Apr 15, 2010

The Hope of Trees Part I: Biochar

looks like coal but it’s different in a key way. It’s charcoal, also called char, or biochar, and like coal is mostly carbon. Unlike coal, which is carbon sequestered 300 million years ago, biochar is carbon recently pulled from the atmosphere by trees. Buried in the ground it will foster plant growth and likely remain as is for many hundreds of years. Biochar is produced by combusting wood with limited oxygen. Done in a controlled way, emissions are minimal and energy is a byproduct. The Earth’s forests accumulate about 60 gigatons (Gt) of carbon per year, but they give almost all of it back to the atmosphere on a short time scale as leaves and wood decay. Biochar is virtually immune to microbial attack and so if some of the carbon captured by plants can be siphoned off into the form of biochar, it will stay out of the air.

If produced and buried on a large scale, biochar could significantly lower the carbon dioxide content of the atmosphere. James Lovelock, in his book, The Vanishing Face of Gaia, (p. 151) says that massive production and burial of biochar, which would also produce, not consume, energy is “the only realistic proposal by which we have even a chance of restoring the Earth to the state it was in before we started using fossil fuel.”

The key question is whether biochar production could be scaled up to a dimension of planetary significance. Some are skeptical. Stuart Staniford, in his blog Early Warning, (http://earlywarn.blogspot.com/2010/03/scalability-of-biochar.html)
argues that only a few gigatons of carbon could reasonably be sequestered each year in the form of biochar, and this would not be a panacea for even the current level of 8.5 gigatons of fossil fuel emissions. Johannes Lehmann, in his 2007 article in Nature (vol. 447, pp. 143-144) provides some numbers that are more optimistic. He points out that in the U.S., the carbon in forest residues and crop residues amounts to about 0.5 Gt/y, and if currently idle farmland was converted to high production woody plants, another 0.25 Gt/y carbon could be harvested. This is for the U.S. only, which accounts for a relatively small portion of global biomass production.

It is possible that not only crop and forestry residues and production from marginal lands, but also production from some of the massive acreage that’s currently devoted to feed grains or corn for high fructose corn syrup could be converted to biochar production. And fossil fuel emissions could be cut by major conversion to nuclear power. Then, biochar could cancel out much of humanity’s fossil fuel emission, or even go beyond that and start to bring down the carbon content of the atmosphere, while at the same time producing useful energy. On paper, this seems do-able with existing technology, and that’s reason enough to take a closer look.

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