The Earth's plants and soils already hold three times as much carbon as the atmosphere does.

halfrainThe Earth’s plants and soils already hold three times as much carbon as the atmosphere does.

A gigantic, steaming-hot mound of compost is not the first place most people would search for a solution to climate change, but the hour is getting very late. “The world experienced unprecedented high-impact climate extremes during the 2001-2010 decade,” declares a new report from the United Nations’ World Meteorological Organization, which added that the decade was “the warmest since the start of modern measurements in 1850.” Among those extreme events: the European heat wave of 2003, which in a mere six weeks caused 71,449 excess deaths, according to a study sponsored by the European Union. In the United States alone, 2012 brought the hottest summer on record, the worst drought in 50 years, and Hurricane Sandy. Besides the loss of life, climate-related disasters cost the United States some $140 billion in 2012, a study by the Natural Resources Defense Council concluded.

We can expect to see more climate-related catastrophes soon. In May, scientists announced that carbon dioxide had reached 400 parts per million in the atmosphere. Meanwhile, humanity is raising the level by about 2 parts per million a year by burning fossil fuels, cutting down forests, and other activities.

At the moment, climate policy focuses overwhelmingly on the 2 ppm part of the problem while ignoring the 400 ppm part. Thus in his landmark climate speech on June 25, President Obama touted his administration’s doubling of fuel efficiency standards for vehicles as a major advance in the fight to preserve a livable planet for our children. In Europe, Germany and Denmark are leaving coal behind in favor of generating electricity with wind and solar. But such mitigation measures aim only to limit new emissions of greenhouse gases.

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That is no longer sufficient. The 2 ppm of annual emissions being targeted by conventional mitigation efforts are not what are causing the “unprecedented” number of extreme climate events. The bigger culprit by far are the 400 ppm of carbon dioxide that are already in the atmosphere. As long as those 400 ppm remain in place, the planet will keep warming and unleashing more extreme climate events. Even if we slashed annual emissions to zero overnight, the physical inertia of the climate system would keep global temperatures rising for 30 more years.

We need a new paradigm: If humanity is to avoid a future in which the deadly heat waves, floods, and droughts of recent years become normal, we must lower the existing level of carbon dioxide in the atmosphere. To be sure, reducing additional annual emissions and adapting to climate change must remain vital priorities, but the extraction of carbon dioxide from the atmosphere has now become an urgent necessity.

Under this new paradigm, one of the most promising means of extracting atmospheric carbon dioxide is also one of the most common processes on Earth: photosynthesis.


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Which is how I came to find myself plunged forearm-deep into the aforementioned mound of compost. It was a truly massive heap, nearly the length of a football field, five feet tall and 10 feet wide, and a second equally large pile lay nearby. It all belonged to Cornell University, one of the powerhouses of agricultural research in the United States. Michael P. Hoffmann, the associate dean of Cornell’s College of Agriculture and Life Sciences, told me it was comprised mainly of food scraps from Cornell’s dining halls and detritus from its groundskeeping operations.

“You don’t want to leave your hand in there too long,” Hoffmann cautioned as I felt around inside the steaming mass of brown. Sure enough, although it was a cool, cloudy day, my forearm soon felt almost uncomfortably warm. “The microbes in there generate a fair amount of heat as they break down the organic materials,” he explained.

 The author, investigating the smoking hot compost at Cornell.

The author, investigating the smoking hot compost at Cornell.

Compost is but one of the materials that can be used to produce biochar, a substance that a small but growing number of scientists and private companies believe could enable extraction of carbon dioxide from the atmosphere at a meaningful scale. Biochar, which is basically a fancy scientific name for charcoal, is produced when plant matter — tree leaves, branches and roots, cornstalks, rice husks, peanut shells — or other organic material is heated in a low-oxygen environment (so it doesn’t catch fire). Like compost, all of these materials contain carbon: The plants inhaled it, as carbon dioxide, in the process of photosynthesis. Inserting biochar in soil therefore has the effect of removing carbon dioxide from the atmosphere and storing it underground, where it will not contribute to global warming for hundreds of years.

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 biomimicry of fire,” says 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 U.S. 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 [PDF] 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.

 This is the kiln, manufactured by Full Circle, where Prof. Johannes Lehmann of Cornell produces biochar.

This is the kiln, manufactured by Full Circle, where Lehmann of Cornell produces biochar.

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 [PDF]. 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. Future Tense explores the ways emerging technologies affect society, policy, and culture. In July, Future Tense will be publishing a series of pieces on agriculture and climate change.

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