Biochar is being promoted as a way to save the world. (I admit to being optimistic about this myself for a very brief time.)
It certainly sounds good. Take agricultural or forestry waste that is pretty much pure carbon, with almost none of the other nutrients plants need. Burn it without oxygen, producing a bit of bio-gas for fuel, and a bit of high value pyloric oil suitable for a number of advanced uses. What is left is charcoal which can help build soil and permanently store carbon. As a a bit of added glamor it is based on the Terra Preta methods of ancient South American Indians.
Unfortunately much of this is untrue, and the rest highly exaggerated. According to a Biofuels Watch Study(pdf), the science on this highly uncertain, and we are moving far ahead of our understanding in our haste to implement. For example if the charcoal is simply dumped onto the soil surface, much of it rises into the air as carbon dust, a sort of cold soot, and thus acts as black carbon, a much stronger greenhouse forcing than simple CO2. If it is worked deep into the soil, the root systems of plants are disturbed and existing soil carbon is released. There are other studies which indicate that in many circumstances adding charcoal to soil displaces existing CO2.
Another related problem: charcoal seems to be thought of by Terra Preta advocates as permanent and stable. But it turns out that quite often it can be as unstable as any other type of soil carbon. The report outlines many more reasons for caution in contemplating Terra Preta. Incidentally, modern Terra Preta has yet to duplicate the results found in ancient South American soil.
Now I’m not as sure as the good folks at Biofuels Watch that the problems they outline can’t be gotten around. But they certainly make a case that we damn well better know how to to do Terra Preta right before we experiment with it on a large scale And I think the various caveats in this report do show that at best Terra Preta will make a modest contribution to solving the problem rather acting as a magic silver bullet. And if Terra Preta is implemented in our current state of ignorance, then that contribution will be zero, or negative and Terra Preta will end up being part of the problem rather than part of the solution.
At any rate, I urge you to read the whole report, and judge for yourself: Biofuels Watch Study(pdf).
If the report has anything wrong, or has taken anything out of context then maybe some of the Terra Preta advocates at Grist will want to write a rebuttal. I don’t speak for David Roberts, but my guess is that if you are not a Grist blogger, but can produce a worthwhile reply, Grist will be happy to provide the space. Or, if you prefer, send it to me, and I’ll ask David for permission to post it.