Compared with trying to counteract climate change resulting from greenhouse gas pollution through solar radiation management (SRM) — essentially reflecting sunlight away, as with stratospheric sulfate injection — actually removing CO2 from the atmosphere by weathering rocks which form carbonates seems more attractive in many ways. The SRM approach may cause major side effects in terms of changes in precipitation, and any cessation in the injection of reflective aerosols in the upper atmosphere would lead to very abrupt climate change.
I asked David Keith about the idea when he was in Toronto talking about SRM-based geoengineering and he said that the problem is simply one of reaction rates. Even if we used zero-carbon energy to grind up vast amounts of ultramafic rock to absorb CO2, that process of absorbtion would happen so slowly that it would not counteract human-induced climate change on reasonable timescales.
I learned about the idea from Wallace Broecker and Robert Kunzig’s book Fixing Climate. Another oft-touted means of removing atmospheric CO2 is biochar. More recently, I read about the idea of speeding up natural rock weathering by biological means. I don’t know if this could somehow overcome Keith’s objection about reaction rates.