Is it really possible to suck out thousands of tons of carbon-dioxide from the air simply by stirring some charcoal into the soil? Or is so-called "biochar" just a crazy idea that's too good to be true? The Economist recently reported from the North American Biochar Conference in Boulder, Colorado, and the research sounded pretty promising, though there were some heavy caveats thrown in.

The basic concept behind biochar is pretty simple. 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? Well, 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 for thousands of years. And this doesn't take any advanced technology, since the stoves are pretty easy to make.

But it gets even better. When you mix biochar in with certain soils, the soils sometimes 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.) Oh yes, and there's a third benefit—the pyrolysis process itself creates a few useful by-products, like syngas that can be burned as fuel.

So what's the hold-up? Shouldn't we be doing this everywhere? After all, according to Cornell's Johannes Lehmann and PNNL's Jim Amonette, if we both buried the biochar and displaced some of our fossil-fuel use with syngas, we could potentially decrease manmade carbon emission by about one or two gigatons per year—out of about 9.7 gigatons we're currently adding to the air each year. That's enormous. But, as The Economist reports, those projections are still very tentative, there's still a lot of research to be done on how biochar interacts with different soil types, and there's the possibility that biochar could be handled as badly as biofuels have been:

Biofuels Watch, a British lobby group, worries that a craze for the stuff could see virgin land tilled specifically to grow crops such as switchgrass, whose only purpose was to be pyrolised and buried. That tillage would release carbon dioxide and methane. But the alternative, growing those crops on existing farmland, would encourage the clearance of more land to grow the food crops that had been displaced.

Indeed, Kelli Roberts, another researcher at Cornell, told the meeting that, taking all factors into account, growing switchgrass for biochar may do more harm than good. Corn stover, garden waste such as grass clippings, and offcuts from forestry and timber production are better bets, she reckons.

There's also the big question of how to finance a massive biochar push. In theory, this is one thing international carbon offsets could help with: Emitters in industrialized countries could pay for pyrolizing stoves in developing countries—stoves that, as yet another bonus, don't emit the Arctic-melting, disease-causing black carbon soot that many wood-burning stoves in the Third World do—as well as paying farmers to bury the charcoal. Still, the concerns from Biofuels Watch are well taken, and while the potential for biochar is too huge to ignore, doing it the wrong way could potentially make the climate problem worse, not better.

(Flickr photo credit: kelpiew)