Carbon Dioxide (CO2) or Carbon Sequestration

Carbon dioxide sequestration is the term given for locking up CO2 somewhere other than the atmosphere, it may also be called carbon sequestration because the carbon dioxide may be broken down into its component parts, and the carbon stored away while the oxygen is released to the atmosphere. There are two kinds of sequestration, biological and geological. In biological sequestration, carbon is naturally sequestered in plants, soils, and in ocean life. Geological sequestration is the storage of carbon dioxide directly in rocks or underwater. If all this sounds far-fetched, consider that until recently huge amounts of carbon were safely sequestered in underground hydrocarbons such as coal, oil and natural gas, and of course climate change is a result of humans undoing this process. Carbon sequestration (or re-sequestration) will probably be a useful tool in reducing climate change, but it must not be looked at as an alternative to other measures. Sequestration alone can't solve this problem.

Geological Sequestration

In 2004, the CEO of Royal Dutch Shell Ron Oxburgh told the Guardian newspaper that "no one can be comfortable at the prospect of continuing to pump out the amounts of carbon dioxide that we are at present." His answer to the problem? Carbon sequestration. Oxburgh went on to say that "sequestration is difficult, but if we don't have sequestration then I see very little hope for the world. I don't see any other approach."

What Oxburgh and many others are suggesting is that carbon dioxide (CO2) released from burning fossil fuels (usually at large point sources such as power plants) be pumped underground, or underwater, in areas where it is likely to remain stored for millennia. Candidate areas include depleted oil and gas reservoirs, saline formations (layers of porous rock that are saturated with brine), and deep in the ocean. The most promising areas are depleted reservoirs that once held oil and gas securely for millions of years. It is believed that these areas will be able to safely contain carbon dioxide for very long time periods, as they once held hydrocarbons. This still needs to be tested, and a number of pilot projects are injecting carbon dioxide into abandoned wells and monitoring the results. To date, this type of sequestration looks promising. Saline formations are less well understood, but more widespread than oil and gas reservoirs. Using saline formations to sequester CO2 would mean less piping of the gas over long distances, but it isn't known whether it would be contained for the very long term. The other potential long-term storage place for CO2 is deep in the ocean. If it were deep enough in the ocean, carbon dioxide would actually turn into a liquid that sinks beneath the water. The technological challenge of doing this on a large scale might rule it out, and there is a problem of ocean acidification when carbon dioxide is dissolved in water, though this is likely to be a greater problem if it is dissolved in sea water at shallower depths vs as a liquid in deep water. No matter how it is done, injecting CO2 into the ocean could have complicated and unpredictable effects.

More information: U.S. Department of Energy

Biological Sequestration

Carbon is also stored in forests, soils, bogs, in ocean ecosystems, among other places. In many cases we are undermining carbon storage rather than enhancing it, by degrading soils, cutting down forests, polluting and acidifying the ocean, etc. Other than the important task of trying to minimize the damage to ecosystems, there are a few promising ways we might try to enhance nature's ability to trap carbon dioxide. One is to fertilize the ocean with micronutrients to promote algal growth. For instance, parts of the ocean are rich in nutrients like nitrogen and phosphorus but low in iron, and as a result, low in phytoplankton. Adding even small amounts of iron can allow the plankton to grow and fix carbon dioxide, using the power of the sun. Some commercial fisheries already drop iron filings overboard to feed the ecosystem that the fish feed on, and may inadvertently be fighting global warming. A far less high-tech way to promote biological sequestration is to encourage organic farming practices, increased organic inputs to farm soils, and low-tillage farming systems. Not only do organically rich soils sequester CO2, they also have higher crop yields and lower fertilizer input requirements (also reducing CO2 emissions). A 2004 article in Science concluded that "As well as enhancing food security, carbon sequestration [in soils] has the potential to offset fossil fuel emissions by 0.4 to 1.2 gigatons of carbon per year, or 5 to 15% of the global fossil-fuel emissions."