The Island of Doubt

"In situ carbonation of peridotite for CO2 storage" appears this week in PNAS. The authors, Peter B. Kelemen and Jurg Matter of Columbia University's Lamont-Doherty Earth Observatory enthusiastically describe the potential of a great mass of common rock called peridotite in Oman. The stuff is everywhere in the Earth's mantle, actually. It's just that in Oman it's right on the surface, sucking up carbon dioxide through a natural weathering process that's been responsible for removing CO2 from the air for billions of years at a relatively leisurely rate.

But what if we could accelerate the process? Kelemen and Matter estimate that some well-placed bore holes and pressurized water containing some of that troublesome CO2 could just do the trick. In fact, they use their paper to:

show that under certain circumstances exothermic peridotite alteration (serpentinization, carbonation) can sustain high temperature and rapid reaction with carbonation up to 1 million times faster than natural rates, potentially consuming billions of tons of atmospheric CO2 per year.

As we're pouring about 30 billion tons of CO2 into the air every year, it would be most convenient if there was an easy way to remove a gigaton or two, wouldn't it? So just how much could that Omani peridotite absorb? Our authors fudge that a bit, talking about a solution that offers the potential to suck up a quarter of all the gas in the air now — equivalent to everything we've emitted since we started burning fossil fuels — and making a somewhat more fanciful reference to a theoretical maximum of 77 trillion tonnes, which is 25 times as much as what's in air now.

That, of course, would be a bad idea, as it's CO2 that keeps the planet warm enough for life. But the great thing about this particular form of geo-engineering (and it literally is rock-changing), is that we could easily turn on and turn off the switch, unlike so many other wild schemes, like seeding the oceans with iron.

But back to what's practically possible. Again, though the has plenty of calculations, Kelemen and Matter don't offer a single number. They write that the natural weathering process absorbs 10,000 to 100,000 tons a year, and that it should be possible to increase that rate by a factor of a million. That would imply a potential of at least 10 billion tons a year. But the abstract mentions ">1 billion tons of
CO2 per year" and in an interview with Reuters, they are more conservative, offering something like a total of 2 billion tons/yr. That and "more research needs to be done before [it] could be used on a commercial scale."

The Reuters piece notes that the authors have "a preliminary patent filing for the technique." There's clearly a lot of work to done before they get rich off that patent, but there's no reason not to wish them luck, and maybe send a few research dollars their way. The Omani peridotite isn't the only formation on Earth that offers such an opportunity. If this thing works, we just might be able to take care of a significant fraction of the CO2 problem. Not all of it, by any means, but enough to buy us some time as we make the shift to the post-fossil-fuel society. As Kelemen and Matter write in the obligatory "more research is needed" closing:

Such processes are all-but-impossible to simulate in the laboratory. Large-scale field tests should be conducted...

-- P. B. Kelemen, J. Matter (2008). In situ carbonation of peridotite for CO2 storage Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0805794105