Carbonizing the Crust
One of the hopes of those who want to continue to burn hydrocarbons until there are no hydrocarbons is some kind of carbon storage. There are loads of problems with this, such as somehow pulling out CO2 from the atmosphere or from smokestacks (seems an unlikely way to reduce CO2 emissions from cars, for example). A large one is the nature of the stuff: it is a gas, and while there are natural gas reservoirs in the crust that apparently have been present for millions of years, it is unclear how many of them can be robust recipients of the large volumes of carbon dioxide we’d pump down: basically, by increasing the pore pressure, the CO2 would generate fractures in nearly the same way that fracking is used in oil and gas development; such fractures risk breaking the seal isolating the gas from the surface (see, for instance, the article by Zoback and Gorelik in 2012).
If you can’t simply pump CO2 into the ground, maybe you can make it react with something to form a more stable material. For instance, one proposal is to fertilize the ocean with iron and hope that the CO2-rich shells from a bunch of tiny sea creatures (mainly diatoms) would then sink to the ocean bottom; while such experiments have yielded biological results, the overall rate of CO2 removed from the atmosphere depends critically on the biology of the site used; the long-term residence of the marine carbonate remains to be calculated. Other materials will react with carbon dioxide, either in the atmosphere directly or in a carbonated fluid. Some of these proposals seem doomed simply by scale: the rate of carbonization of olivine sand, for instance, is pretty darn low, so making a lot of olivine sand (which would take a lot of energy) is hardly likely to help.
If you want to go this route, you need something that will react fast. The New York Times just covered the ongoing CarbFix experiment in Iceland which pumps CO2 rich water into basalt. Their claim and hope is that these reactions are so fast that they are potentially economically feasible. This approach has some points in its favor. Basalt is a pretty widely available rock, unlike olivine. Injection of brines into the ground is a very mature technology (and growing more mature with the applications for oil and gas development in the past decade). There are no dramatic surface impacts (such as altering marine ecosystems or mining enormous quantities of olivine) or effects that would tend to place other nations at a disadvantage (can’t use it as a sort of climate weapon). But it would still require harvesting CO2 from emissions or the atmosphere itself. If you rely on emissions, you want your power plant on a bunch of basalt.
Will it work? Obviously the test plants are to try and answer that, but the real test would be trying to make it pay. Right now, there is no money in the US for sequestering carbon. Enact a carbon tax that includes a carbon sequestration credit, be careful to be able to verify sequestration, and then let the marketplace help you out. Maybe this is cheaper than ending carbon emissions directly, maybe not. Right now it is awfully hard to know, and as many have suggested, trying as many avenues as we can is probably the best way forward.