Friday, August 25, 2006

A Bigger Sink

The idea of capturing our carbon dioxide emissions before they get into the atmosphere is becoming increasingly mainstream. The prospect of practical carbon sequestration has changed the view of at least one major environmental group towards the use of coal, otherwise the most carbon-intensive of our fossil fuels. Historically, there was only one way to tie up carbon dioxide, by growing vegetation. More recently, two other avenues have shown potential, by transporting compressed or liquefied CO2 into geological formations or to deep ocean waters. A recent article from MIT's Technology Review suggests an improvement on undersea storage, injecting it into porous seabed sediment. If the long-term stability of this method proves out, it could greatly enhance the economics of undersea sequestration and dramatically expand its potential capacity.

It appears likely that no single sequestration strategy will dominate; sources of greenhouse gases are too diverse. Re-forestation remains a useful tool, with benefits beyond the capture of carbon, but its duration is typically limited to a century or two. Geological sequestration depends on underground formations such as depleted oil or gas reservoirs that aren't always conveniently located. Having a non-geological, sea-based option makes enormous sense, given the global distribution of coastal population and industrial centers.

Earlier versions of this approach raised serious concerns among environmentalists. Simply depositing compressed CO2 into the deep ocean could make the local ecology more acidic, and some of this CO2 would ultimately find its way into the atmosphere, reducing the sequestration benefit. My favorite antidote to a runaway greenhouse effect, seeding the oceans with iron and tying up CO2 in the resulting plankton "bloom", could have even more serious side effects.

The idea of depositing CO2 into seabed sediments, at temperatures and pressures that would keep the CO2 inertly in place, avoids most of the pitfalls of these other approaches. It is only feasible because of technology developed for deepwater oil drilling. The article makes it clear that this technique won't be inexpensive. For that matter, separating carbon dioxide from power plant exhaust isn't cheap, either, nor is compressing it sufficiently to ship it to the nearest reservoir or to pump it under the sea. But it could keep the fossil fuels upon which we rely economical enough in a carbon-constrained world for us to successfully shift to low- or zero-carbon sources of energy.

Therein lies the biggest argument that will be used against this approach, and against sequestration in general. Not only does it fail to force us off fossil fuels rapidly, but it enables their continued use during a longer, less disruptive transition to something else. Sequestration is a crutch for the carbon economy, no question, and some see that as inherently bad. I prefer to see it as the sine qua non of adapting our high-energy society to a warming world, without cutting the food-and-fuel lifelines of millions or billions of people.

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