THE VINE AUGUST 11, 2010
Biochar has always sounded like a whimsical climate solution that's too good to be true. Simply stir a little charcoal into the soil and—voila—it's supposedly possible to suck thousands of tons of carbon-dioxide out of the air. Sounds suspicious, no? And yet it just might work. A new study in Nature Communications finds that the world could, in theory, sustainably offset a whopping 12 percent of global greenhouse-gas emissions by producing biochar.
The basic idea is easy enough to follow. Plants, as every eighth-grader knows, absorb carbon-dioxide as they grow and then release it back into the air when they die and decompose. All part of the natural carbon cycle. But what if there was a way to block that second part? As it turns out, if dead plants are burned in a controlled, low-oxygen atmosphere—a process known as pyrolysis—you get charcoal, which holds its carbon in a stable state for thousands of years. And this doesn't take any advanced technology, since the stoves are pretty easy to make.
But it gets better. When you mix biochar in with certain soils, the soils can end up releasing less methane and nitrous oxide—two potent greenhouse gases—than they otherwise would. What's more, infusing select types of soil with biochar can make the soil more fertile and cause crops to grow faster, since the charcoal seems to prevent nutrient leaching and increase water retention. (In the Amazon, much of the soil on the forest floor is poor quality except for patches of "terra preta"—black soil that appears to have been deliberately mixed with charcoal centuries ago.)
The big question, though, is whether you can grow the plants needed for biochar sustainably. As groups like Biofuels Watch have warned, if farmers start tilling virgin land to grow switchgrass, with the intention of creating biochar, then that could end up releasing additional carbon-dioxide and methane into the air. Alternatively, if biochar crops are grown on existing farmland, then that might encourage farmers elsewhere to hack down forests for space to grow the displaced food crops. This could be like the destructive ethanol craze all over again.
So for the Nature Communications study, the researchers just looked at the world's supply of crop leftovers: corn leaves and stalks, rice husks, livestock manure, yard trimmings. If virtually all of that biomass was used to make biochar, we could conceivably offset 12 percent of global carbon emissions. Trouble is, it would take a massive shift in production: "Using biochar to reduce greenhouse gas emissions at these levels is an ambitious project that requires significant commitments from the general public and government," said study co-author Jim Amonette of the Energy Department's Pacific Northwest Laboratory. "We will need to change the way we value the carbon in biomass." Still, as good ideas for averting global warming go, this one seems well worth it.
(Flickr photo credit: kelpiew)