Geoengineering via rock weathering

2016-01-04

in Science, The environment

Compared with trying to counteract climate change resulting from greenhouse gas pollution through solar radiation management (SRM) — essentially reflecting sunlight away, as with stratospheric sulfate injection — actually removing CO2 from the atmosphere by weathering rocks which form carbonates seems more attractive in many ways. The SRM approach may cause major side effects in terms of changes in precipitation, and any cessation in the injection of reflective aerosols in the upper atmosphere would lead to very abrupt climate change.

I asked David Keith about the idea when he was in Toronto talking about SRM-based geoengineering and he said that the problem is simply one of reaction rates. Even if we used zero-carbon energy to grind up vast amounts of ultramafic rock to absorb CO2, that process of absorbtion would happen so slowly that it would not counteract human-induced climate change on reasonable timescales.

I learned about the idea from Wallace Broecker and Robert Kunzig’s book Fixing Climate. Another oft-touted means of removing atmospheric CO2 is biochar. More recently, I read about the idea of speeding up natural rock weathering by biological means. I don’t know if this could somehow overcome Keith’s objection about reaction rates.

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. September 18, 2016 at 2:14 pm

A paper just published in Science offers a possible solution. By burying CO2 in the right sort of rock, a team of alchemists led by Juerg Matter, a geologist at Southampton University, in Britain, was able to transmute it into stone. Specifically, the researchers turned it into carbonate minerals such as calcite and magnesite. Since these minerals are stable, the carbon they contain should stay locked away indefinitely.

Dr Matter’s project, called CarbFix, is based in Iceland, a country well-endowed with both environmentalism and basalt. That last, a volcanic rock, is vital to the process, for it is full of elements which will readily react with carbon dioxide. Indeed, this is just what happens in nature. Over geological timescales (ie, millions of years) carbon dioxide is removed from the air by exactly this sort of weathering. Dr Matter’s scheme, which has been running since 2009, simply speeds things up.

Between January and March 2012 he and his team worked at the Hellisheidi geothermal power station, near Reykjavik. Despite its green reputation, geothermal energy—which uses hot groundwater to drive steam turbines—is not entirely emissions-free. Underground gases, especially CO2 and hydrogen sulphide (H2S), often hitch a ride to the surface, too. The H2S, a noxious pollutant, must be scrubbed from the power-station exhaust before it is released, and the researchers worked with remainder, almost pure carbon dioxide.

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