Reducing Trapped Radiation: Fossil Fuels Without the Emissions
An entirely different set of strategies to reduce the amount of trapped radiation focuses on the greenhouse effect rather than surface or oceanic modification. These strategies would effectively reduce the quantity of greenhouse gasses that make it to the earth's atmosphere. This is not to say that overall production of greenhouse gasses would decrease, but rather the emissions into the atmosphere would decrease. These strategies focus on sequestering carbon dioxide in terrestrial or geologic formations.
Terrestrial sequestration is pretty straightforward, at least in concept, and simply takes advantage of the fact that most plant life on earth demands carbon dioxide to survive. The more carbon-consuming plant life, the less carbon dioxide reaches the earth's atmosphere. Thus, there should be less warming. In practice, measuring the amount of carbon sequestered through terrestrial means is complicated. Because terrestrial sequestration is considered a "Clean Development Mechanism" under the Kyoto protocol, the process of determining which terrestrial practices actually sequester carbon (and how much carbon has been sequestered) has become somewhat politicized. Increasing forest cover, for example, certainly helps to sequester carbon. But what about agricultural land? Some have argued that simply having arable soil counts as a method of carbon sequestration since the soil itself will absorb some amount of CO2 emitted into the atmosphere (how much is, again, a subject of some debate).
Even more contentious has been the proposition that raising crops for fuel should be considered a method of sequestering carbon. If you think about it for a minute, you can see where the controversy comes from -- if bio-fuel crops absorb CO2 while they are growing, but then that CO2 is released when the crops are burned, isn't the effect a "net zero" greenhouse gas emission rather than an absolute reduction? Even more complicated, there are some types of agricultural practices that themselves produce large volumes of CO2 (examples would include the heavy use of fertilizers, which are produced using natural gas, or extensive tilling of arable land, which releases CO2 stored in the soil). The University of California at Berkeley has a paper that describes in some detail how market structures and agricultural practices can affect the "sequestration value" of terrestrial carbon storage, specifically for biomass transportation fuels.
A. Farrell, et al., "Ethanol can Contribute to Energy and Environmental Goals," Science vol. 311 (2006), pp. 506-508.
Remember in our discussion of albedo enhancement that simply reducing the amount of radiation that is absorbed by the earth is probably not, in and of itself, sufficient to mitigate the effects of CO2 emissions on the environment (remember the example of coral bleaching). That sort of mitigation would require reducing emissions themselves. Utilizing alternatives to fossil fuels is certainly one way to achieve this goal. Another is to continue to burn fossil fuels but to capture the CO2 and then store that CO2 in a geologic formation for a long period of time.
While there are some emerging technologies that would remove CO2 from the atmosphere directly, most proposals for Carbon Capture and Sequestration (CCS) would capture carbon dioxide from large sources like power plants or industrial facilities. Since there are relatively few of these facilities (compared to, say, automobiles) and each facility generally produces a lot of carbon dioxide -- a single coal-burning power plant might produce several million tons per year -- they represent the low-hanging fruit for carbon capture. Carbon capture can be achieved by chemically separating the CO2 from a fossil fuel (usually coal or natural gas) either after the fuel is burned or before the fuel enters the power plant combustion chamber. The first of these options, "post-combustion" CO2 capture, is generally employed in the few carbon capture sites currently operating.
Earth: The Operators' Manual
Carbon sequestration is one of the few geoengineering technologies with which society has a lot of experience. But the deployment of technologies to capture carbon dioxide from large polluting sources, like power plants or industrial plants, has been slow. The following video shows how one of the largest greenhouse-gas emitters on the planet - China - is taking the initiative to pilot the use of carbon capture on its coal-fired power plants.
Video: The New Empire of Cleantech (10:58)
Once the CO2 is chemically separated, it must be shipped, usually via pipeline, to the point where it will be injected deep underground. People often call these underground storage sites "reservoirs" but they aren't reservoirs in the sense that you might think of it (like a reservoir is a lake of water behind a dam). Carbon sequestration is not that like putting CO2 molecules in a jar. Once the carbon dioxide is injected deep underground we rely on chemical reactions or other geologic processes to keep the CO2 from sneaking back into the atmosphere. There are generally two types of underground places that are considered to be good candidates for carbon sequestration.