Red, theory; black, fact.
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.
I would add here that 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.
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.
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.)
Note that the use of any terrestrial artificial mirror membrane has the drawback that the membrane will get dirty rapidly from dust, pollen, and plant parts, thereby reducing its efficiently. However, a living means of light reflection would renew itself, gratis, each year. Orbiting space mirrors have also been proposed as the solution to global warming. They wouldn't get covered in pollen anytime soon -- just shot full of holes by micrometeorites.
