A sprinkle of limestone could help oceans absorb CO
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14 May 2008
From New Scientist Print Edition.
Kate Ravilious
GRIND it down, pour in a sprinkle here and a dash there, and wait for results. That's the recipe for
helping the oceans to absorb more of our carbon dioxide emissions: add limestone. It may not only
help reduce global warming but could even reinvigorate ailing coral reefs.
When atmospheric CO2 dissolves in the ocean, it reacts with carbonate ions in the surface waters to
form bicarbonate ions. While this helps keep the acidity of the ocean constant, it lowers the
concentration of carbonate ions. This makes the rise in atmospheric CO2 bad news for corals and other
organisms which build their exoskeletons by absorbing carbonate ions along with calcium. Ultimately
the oceans could also become less able to absorb atmospheric CO2, as there are fewer carbonate ions
around to mop up the CO2.
Danny Harvey, a climate scientist at the University of Toronto in Canada, looked at whether this
carbonate depletion can be prevented by sprinkling the oceans with powdered limestone, which is
made of calcium carbonate. Limestone is only slightly soluble in water, so it sinks and slowly dissolves
deeper down, where the concentration of carbonate ions is low. Years later, this water, now carbonaterich, returns to the surface via upwelling currents, reviving its ability to absorb CO2.
Harvey says that the best place to sprinkle the limestone is the northern Pacific. Here the carbonatesaturated surface layer has already become rather thin, and the upwelling currents are strong. Harvey
calculates that if 4 billion tonnes of limestone were sprinkled over the northern Pacific every year, then
after 50 years the ocean would be able to absorb 600 million tonnes of CO2 annually - 7 per cent more
than today. This will increase to 900 million tonnes per year after 100 years - equal to 3 per cent of
current annual CO2 emissions derived from fossil fuels, says Harvey (Journal of Geophysical
Research, DOI: 10.1029/2007JC004373).
However, adding limestone to the oceans might reduce the water's transparency to solar radiation.
This could have a negative effect on marine organisms which rely on sunlight, says Corinne Le Quéré,
an environmental scientist at the University of East Anglia, UK. And mining and transporting the
necessary limestone, which Harvey says is quite feasible, will have its own environmental impact.
While not questioning the chemistry behind Harvey's calculations, some scientists say it will bring relief
too late. "It's unlikely to be of much help in preventing climate damage over the next decades at least,"
says Ken Caldeira of the Carnegie Institution for Science in Stanford, California, who has explored the
idea in the past.
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Weblinks
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Danny Harvey, University of Toronto
http://www.cgcs.utoronto.ca/research/CGCS_Research__Climate_Models_and_Dynamics/C
GCS_Research__Danny_Harvey.htm
From issue 2656 of New Scientist magazine, 14 May 2008, page 16