I lack all relevant scientific expertise, but biochar sounds very promising to me. Basically, the idea is that you burn “biomass” (crops, trees, used paper, kitchen waste, etc.) with minimal oxygen. You can accomplish this by getting a fire going and then covering the biomass with soil while it smolders–the ancient technique–or by burning it in a special kiln, or even by microwaving it. This process produces the following products, in a ratio that you can control:
1. A stable form of carbon that will not return to the atmosphere for at least hundreds of years. This product also makes an excellent fertilizer.
2. Various valuable chemical byproducts.
3. Three forms of fuel: solid (charcoal), liquid (oil), and gas.
If all we wanted to do was mitigate global warming by removing carbon from the air, we could grow crops (which pull carbon out of the atmosphere), burn them in kilns, and store vast quantities of biochar in its stable form. But that’s expensive to do on a massive, global scale. The other uses of biochar–as fertilizer and fuel–make it economically valuable.
When people burn biochar as fuel, they do put carbon back in the atmosphere. But the fuel is highly efficient, and you can keep the residue as stable carbon. The result is a fuel that actually lowers atmospheric carbon when you combine its production and its use. If we substitute biochar for coal or oil extracted from under the earth and then burned, the benefit is huge. Likewise, if instead of creating arable land by setting rain forests on fire, we turn trees into biochar fertilizer, we can produce productive farmland with dramatically less damage.
Biochar could be produced on an industrial scale by firms or agencies that would sell biofuel to replace fossil sources, such as coal and oil. It could also be produced by households or villages for their own local use. That opens the possibility of a decentralized process that could be socially empowering. To get this going, it might be helpful (I’m speculating here) to invest public funds in developing new kilns and processes.
Nothing is a panacea, and some skeptical points are listed here. But overall, this sounds like the most promising strategy I’ve heard of.