Red, theory; black, fact
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| Dusty miller, Senecio cineraria, annual, prefers full sunlight. |
Bury Charcoal to Mitigate Climate Change
Reading "Just Cool It!" by Suzuki and Hanington introduced me to the ancient terra preta agricultural technology, given as a possible solution, or part of the solution, to the global warming problem. The term is apparently Portuguese for "black earth" and the technology involves enriching the soil by ploughing it full of charcoal. Suzuki and Hanington make the point that this should sequester a lot of carbon in the soil, thereby taking it out of the atmosphere. Charcoal is nearly pure carbon. Moreover, charcoal, being indigestible to decay organisms, should stay in the soil for a very long time. The logical raw material for making the charcoal would be either wood from clearing the land for agriculture, or crop residues, the parts of the crop plant that people cannot eat.
Making Charcoal at Scale
In modern pyrolysis plants, not only is charcoal produced, but also flammable off-gasses, which could be used for fuel directly in some future scenario, or catalytically reformed to a liquid fuel for running the tractors and combines. In gaseous form, the fuel could run a steam turbine to produce electricity to supplement that from wind farms, hydro, tidal, geothermal, thorium-nuclear, and photovoltaics.
However, the off-gasses are also used to fuel the pyrolysis plant itself. Whether any would be left over for other uses would depend on careful plant design for energy efficiency and on avoiding fuelled drying operations. Thus, the feedstock should be sun-dried.
Competing Schemes
Schemes like second-generation power ethanol are touted as carbon-neutral, but in terra preta with these additions, we have one that is actually carbon-negative.
Unintended Consequences
However, the soil ends up black. No other color is as efficient at converting sunlight into heat, which we don't need more of at this point. This seems to be a problem with the terra preta solution. (The ideal color for avoiding heat production would be white.)
Mitigation
The use of any terrestrial artificial mirror membrane to pull the average reflectivity of the earth back up after terra preta implementation has the drawback that the membrane will get dirty rapidly from dust, pollen, and plant parts, thereby reducing its efficiency. (Commercial photovoltaics have the same problem.) Orbiting space mirrors have also been proposed as the solution to global warming but maintenance would be an issue there also; even in space, there are high-velocity particles that can cause wear and tear over time.
However, a living means of light reflection, like a low understory of predominantly white plants (e.g., lamb’s ear, dusty miller, or bugleweed), would renew itself, gratis, each year. Plants on land have always had the problem of keeping their leaves clean and by now natural solutions will have evolved (Photovoltaics manufacturers, take note).
An alternative mitigation would be to set aside a portion of the crop residues with which to cover the soil for improved light reflectivity. If these residues are left on the field over the winter, they might become snow-bleached in the spring when the snow melts, which releases the bleaching agent ozone. This would further improve the reflectivity of the fields.
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| Bugleweed, Ajuga reptans, “Princess Leia” cultivar, perennial, chokes out weeds, prefers shade. |
