Direct injection of CO2 into the deep ocean has been proposed as a way to slow the accumulation of carbon dioxide in the atmosphere, one of the causes of global warming. In the direct injection scenario, fossil-fuel carbon dioxide is injected into the ocean interior, bypassing the mixing processes that would otherwise cause a relatively slow transfer of excess atmospheric CO2 in to the deep ocean.

In a study released today in Geophysical Research Letters, Ken Caldeira and Philip Duffy of the Climate and Carbon Cycle Modeling Group and Michael Wickett of the Center for Applied Scientific Computing, all at Livermore, show that the depth, rather than radiocarbon, is a relatively good predictor of the effectiveness of CO2 injection.

The researchers studied both radiocarbon dating (typically used to date anthropologic items) and the depths of injection to determine the effectiveness of direct CO2 injection as a carbon sequestration strategy.

Scientists used one-dimensional box-diffusion models and three-dimensional simulations run under the radiocarbon and sequestration scenarios described in Livermore's Ocean Carbon-cycle Model Intercomparison Project protocols.

"These simulations indicate that the amount of time it takes for a water parcel to return to the ocean surface increases with depth, but is not related to the amount of time since that parcel was last at the surface," Duffy said.

Injections were simulated at 800 meters, 1500 meters and 3000 meters for 100 years near the Bay of Biscay, New York City, Rio de Janeiro, San Francisco, Tokyo, Jakarta and Bombay.

The models showed that injection at 3000 meters is quite effective at sequestering carbon from the atmosphere for several centuries while injections at shallower depths are less effective. In general, injections into the Pacific Ocean (San Francisco and Tokyo) were more effective than injection at the same depth in the Atlantic Ocean (New York City, Rio de Janeiro and the Bay of Biscay).

"We showed that radiocarbon content alone at a given location in the deep ocean is a poor predictor of how effective CO2 injection at that location is at sequestering carbon away from the atmosphere," Caldeira said. "In contrast, our models showed that the depth of injection alone is a good predictor of the effectiveness of the CO2 injection."

Researchers have yet to determine the effects of direct injection on marine life.

Whether carbon dioxide is released in the atmosphere or the ocean, eventually about 80 percent of the carbon dioxide will end up in the ocean in a form that will make the ocean more acidic.

While the carbon dioxide is in the atmosphere, it could produce adverse climate change. In previous studies, Caldeira showed that unless carbon dioxide is converted to some other form before injection, it will degas back to the atmosphere when diffusion or ocean circulation returns it to the ocean surface.

Other methods of carbon sequestration include ocean fertilization, in which the biology of phytoplankton (which grows close to the ocean surface) is changed so that it increases the conversion of carbon dioxide to biomass. The conversion is likely to transport acidity from the surface ocean to the deep ocean.

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