The Coal That’s Good for the Climate

In northern Senegal, near Ross-Bethio, it’s mid-day, the time when the sun is at its hottest. I’m meeting Alioune Diatta in the middle of a field, and he’s wearing the pinched expression of someone wrung out by too much sun and too much sweat. “The farmer is the unluckiest person in the world,” he tells me right away after introducing himself.

Almost 20 years ago, Diatta moved from the lush and semi-tropical Casamance region to the arid northern part of Senegal, a landscape marked by its sandy soils, its dry earth-moving winds, and its short rainy seasons. I ask him, why had he wanted to leave the paradise that is the Casamance in order to come to this place? He says there was the conflict—a low-grade civil war waged by the region’s separatists that was especially disruptive in the 1990s when Diatta made the move. That was one consideration, but not the principal one.

“I was mostly drawn by the onions,” he says. This region produces much of the country’s onions, a high-value vegetable crop in a culture where onions are at the base of almost every dish. They’re so valuable that the country has set a goal to be self-sufficient in onion production by next year.

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A Senegalese farmer tends his irrigated onion crop.

What the north had that Casamance did not was irrigation water from the nearby Senegal River, which Diatta could use to grow rice and vegetables in addition to onions. And that’s what Diatta does, sometimes growing onions or tomatoes or other vegetables continuously. So it’s no surprise when he tells me that some of his fields had become worn out. “They are dead lands,” he says. “They don’t even yield half of what they yielded before.” Even with fertilizer, the fields were only somewhat productive since the sandy soil could not hold onto nutrients. On one of them, his seeds would not even germinate.

A few years ago, Diatta heard about biochar as a possible solution to his woes. Biochar is a type of charcoal made from plant and animal waste that has been heated intensely in an environment with little or no air, a process known as pyrolysis. Unlike combustion, which releases carbon dioxide, the process allows the organic matter to retain its carbon as it’s transformed into charcoal, preventing the gas from leaking into the atmosphere. It also boosts the fertility of poor soils like Diatta’s.

But biochar has another trick up its sleeve. By retaining carbon even after combustion, it may not just help boost Diatta’s low yields. Some researchers believe it could be part of a solution to climate change, one of the most intractable problems of our era.

Crutch or Cure?

For climate scientists, the biochar solution falls under the broad rubric of geoengineering. For a long time, climate change circles regarded geoengineering with skepticism. Many schemes were treated as the equivalent of the crazy, or—more charitably—the lazy person in the room. The idea that—instead of simply trimming greenhouse gas emissions—you could develop large-scale, technological fixes to tweak the Earth’s natural processes, well, it seemed a little like taking heart medication so that you could continue to stuff yourself with steaks and cupcakes. It might work in a pinch, but shouldn’t you just eat some vegetables and cut back on the things that could kill you?

Now, many climate scientists acknowledge that it is probably too late to stop climate change just by reducing emissions alone. Sooner or later, we may need a plan B. The latest assessment report from the Intergovernmental Panel on Climate Change (IPCC), which has historically resisted geoengineering, featured several references to the concept, a sign that the panel, even if still skeptical, is now open to talking about it.

What get the most press are the wildest geoengineering ideas: using gargantuan lenses to shield the Earth from the sun; dumping tons of iron ore into the ocean to fertilize phytoplankton, which absorb carbon dioxide; and injecting sulfur dioxide into the atmosphere to create aerosols to dim the sun’s rays.

Biochar is less bold, but also potentially effective, and it earned a mentioned in the IPCC report. Scientists believe that the carbon locked in biochar will remain stable for some time, though for how long that might be, they’re not exactly sure. “Some biochars may live in soil for 30, 40 or 100 years,” says Bhupinderpal Singh, a research scientist with the Australian government who has been studying the stability of biochar in soil and its greenhouse gas mitigation potential. “Some biochars may live in soil for millennia.” But what eventually happens to biochar remains a divisive question.

We know that thousands of years ago people from the Amazon, Australia, Asia and Africa were adding charcoal to the soil. Stephen Joseph, a biochar trailblazer from the University of New South Wales in Australia, says calling it biochar may be the latest trend, but it is an old technology. “I was up in Nepal, up in the mountains, and we came across a group of people who made biochar very crudely in the field,” Joseph says. “I asked them how long they had been making it for, and they said, probably for centuries.” But the traditional cultures that used biochar were not worried about climate change when they burned their crop wastes and spread the residues on their fields. They used it because it seemed to help their soils become more fertile.

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A small sample of a granular biochar

Despite its long history, we do not really know how long a biochar’s carbon will remain sequestered. “Once it’s in soil, you can’t take it out,” Singh says. “So we need to know the long term benefits and the long term implications of biochar applications. Does it regulate nutrient supply to plants? What’s the mechanism? Does it improve nutrient use efficiency? Are there any nasty chemicals in biochar, for example? So basically, thinking beyond carbon sequestration.”

People on the fence about biochar—and there are many—say that we should be careful about putting too much faith in the technology. Some detractors say that biochar might eventually break down or cause other forms of carbon to degrade in turn. Still others worry about the socioeconomic impact of large-scale biochar production. The most outspoken critics are concerned that land used to grow plant material for biochar production could displace food crops, similar to worries some had about biofuels.

Still, it may be one of the least bad options. “As we think about geoengineering techniques, it certainly feels like one of the least scary of them. We’re not talking about something that is a radical change for the Earth system,” says Caroline Masiello, a biogeochemist at Rice University in Houston, Texas. She has been studying the carbon cycle and charcoal for most of her career. “Although, you can make a mess with anything. Humans are fully capable of getting it wrong with any kind of process, so I guess I would not say that this is especially safe or especially foolproof.”

But, she adds, biochar has a certain intuitive feel to it. “It’s already a part of the Earth’s stable soil carbon pool, and people have been doing this in other cultures for a long time. So why not look at it and consider it as an option?”

The Unknowns

When Sharon Weyers, a research soil scientist with the USDA’s Agricultural Research Service, started hearing about the wide use of biochar a few years ago, she says that she was concerned. “You can not just put this blanket statement out there that biochar is good when there are some biochars that might be good but some that aren’t,” Weyers says. (The properties of biochar can vary according to the source material and the method by which it is made.) Weyers had done most of her dissertation research on earthworms, which perform key functions in the soil ecosystem, and she knew that some biochars were toxic to them.

In fact, her original concerns were justified. Biochar-treated soils did tend to make earthworms migrate away, possibly as a result of the change in pH. Weyers says that this effect dissipates over time. Eventually, after a couple of years or so, the earthworms will come back.

The ease of use can be tempting for practitioners, says Franco Miglietta a researcher at Italy’s National Research Council. “Biochar may create a lot of enthusiasm,” he says. “But we have to wait for the proper answers that science will provide before going straight into this agronomic strategy, to use it everywhere, and to produce a lot of the charcoal, to sell it like gold. We have to be very careful.”

Miglietta was the project coordinator of EUROCHAR, a European Union research project on biochar that ended in 2014. One paper published out of his project showed that more research is needed to understand how biochar affects plant growth and plant resistance to disease and pests. Miglietta says that the researchers discovered that biochar seemed to stimulate the production of growth hormones, but they also found that plants seemed to dial down the expression of the genes that control resistance to pests and disease.

It could be that when the plant is doing well, it does not invest as much in defending itself, he says. But there’s also the possibility that a weakened defense is the result of chemical signals the charcoal elicits from plants. In a natural setting, they usually only encounter charcoal after a fire, which tends to clear the landscape of pests. “It could be a post-fire strategy that plants have already in their genes,” Miglietta says. “Now when we apply biochar into the field, we expose plants to something they potentially know already. They get it like a signal that they can grow and must grow faster because there are less pests around.” He cautions, though, that these were lab tests and not field data and that we should not extrapolate too much from the results. “We are just warning the scientific community that we should carefully look at the effect of biochar on pest tolerance of plants. This is an area that requires investigation.”

At the same time, research from Masiello’s lab at Rice University suggests that biochar might help fend off root pathogens. Some soil bacteria can act together, coordinating their attacks on a plant. She thinks that biochar may disrupt the ways that these microbes signal each other. “Some biochars are effective at severing that communication,” she says. In fact, it could give agronomists new ways to treat root pathogens.

Research on biochar is progressing rapidly. “Every day, there are new papers coming out. So the rate at which we are accumulating knowledge in this area is really exorbitant,” Miglietta says. “I bet that the progress we will make in the next 24 months will be larger than the progress that we have made over the last ten years.” But farmers haven’t been waiting for a final verdict. They’re starting to use biochar now, especially in developing countries, where the amendment can be an inexpensive way to rehabilitate tired soils.

Fixes on the Farm

Back when Diatta, the Senegalese onion farmer, first considered using biochar, a nongovernmental organization called Pro-Natura International was producing the stuff from both rice hulls and typha, a rampant semi-aquatic reed, known in the U.S. as cattail, that chokes the region’s fresh waterways.

Pro-Natura was first founded in Brazil in the 1980s. Their main target was originally deforestation, not climate change. “At the beginning, we developed this technology to replace charcoal made from wood,” says Guy Reinaud, the president of Pro-Natura International. In parts of South America, as well as in Africa and Asia, many households still use wood or wood-based charcoal to cook. Pro-Natura’s “green” charcoal briquettes were made from agricultural waste, not trees, and provided an alternative. In 2008, they started producing charcoal powder from rice hulls and typha to use as a soil amendment in Senegal, as well. It was light and feathery like the source materials.

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A lighter, flakier biochar

Diatta says that he first tried it in 2011, spreading the fluffy biochar on just over 4,300 square feet of his worst land. “After I put on the biochar, I had a good harvest,” he says, noting that he harvested 130 sacks of onions that year.

In some ways, it is no surprise that the biochar worked so well on Diatta’s poor, sandy soils. “When we look at the hydrological effects of biochar, we see the biggest improvements in water-holding capacity in these sandy soils,” Masiello says. Her research suggests that biochar can change how fast the water moves through the soil. By improving the soil structure, biochar also can help improve how well a poor soil absorbs nutrients. “The biochar itself is porous, and adding smaller particles to the big sand particles changes the ability of the soil matrix to hold water.”

In western Kenya, Andrew Crane-Droesch, a PhD candidate at the University of California, Berkeley, found that biochar could help increase maize yields. “On average, the biochar plots yielded something like 37% higher than the control plots, so a huge jump in yields,” he says. Other field studies have reported more modest gains. Crane-Droesch says that biochar tends to increase yields more when used on poor soils, like the highly-weathered soils found in tropical and semi-tropical zones. Weathered soils often lack organic matter and have trouble holding onto nutrients—even those from fertilizers. Crane-Droesch says those soils are almost like a sieve. “If you’re shoveling sand into a sieve, you could either shovel faster or you could plug the holes. Biochar sort of plugs the holes.”

On good soils or soils found in temperate zones, though, biochar might have little or no effect. That was the experience that Senegalese agronomist Moda Gueye had when he tested biochar on a heavier soil. He says he saw almost no difference in yields. The effort just wasn’t worth it. “For me, the problem with biochar—or at least the number one problem—is that it is very voluminous,” Gueye says. “Biochar was not very expensive, but you needed two or three trucks of it per hectare.” It’s not uncommon to have to pay twice as much to transport the biochar as for the biochar itself, so at that scale, logistical costs quickly add up.

Nonetheless, Diatta sees biochar as a solution to the overuse of synthetic fertilizers by farmers in the region. “We can’t continue to use chemicals,” he says. He adds that he would use biochar on some of his other fields, too, if he could get his hands some. There’s just a small pile of it left now since the Pro-Natura factory shut down in 2013 pending the outcome of a legal dispute and the creation of a new business, Biochar Senegal, to handle it. Awa Diop Toure is the director of Biochar Senegal. She says the demand is there. “The soil amendment is not difficult to sell because the farmers will invest in the soil, if it’s profitable.”

Andrew Crane-Droesch’s research in Kenya, though, has shown it is a little more complicated than that. He wanted to know how social networks affected villagers’ adoption of biochar: If your neighbor used biochar, would that affect how likely you were to use it yourself? It didn’t seem to make much of a difference. Biochar adoption among farmers remained low despite the demonstrated increases in yields. Farmers were reluctant to pay the market price for biochar, but they were more open to it when the price of biochar declined. “Social learning seems to be a useful tool for disseminating the technologies, but without subsidies, it’s often just going to be worth nothing,” Crane-Droesch says.

Stephen Joseph says subsidies are already in place in China, for example, where the government banned burning straw in the open. That created an opportunity for biochar producers to make use of what was a waste product. Joseph says that it seems to be working, if slowly. “It’s like bringing in any new innovation. It takes a long time, and what you think is going to work in the beginning is usually not what actually works in the end.”

Take the example of an Australian farmer, he says, who, instead of burying biochar, started feeding it to his cattle. The biochar still comes out the other end, a rich biochar-manure mix, which Joseph thinks might be more effective on plant yields than biochar or manure alone. Native dung beetles then mix the biochar and manure into the soil profile. “He has had major success in terms of improving the quality of his meat and the quality of his pastures,” Joseph says.

The back-and-forth over biochar is starting to calm down, and a clearer picture is emerging. Biochar probably won’t save the world from climate change, nor is it going to be the key to increasing agricultural productivity everywhere. But used wisely, it is a tool that may help in both situations. “Biochar is not going to work in all places,” Joseph says. “It’s not the magic bullet. But it potentially can have a big impact.”