drill, leaving the earth basically undisturbed. A calculation by the Rodale Institute, a nonprofit agricultural operation in Pennsylvania, found that if no-till were used on all 3.5 billion acres of the Earth's tillable land, it would sequester more than half of humanity's annual greenhouse gas emissions. "If ideas such as biochar emerged recently," Lehmann asks, "what other ideas might still be out there?"

Climate change policy traditionally has focused on the energy sector, but under the new paradigm advocated here, the agriculture sector would gain prominence as well. Earlier in this monthlong Slate series on climate change and agriculture, Michael Pollan and I discussed how taking advantage of photosynthesis could turn eating meat from a climate sin into a blessing by relying on the same ecological principles that make biochar possible. The key is not meat versus no meat. The key is to reform agricultural systems away from the current industrial approach that uses vast amounts of petroleum to produce food in favor of systems that rely on natural processes such as photosynthesis. Pollan calls it the "oil food" versus "sun food" choice.

Critics are right that much practical work remains to be done to demonstrate whether a "sun food" system can actually succeed in both feeding humanity and fighting climate change. But there is good reason to think that humans can indeed harness photosynthesis to draw down the rising level of CO2 in the atmosphere. If we can then safely store that extracted carbon in places where it will not contribute to global warming, we could significantly reduce the 400 ppm of CO2 that are currently overheating our planet (assuming that we limit the 2 ppm of annual emissions as well). In short, we might begin to turn back the clock on global warming. And not a moment too soon.

This article arises from Future Tense, a collaboration among Arizona State University, the New America Foundation, and Slate. On July 25, Future Tense will be hosting an event on agriculture’s role in climate change at the New America Foundation in Washington, D.C. For more information and to RSVP, visit the New America Foundation website.