As everyone knows, a lot of scientists are extremely concerned about global warming. Evidence suggests that the high levels of so-called greenhouse gases produced over the past half-century or so will result in higher temperatures worldwide over the coming decades. The additional heat could melt polar ice and raise the level of the ocean, causing flooding and eroding coastlines; it could also lead to more severe climate change with potentially devastating effects. Other scientists say that worries about global warming are overblown—that the temperature will not rise significantly (at least, not due to human activity), and that in any case, the results of a slightly increased average temperature would be mild rather than disastrous.
But no one disputes that the air has become quite polluted—you can verify this easily by looking out your window. One major component of air pollution is carbon dioxide (CO2), which is produced as a waste product when fossil fuels are burned. The level of carbon dioxide in the atmosphere has risen markedly since the beginning of the industrial age, and even if that change is not completely attributable to human progress, it’s not a good thing. Whether or not human-generated CO2 contributes to global warming, it clearly causes other problems.
Convincing the people and governments of the world to reduce the use of fossil fuels is essentially a lost cause, so the latest trendy approach to dealing with all that excess CO2 is capturing it as soon as it’s created and then disposing of it somewhere. Equipment installed at power plants and other major industries can separate the carbon dioxide from other waste products and liquefy it for temporary storage; finding a long-term home for massive amounts of the gas is the real problem. The process of storing carbon dioxide permanently in such a way that it cannot escape back into the atmosphere is known as carbon sequestration (or, sometimes, carbon dioxide sequestration). Broadly speaking, there are two places one might put large quantities of unwanted carbon dioxide—in the oceans or underground. Techniques for getting the CO2 to its putative final resting place (and keeping it there) are in varying stages of experimental development.
The world’s oceans already absorb unfathomable amounts of CO2; some researchers believe they could hold a great deal more with a little help. The upper part of the ocean typically has a fairly high concentration of CO2 (absorbed directly from the atmosphere), but at greater depths, the concentration is much lower. So one way to dispose of CO2 is to inject it into deep ocean water. At depths over 3,000 meters or so, liquid or solid CO2 is denser than the surrounding water, meaning that it could sink all the way to the ocean floor. Closer to the surface, it will dissolve into the water. Dissolved CO2 makes the water acidic, with unknown (but likely detrimental) effects on marine life. Liquid CO2 on the ocean floor may react with minerals there and form solid precipitates—or it may simply kill off organisms already living there.
Merely burying CO2 is not good enough; in order for it to stay put, it has to be stored very deep in the ground, and somewhere that the gas cannot escape into the atmosphere. Some possibilities include:
- Saline Aquifers: An aquifer is a porous layer of rock that holds a large quantity of water—often saltwater. Inject CO2 deeply enough into an aquifer, and the surrounding pressure keeps it in liquid form. Meanwhile, an impermeable layer of solid rock above prevents the gas from being released back into the atmosphere. Although aquifer storage is expensive, it is likely to have less impact on the environment than ocean storage—and the CO2 can remain safely underground, theoretically, forever.
- Oil and Gas Reservoirs: If you can put carbon dioxide into an aquifer, you can also put it into a depleted gas or oil well. In fact, the technology to deliver CO2 into such wells has been in use for decades; pump CO2 into an oil well, for instance, and you can push out extra oil that would otherwise be unreachable. As long as the CO2 is stored deep enough, it will remain as a liquid.
- Coal Seams: Most of the world’s coal deposits are located too deep in the ground for mining to be practical. When CO2 is injected into coal seams, the coal absorbs the gas. Meanwhile, in a manner similar to enhanced oil recovery, the process also pushes out methane gas, which can be used as a fuel.
And then, of course, there’s a natural CO2 storage apparatus: forests. Trees are incredibly effective at absorbing carbon dioxide and creating oxygen, so planting (or replanting) millions of acres of forest could go a long way toward solving the CO2 problem—no drilling or high-tech research required. This is not technically sequestration, as you wouldn’t manually inject previously collected carbon dioxide into a tree—but it does have essentially the same net effect.
Politicians hope carbon sequestration—of one kind or another—turns out to be a magic bullet that can appease consumers, energy companies, and environmentalists alike. Although all the potential terrestrial CO2 storage spots show some promise, the safety, capacity, and long-term effectiveness of carbon sequestration is ultimately unknown. At best, it will address only a small portion of the world’s pollution problem; at worst, we may find that something we thought we buried comes back to haunt us. On the other hand, we certainly take a lot of carbon out of the ground. Putting some back in just may balance the scales a bit. —Joe Kissell
Thanks to Carole Walker for suggesting today’s topic!
Overviews and comparisons of major carbon sequestration methods can be found at:
- Capture and Storage of CO2 at the the IEA Greenhouse Gas R&D Programme (IEAGHG)
- Where CO2 Goes to Die in Wired
- What Is CO2 Sequestration? at CO2 Capture and Storage
For more information on carbon sequestration, see: