Danny Harvey of the University of Toronto wanted to check it out. In a new study, published in the Journal of Geophysical Research, he calculated the effect of dumping massive amounts -- 4.4 billion tons per year -- of powdered limestone, or calcium carbonate, into the ocean to counteract the acidification caused by increased atmospheric CO2 dissolving in the ocean.
"I've been concerned for a long time about how we might draw down the atmospheric CO2 once we've
phased out fossil fuels," Harvey said.
"The natural recovery would be very slow," he added. "If you can find ways to enhance uptake of CO2, you could draw it down faster. But then there's the question, is there any way we could accelerate the natural recovery of the ocean's acidity?"
It turns out that adding limestone could do both, but -- perhaps surprisingly to the chemistry student -- the dominant effect of adding limestone, a base, would be to sequester carbon, not to change the pH.
Here's how it would work.
The upper layer of the ocean is supersaturated with carbonate ions (a necessary situation for corals, certain plankton and other tiny organisms to make their shells).
As CO2 from the atmosphere dissolves in the surface layer, the interaction of various types of carbonate molecules decreases the degree of supersaturation of carbonate ions (which threatens the health of these organisms), and increases acidity.
Since the surface layer is supersaturated, limestone powder can't dissolve until it sinks into deeper water, where higher pressures allow the water to hold greater amounts of carbonate ions while still remaining unsaturated. If that water returned to the surface through upwelling, it would be recharged with carbonate and able to take up more CO2.
The net effect is that more CO2 is pulled into the ocean -- about one molecule for every molecule of calcium carbonate added. But since more acidifying CO2 gets pulled in with this cycle, the acidity decreases only slightly as limestone is added.
Because this process relies on sinking, dissolving and upwelling, Harvey's calculations predict it would take decades for the limestone to fulfill its potential for absorbing CO2.
Still, Harvey calculates that adding 4.4 billion tons per year of limestone to regions of the ocean where strong upwelling occurs would eventually result in the uptake of about 0.3 billion tons of carbon per year.
oday we emit more than 3.3 billion tons of carbon from coal burning alone. That number is projected to climb to 16-28 billion tons per year by 2100 if we continue "business as usual."
Harvey also examined the energy requirements of mining, grinding and shipping the limestone.
"You wouldn't want to be supplying grinding energy from an inefficient coal-fired power plant," he said. He imagines putting this approach into place after society has made the switch to renewable energy, when energy needs would not offset the carbon gains of adding limestone.
And would the approach have unwanted ecological consequences? Large zooplankton would be likely to eat the limestone as it fell through the surface layer, which would probably be harmful to them, but Harvey noted that the limestone would only be present in their habitat for a few hours a year.
"I look at this paper as detailing a theoretical possibility, but not one that's likely to be implemented," said Ken Caldeira of the Carnegie Institution of Washington in Stanford, Calif. "You could imagine engineering the water of a specific bay to maintain coral reef growth, but the attempt to use calcium carbonate to change the whole ocean chemistry is a bit unrealistic."
To engineer a bay would probably require adding already-dissolved calcium carbonate, he added, because reefs occur at depths too shallow to allow limestone powder to dissolve. Caldeira and others have considered using CO2 from power plant emissions to dissolve calcium carbonate, and then to apply the resulting solution to the ocean.
Such an approach might be useful for a small-scale project like altering the chemistry of a bay, Caldeira noted.
Harvey has mixed feelings about the feasibility of his scheme.
"My conclusion is that it might work," he said. "Sometimes I've thought this was a crazy idea, but it might be a valid thing to do once you've dealt with the root source of the problem" -- burning fossil fuels and emitting CO2. "If you don't, the problem is hopeless."
Source: discovery.com
| Comments |
|
