Now that the Bush administration has admitted something every thinking person already knows--that the world is warming--and now that the administration is fully behind the search for alternative fuels, we can expect more talk about the benefits of carbon sequestration.
For some people carbon sequestration offers the best of all possible futures. Presumably, we could continue to burn plentiful coal supplies long after oil and natural gas production declines. With the proper technology we could then capture the resulting carbon dioxide (the main greenhouse gas) before is it released into the atmosphere. This would supposedly give us the necessary breathing room to make a gradual transition to a renewable energy economy with a minimum of disruption.
The initial schemes for sequestering carbon such as placing it in caverns underground sound easy enough. But several concerns arise. Can leaks be avoided, not just immediately, but over a period that might last many thousands of years? Will the carbon dioxide react with the surfaces of the cavern opening up pathways to the air above? Such reactions have been noted, but whether they would ultimately result in substantial leaks remains unknown. Can all the existing drill holes into such a cavern be found, adequately sealed and monitored for an indefinite period of time? In other words, who will be babysitting these sites 100, 200, even 500 years from now?
One sequestration scheme already in use stores carbon dioxide in an aging oil field. The injected CO2 is used to force out the remaining hard-to-get oil. As clever as this seems, it's worth asking whether any net reduction in greenhouse gasses occurs since the recovered oil is ultimately burned.
Yet another land-based method involves injecting carbon dioxide into underground saline aquifers which will presumably never be used for human consumption. The supposed advantage of this method is that saline aquifers are widespread whereas suitable caverns and old oil fields are not nearly as widely or conveniently located. No one really knows whether this would result in permanent storage or whether the drill holes used to reach the aquifers or other holes intentionally or inadvertently drilled in the past or the future can be adequately monitored. Even the extent of an aquifer and its communication with other aquifers and with the surface are difficult to ascertain.
Another often touted method is to use the mineral serpentinite which will combine with CO2 from power plant flues to form a stable magnesium compound. Energy costs and storage problems could be substantial. And, while serpentinite is relatively abundant, it's not clear whether there would be enough of it with sufficient concentrations of magnesium to handle the projected need.
The open sea apparently offers even greater possibilities including pumping carbon dioxide to the bottom of the ocean. At great depths the pressure and low temperature would maintain the CO2 in liquid form which is heavier than water. Presumably the liquid carbon dioxide would simply hang out for millennia. The financial and energy costs of gathering and transporting the CO2, perhaps in liquid form using an elaborate pipeline system, seems to be the major hurdle. In addition, what we know about the bottom of the oceans is probably less than what we know about the nearest star. Can we really be sure that the CO2 will stay where we put it?
As I said above, all of this effort is focused on allowing us to use coal for the foreseeable future. The coal infrastructure is already in place, and it works to produce more than half the electricity in the United States. What is not considered is the use of coal to make liquid fuels which would replace those now provided by oil. Coal-to-liquids, as it is called, is a very carbon intensive process. Even if the carbon emitted from the coal-to-liquids refineries were captured and sequestered, there is currently no practical way to capture and sequester carbon from a moving vehicle. This is no small matter. In the United States vehicles create 27 percent of all greenhouse gas emissions.
Given the uncertainties, the costs and the alternatives available to us--conservation, efficiency, wind and solar--does it make sense to build a hugely expensive sequestration infrastructure that will essentially be a giant subsidy for the coal industry? Can we even be sure that exponentially increasing rates of coal production could be sustained? In other words, would we not be bringing forward a peak in coal production, perhaps by the middle of this century and then face all the same questions about a transition to a non-fossil fuel economy? Finally, given that we cannot guarantee the successful long-term sequestration of CO2, would it be moral to commit future generations to such a risky path?
When you think all this through, there is one method of carbon sequestration that stands head and shoulders above the rest. Leave the carbon in the ground to the greatest extent possible and get on with the project of creating a genuinely sustainable society.