It's tough slogging these days for people interested in capturing greenhouse-gas emissions at the source and burying them in deep rock formations, or, short of that, figuring out how to suck CO2 out of the atmosphere once it gets there.
What to do while research continues on the feasibility (or infeasibility) of these approaches and the world still struggles to reduce emission in the first place? Bundle up crop residue -- straw, corn stalks, and their ilk -- and heave it overboard in the deep ocean.
That's the reply that scientists Stuart Strand and Gregory Benford have offered up in a recent policy paper in the journal Environmental Science and Technology. They calculate that deep-sixing crop residue worldwide could slow the current global annual increase in CO2 emissions by 15 percent.
The benefits, they argue? The approach sequesters far more carbon per unit of carbon emitted by the process than do alternative "green" uses for the residue. The carbon goes to deep-sea sediments. It stays put for millennia. And no new technology is needed. That means it's workable today for developing countries as well as rich countries. Just bundle, ship, and dump.
If you've made it this far, you probably have guessed at some of the reactions already. In many circles, the idea is receiving the kind of welcome that swimmers in an overcrowded pool would offer to a hefty diver who just finished a cannonball.
The biofuels folks have their sights set on crop residue as one source of feedstock to make cellulosic ethanol. Soil scientists argue that residue plays an important role in locking carbon up in soil. And some marine scientists respond with a variation on the old Jimmy Durante line (spoken with a disapproving shake of the head): "Everbody wansta get intta the act." In this case, the act of dumping humanity's waste in the ocean.
But through their paper, the two scientists in effect say: Hear us out.
During a phone chat, Dr. Strand, a professor at the University of Washington who works on ways to use plants and microbes to clean up pollution, notes that the idea has been around for most of this decade. (For the record, Dr. Benford is a physics professor at the University of California at Irvine and a prolific science-fiction author.)
But crop-residue sequestration in the deep ocean has taken a back seat to more high-profile concepts of carbon capture and storage, including fertilizing the oceans with iron to stimulate plankton photosynthesis or turning CO2 back into carbonate rocks (you can read about this approach here).
In large part, he says, that's because early on, no one had worked out the carbon balance sheet for this plan versus other potential uses for crop residue. Nor had the carbon costs been worked out.
"That left doubt in peoples' minds [about] how efficient it would be to get crop residues transported to the place they needed to be," he says.
How they figure that
He and Benford calculate "carbon-sequestration efficiency" by tallying up the amount of carbon emitted to replace the residue-based soil nutrients now headed for the ocean, to bale the residue, and ship it to the dump site. Then they divide that number by the total amount of carbon sequestered. They have a slightly different formula for cellulosic ethanol, which they also describe in the paper.
By their estimate, burial at sea is 92 percent efficient, versus 32 percent for making cellulosic ethanol out of the residue, or 14 percent for using the residue for soil sequestration. And one could still leave enough residue on farmland to meet soil sequestration needs.
The most ideal dump sites, they hold, would be in the deep ocean, off the mouths of major rivers.
This would hold down shipping costs. Even so, the costs aren't trivial, Strand acknowledges. It would vary with distance. One could start with farms close to the river, then as economies of scale begin to kick in, expand the approach outward. Their ballpark estimate for sequestration costs along the Mississippi River Basin is $74 per ton of carbon sunk.
And river's-end dump sites already receive a fair amount of vegetable matter that flows down river. So any ecological effect would likely be minimized compared with other parts of the deep ocean. Still, the duo continues, there is much to learn about potential ecological effects. So they make a pitch for more-intense research to cover that base.
Yes, but ...
Gentlemen, meet Robert Carney. He's a biological oceanographer at Louisiana State University in Baton Rouge. He's heard about the proposal. And he offers some cautions (which Strand and Benford recognize in their paper):
Residence time: The Gulf of Mexico is an odd beast. It's known as a silled basin. Water glides in from the Caribbean through the relatively shallow Yucatan Straight, then moves over that sill and into the Gulf's basin. Although the estimate is somewhat controversial, Dr. Carney says, the Gulf is small enough and circulation vigorous enough to give deep water there a residence time from 200 to 300 years before it resurfaces.
Ecological changes: Meanwhile, his own experiments placing containers of rabbit food (a.k.a compressed alfalfa) on the deep-sea floor suggest that over several years, the presence of high concentrations of crop-like nutrients allow colonies of deep-sea chemosynthetic worms to take hold. Even without them, bacterial will break down the vegetable matter, with CO2 as a byproduct. Assuming the sea-water residence estimate holds up, that falls short of the millennial time scales needed for keeping the carbon out of the atmosphere.
And society may have other ways it wants to value crop residue, adds James Dooley, a senior scientist at the Joint Global Change Research Institute. It's a joint project of the US Department of Energy and the University of Maryland.
He cites cost or sustainability as two other values society might place on the same crop residue.
"Our research tells us that, over time, society tends to use things for their highest-value uses," he explains in an email exchange.
Other metrics besides efficiency
In other words, as policies are adopted that make high-carbon fossil fuels more expensive, biofuels made from renewable crop residue will become relatively cheaper; it has a far lighter carbon footprint than fossil fuels. Consumers will go for the cheap(er) stuff. And if biofuels come to provide the energy to make electricity, especially if the generating stations are equipped with carbon-capture and storage, the value of crop residue for energy generation rises even further.
So as a consumer (individual or corporate), one might then ask: Why dump this stuff in the ocean if I can use it to reduce the growth in emissions (perhaps at a less efficient rate than pure dumping) and keep my motor-fuel or electricity bills lower than they otherwise would have been?
Such are the discussions surrounding a range of semi-natural or steel-and-concrete sequestration approaches.
But if they are not ready for prime time, or even late-night, they are concepts that we may not want to discard entirely.
Some scientists -- most recently, noted atmospheric chemist Susan Solomon -- are making an increasingly vocal case that when we finally come to stabilize greenhouse-gas concentrations in the atmosphere at whatever level (especially CO2), that level will remain essentially constant for 1,000 years or more. You can read more about it here.
If emissions aren't brought essentially to zero fast enough to avoid the more-dire emissions trajectories (zero emissions are needed to stabilize atmospheric concentrations), minimizing the effects later may require some approach to recapturing that carbon from the air and squirreling it away for a very long time.