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11 February 2009 3:33 pm
There's too much carbon dioxide in the air. Why not just remove it manually?
A handful of scientific teams at universities and companies are trying to build machines to do just that, creating what are essentially mechanical trees that manually lower greenhouse gas concentrations. But it's an extremely expensive proposition, with experts estimating that the process could cost more than twice as much as directly capturing carbon before its emitted from smokestacks. That's because it's thermodynamically easier to grab carbon dioxide molecules in a concentrated stream—say, coming out of a full-steam-ahead 500 MW coal plan—than out of thin air, where makes up only about 400 ppm.
So with thousands of coal plants around the world spewing carbon dioxide with impunity and a Herculean effort required to grab those emissions, why look at air capture at all? Experts fear talking about air capture could distract the world from the tough political choices it needs to make to stop spewing so much carbon pollution. And would the money be better spent on carbon capture technology, solar panels, or developing other climate-friendly solutions?
Last week science policy polymath Roger Pielke Jr. of the University of Colorado gave an answer in a paper due to be published this spring in Environmental Science and Policy.
The reason is that air capture wouldn't be much more expensive than mitigation—on a time scale of 50 years or more—and might be required in a global crisis. The Intergovernmental Panel on Climate Change and others expect the world economy to grow like gangbusters between now and 2050. One option is to spend a lot of money to green the world energy economy right now. Pielke is raising another theoretical option in his thought experiment: Do research now and use air capture later. Sucking carbon out of the air decades from now means that the technique will be a lot cheaper, as a fraction of the world economy, than it is today. "It's a useful service to be showing what role air capture can play," says Frank Zeman of New York Institute of Technology in New York City, an engineer who has studied air capture. "It's an economic shell game.”
To understand that shell game, it's important to first keep in mind that, while they're expensive, there are already proven methods to suck purified carbon dioxide gas molecules out of the air. The U.S. Navy does it on submarines and NASA does it on spaceships. Both use pricey filters that must be replaced frequently. More experimental methods look promising, though costly. A University of Calgary team uses liquid sodium hydroxide, which reacts with carbon dioxide in a tubular design akin to SO2 scrubbers that power plants use. The process requires a kiln running at 900°C to recycle the reagents. Swiss scientists are experimenting with thin tubes heated by solar reflectors in which chemicals grab carbon. The groups estimate costs between $100 and $140 per ton of CO2 grabbed.
By contrast, scientists at the Massachusetts Institute of Technology estimate that carbon capture from coal power plants will eventually cost $30 a ton of CO2. So how does Pielke give air capture a fighting chance? He calculated how much it would cost to operate air capture facilities to stabilize atmospheric CO2 levels at 450 ppm and 550 ppm, levels climate scientists have set as targets for global stabilization of the problem. His trick was to suppose that devices wouldn't be deployed until they're absolutely necessary—decades into the future, perhaps in response to an absolute climate crisis. Using simple arithmetic, he estimated that using air capture to get to 450 ppm by 2050 would cost between 1.5% and 3% of the global GDP. No doubt, that's many trillions of dollars. "It's likely that air capture will be [ton-for-ton] the most expensive way to tackle the climate problem," added Zeman.
But achieving 450 ppm with other means would be extremely expensive, too. In its last report, the IPCC estimated that the cost would be 5.5% of the global GDP in 2050 on the high end and -1% on the low end (with some mitigation efforts, such as energy efficiency and saving people money). The U.K. government's commissioned Stern Review of Climate Change estimates that stabilizing global emissions at 450 ppm by 2100 would cost 1% of the global GDP, plus or minus 3%. Pielke's estimates for air capture fall within these ranges.
The trick is that by using air capture instead of mitigation in this back-of-the-envelope exercise, Pielke allows the world economy to grow between now and 2050—without the albatross of mitigation on its back. Then, with an economy that has grown robustly in the interim, he'd deploy air capture, what he calls "the brute force backstop."
And there's the possibility that air capture could be much cheaper. Klaus Lackner of Columbia University says that his technique, which he patented this year, could eventually allow air capture at a cost of $30 per ton of CO2. He acknowledges that obtaining CO2 from coal power plants will always be cheaper than pulling it out of the diffuse air, but he says that difference may be small and will get smaller in the future. "Plus, what are you going to do about all the carbon dioxide emitted from cars?" he says. (Theoretically, hybrid cars that power up at night could get carbon-free energy if the power plants providing electricity are capturing their carbon. But that's not likely to happen soon in the United States, let alone in rapidly expanding China and India.)
Pielke is not suggesting that the world should avoid mitigating emissions of carbon dioxide between now and 2050. "We should be trying all forms of mitigation as fast as we can," he says. Many scientists fear that rapidly accelerating global warming, long before the atmospheric level of CO2 reaches 450 ppm, could lead to devastating sudden shifts in the world climate, so Pielke knows it could be foolish to wait. But he thinks his work shows that air capture should “at a minimum … receive the same detailed analysis as other approaches to mitigation." No government is funding research into the technique on any significant level; the handful of scientists who are studying the method are mostly using private funds. It's a backstop, he says, that humanity has yet to develop. Whether or not talking about air capture will dull efforts to mitigate global warming is another story.