Mopping Up the CO2 Deluge

To battle global warming, it's not enough to reduce emissions. We need to soak up what's already there

  • Ian Shive / Aurora

    The oceans aid the sky by absorbing some CO2 in greenhouse gases. If they could be made to absorb more, warming might be curbed.

    The earth will little note nor long remember what we do to it--at least over the course of its own grand time scale rather than our brief, urgent one. Once we stop burning fossil fuels, it could take as long as 100,000 years for the carbon dioxide we've been pouring into the atmosphere to be gone. Most of it will have settled into the ocean, on its way to becoming new limestone beds on the seafloor; the rest will have been absorbed by the land, some of it eventually forming new deposits of coal. Even now, the water and soil are acting like great sponges, soaking up at least some of the carbon our industrial species emits every day and slowing--if not preventing--the climate-changing damage we're doing to our world.

    That's why a paper that came out last October in the Proceedings of the National Academy of Sciences was so alarming. CO2, the scientists concluded, is piling up faster than ever in the air, not only because our emissions continue to rise but also because the ocean and land have quit sopping up as much as they used to. Apparently, they've had enough.

    Dialing back emissions now will thus be less effective than we hope, because a growing share of what we still produce will stay in the sky rather than being absorbed by the oceans and land. The answer may be to quit thinking about solving climate change as only a matter of cutting greenhouse gases off at the source and to start considering how to clean up the mess that's already there. After all, when a busted pipe floods your home, you do more than just fix the leak and let evaporation take care of the water. You get out a bucket and start mopping.

    In small ways, we've been trying to mop up our CO2 deluge for a while. It's true enough that if you plant a tree, you clean the air, because trees do take carbon out of the sky--but only a little and not for long. The moment a tree dies, it usually begins to release the carbon it absorbed, and logging and burning only accelerate that process. So scientists are thinking bigger thoughts: Is it possible to increase the oceans' capacity to absorb carbon--without making the water so acidic it dissolves corals? Is it possible to scrub the atmosphere itself somehow, extracting CO2 the way a filter cleans the air in a home? Macroengineering like this is a fun thing for scientists to dream about, but it usually does not go much further, the scale and risks being simply too great. But that hasn't stopped big ideas from coming--which is fortunate, because any idea that's going to have much effect on global warming is going to have to be big indeed.

    The Iron Ocean

    One of the reasons the oceans soak up so much carbon is that phytoplankton--microscopic floating plants--love it, feasting on it and taking it out of circulation. The problem is, there are vast regions where the water is iron poor and plankton languish. The amount of iron the plants need and aren't getting is tiny--less than 20 lb. per sq. mi. (3 kg per sq km) by some estimates. If this were pumped as a diluted slurry into the wake of a ship steaming back and forth like a tractor seeding a field, the plankton would bloom and global CO2 levels--in theory--would fall.

    Sometime next year, a California start-up called Climos plans to experiment with the technique, fertilizing about 4,000 sq. mi. (about 10,000 sq km) of ocean. The goal is not to prove that the iron makes the plankton grow but to determine how much carbon this takes out of the atmosphere and for how long. "When we add iron, we create plankton blooms," says oceanographer Ken Buesseler of the Woods Hole Oceanographic Institution, who led an earlier, smaller iron-seeding test, "but a lot of that just dies and decomposes" at the surface. Only when organic matter snows into the deep does CO2 get locked away. Climos is in the process of raising the $12 million or so it will need to run its experiment, which will use rain-gauge-like underwater traps and other techniques to capture and measure this precipitate.

    Scientists have plenty of reasons to be skeptical about iron-seeding, not the least being that it will alter the base of the marine food web, with ripple effects that are hard to foresee. Environmental opposition scuttled a similar plan of Climos' chief rival, another California company, Planktos. International law on the matter is murky. In May, the U.N. Convention on Biological Diversity called for a moratorium on everything but "small" experiments "in coastal waters." Climos chief science officer Margaret Leinen concedes that even if the idea works, it won't remotely deal with all the planet's excess carbon. But she says it doesn't have to. "We're not thinking of this as solving the problem," she says. "We're looking at this as one of a whole portfolio of techniques."

    Another part of that portfolio could focus on a component of the ocean far more plentiful than its plankton: its salt. Sea salt, like table salt, is made of sodium chloride. If you break that compound in two, you create an acid and a base. Remove some of the acid, and you change ocean chemistry in such a way that atmospheric CO2 dissolves into the water, where it is taken up in the shells of marine creatures, which fall to the seafloor and become limestone. Essentially, says Kurt House, a Harvard graduate student who came up with the idea when he was jogging by the Charles River, the ocean "could become a giant carbon dioxide collector."

    Easy, right? Well, one part is, yes. Salt-splitting involves old technology--used in manufacturing chlorine--and is done simply by running an electric current through a pure brine solution, causing the positive sodium and negative chloride ions to head toward opposite poles. The technique does not yet work on something as gunky and mineral-laden as seawater, but that could be figured out.

    The bigger problem is scale. According to House's calculations, his plan would require 100 seawater-electrolysis plants, each as large as the largest sewage-treatment plant on Earth, built on shorelines around the world. They would draw out 180 billion metric tons of seawater each year, split the salt, keep the acid and pour back the water. And even that would remove just 10% of the more than 30 billion metric tons of CO2 we put into the air annually.

    What's more, you'd be left with a lot of hydrochloric acid to get rid of on land, while the changed ocean chemistry would surely kill a lot of fish--though only, says House, in the immediate vicinity of the electrolysis plants. "I would bet against any of this happening in the next half-century," House concedes. Still, he adds, "if global warming gets really bad, we could do it." Harvard has applied for a patent on the process just in case.

    Flypaper in the Sky

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