Planting trees and mechanical air capture of carbon are better than injecting SO2 into the stratosphere, which again is better than seeding the oceans with iron dust.
Fundamental thermodynamic principles say that to collect CO2 and store it under high pressure, a system must do the mechanical work needed to compress the gas starting from its low, atmospheric partial pressure. (Doing better than this would be equivalent to building a perpetual motion machine of the second kind.) To capture and compress all the anthropogenic excess CO2 would require about 1021 Joules. That is roughly the amount of energy that all the world's electric power plants produce in a decade.
This would be clean and easy, if we were good at making things like solar-electric power systems, but that's on the other side of a major technological revolution. (By "good at making things", I mean "as easily as producing the same weight of hay.") Words and ideas I offer here may be used freely and without attribution.
Perhaps we could couple it to mechanical air capture, increasing the carbon content of soils, and reforestation, depending upon how quickly we need to hit the break. Boyd says the following in the Q&A on ieee spectrum:
IEEE Spectrum: Q&A With: Ecologist and Geoengineering Expert Philip Boyd
Spectrum: Which schemes are the most promising and why? PB: The one I favor the most is atmospheric carbon capture. That is the direct capture of carbon dioxide using some sort of scrubbing system. There have been proposals from U.S. researchers where they've described a medium-sized water-tower structure with a turbine system. As the wind blows through, there is a scrubbing device, which has a chemical absorbent that absorbs the carbon dioxide. Then the carbon dioxide can be converted into a solid and stored so that it can't impact climate. And with these towers, it is possible that they could be incorporated with wind turbines, so you're actually generating power to help drive the process. The reason that one gets a vote is, although it's one of the more expensive ideas, it is much lower risk. In terms of an emergency stop, it can be shut down quickly, and you can verify how much carbon is being removed.
Spectrum: Which schemes are the most promising and why?
PB: The one I favor the most is atmospheric carbon capture. That is the direct capture of carbon dioxide using some sort of scrubbing system. There have been proposals from U.S. researchers where they've described a medium-sized water-tower structure with a turbine system. As the wind blows through, there is a scrubbing device, which has a chemical absorbent that absorbs the carbon dioxide. Then the carbon dioxide can be converted into a solid and stored so that it can't impact climate. And with these towers, it is possible that they could be incorporated with wind turbines, so you're actually generating power to help drive the process.
The reason that one gets a vote is, although it's one of the more expensive ideas, it is much lower risk. In terms of an emergency stop, it can be shut down quickly, and you can verify how much carbon is being removed.
Unfortunately, the first tentative results I know of have been mixed and the energy needed to pump CO2 underground remains.
The geological packages under consideration are salt and rocks, mostly. It all depends on reaction rates - and finding those out is part of the research. There's very little known on the kinetics of mineral formation at non-atmospheric pressures and temperatures.
Will know more in January; apparently there's a publication in the pipeline.
