A semi-recent issue of High Country News carried a feature on the deep-rock carbon sequestration potential in the northwestern U.S.: it's maybe possible to inject CO2 captured from power plants into the basalt that underlies the region, producing inert calcium carbonate. If so, there's apparently enough basalt to capture centuries of the region's carbon emissions.
It's safe to say the research has its doubters. And carbon sequestration in general deserves the hairy eyeball: even if proven both ecologically and geologically viable and economically feasible, if it leads to the continued destruction of Appalachia and vast tracts of the West for coal, count me out.
Elsewhere, a study's findings added to the body of evidence that shellfish, like clams, oysters, and mussels (oh, and plankton, crustaceans, and corals), will start growing more slowly or dissolving altogether due to anthropogenic ocean acidification (from all of the excess CO2 we produce that goes into oceanic solution), which would dissolve their shells. Fewer/smaller/weaker shellfish would have economic effects, but also much greater impacts on marine life: they're an important food source for everything from fish to whales and birds.
My point? These critters fix carbon ("biosequestration") in their shells, so we could start losing an important piece of the ocean's ability to maintain its natural alkalinity, plus its tendency to sequester carbon, just when they're most needed.
My disinterested and clear-eyed proposal, then, is increased aquaculture of mollusks in bays, sounds, estuaries, sloughs, etc. We're already growing tens of millions of pounds of clams alone each year in the U.S., and unlike most other forms of aquaculture, you don't get the massive energetic losses like with the feeding of fish meal to top-of-the-food-chain finfish.