Many claims have been made over the years about biochar. It certainly sounds like something that can help save our asses from the climate crisis. Take some waste from thinning trees to prevent fires, or straw from growing wheat or rice, or any other high-carbon waste. Partially burn it without air to power cooking stoves or provide some other kind of energy. Bury the resulting charcoal in fields to store the carbon, and improve soil structure. Voila, biopower that on net takes carbon out of the atmosphere and turns some of it into fossil carbon that is permanently removed from the carbon cycle. In essence, it is supposed to be a form of carbon sequestration that is permanent today, and makes agriculture more sustainable besides. Suck it, global warming!
Unfortunately, this is completely unproven. The heart of any claim about biochar is that it permanently and on net stores carbon in the form of charcoal. No net permanent storage, no net sequestration. A review of the literature [PDF] by Biofuels Watch shows there have been a grand total of five peer-reviewed field tests of biochar where soil carbon was measured before and after biochar was applied. None of these tests used charcoal from the low-pollution charcoal-making stoves that have been developed for use by small farmers to create biochar. Nonetheless, looking at what is out there, in two of the five trials, carbon showed no increase after the application of biochar compared to before, or less increase than competing methods of improving the soil, such as adding manure or sawdust. A third trial showed no improvement in two cases, and an increase in one. The results in the other two trials were mixed — showing more carbon in a majority of cases, but less in a minority. So if we look at peer-reviewed field trials, biochar results so far range from outright failure to unreliable. We need to do extensive study to figure out what works and what doesn’t before we move prematurely to deploy. It is a shame, because anything that could stave off oncoming disaster is worthwhile. But there is no point in deploying something like this before we know what versions work, and what versions are quack remedies. Note, by the way, that these trials were in warm climates, and biochar probably is less effective in cold climates than warm.
What about studies other than field tests? Well, those can be divided into three categories, and none of them outweigh field tests.
There are a number of studies based on Terra Preta, the black earth created by a complex system of slash-and-burn agriculture once used in the Amazon. That did indeed work, and it would be wonderful if we knew how to recreate it. Unfortunately, long-ago massacres and destruction of indigenous nations in the Amazon mean that we have no idea of the ancient technologies that produced the Terra Preta biological cycle. Burning wood or straw in high-tech charcoal stoves is NOT the same thing, and so far has not been shown to produce the same results.
There are also studies based on forest fires. Researchers estimate how much carbon there should be in the soil if all the charcoal produced by a forest fire was retained, measure the carbon actually present, and — voila! — find around that much carbon in the soil. Note that this kind of measurement is very sensitive to assumptions about how much carbon should exist. And when field trials are possible, it is definitely not something that should outweigh field trial results.
Similarly, laboratory trials produce charcoal, and then measure charcoal lifespan under various conditions. The problem is that such trials don’t measure how charcoal added to the soil affects existing biological stores of carbon, and whether soil already in the ground will be released by the addition of charcoal. And we don’t really know how comparable laboratory charcoal is to what would be produced in field conditions. These kind of details matter intensely.
There are other problems with biochar. For example, if it is not applied carefully, it can blow away in the form of dust, and as a form of black carbon, constitute a much more powerful global warming “gas” than carbon dioxide. There are real problems with sources too. Peer-reviewed studies have show that forest thinning for fire prevention at too high a level can actually reduce the ability of forests to store carbon in many cases. From a climate standpoint and an energy standpoint, most waste straw may be better used to create single- and two-story buildings (or to produce strawboard) than as biochar feedstocks. There are sustainable levels of waste wood and waste straw that are suitable for biomass use, as well as some animal and urban waste. But the ratio of sustainable wet waste to sustainable dry waste may prove far more suitable for processing in methane digesters to create biogas than to be burned in special stoves to produce biochar.
One technique some of the more subtle global warming delayers and deniers use to delay action is to claim we should do very little now, but instead research and look for a magic solution. (Hi, Breakthrough Institute!) When it comes to efficiency, conservation, solar and wind energy, mature sustainable agriculture, and forestry techniques, and other known means of reducing greenhouse-gas emissions, they are dead wrong. Biochar, on the other hand, is one technique we should NOT be deploying now; it is a technique we should be field testing extensively and massively funding research on. Because if we could make it work, biochar, combined with the other things we already know how to do, really could help save our asses, just as its advocates claim. But that is only true if we figure out how to do it right, and what the proper sources are, and don’t deploy quack remedy versions in advance of knowing what “doing it right” means.