Because the atmosphere interacts with the oceans, the net uptake of carbon dioxide and the oceans' sequestration capacity would be affected by a change in climate. Just how effective carbon sequestration would be, in light of projected climate change, has not been studied before. Indeed, estimating the impact of carbon injection is complicated because of a limited understanding of climate and oceanic carbon cycle feedback mechanisms.

"Through various feedback mechanisms, the ocean circulation could change and affect the retention time of carbon dioxide injected into the deep ocean, thereby indirectly altering oceanic carbon storage and atmospheric carbon dioxide concentration," said Atul Jain, a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. "Where you inject the carbon dioxide turns out to be a very important issue."

To investigate the possible effects of feedbacks between global climate change, the ocean carbon cycle and oceanic carbon sequestration, Jain and graduate student Long Cao developed an atmosphere-ocean, climate-carbon cycle model of intermediate complexity. The researchers then used the model to study the effectiveness of oceanic carbon sequestration by the direct injection of carbon dioxide at different locations and ocean depths.

Jain and Cao found that climate change has a big impact on the oceans' ability to store carbon dioxide. The effect was most pronounced in the Atlantic Ocean.

"When we ran the model without the climate feedback mechanisms, the Pacific Ocean held more carbon dioxide for a longer period of time," Cao said. "But when we added the feedback mechanisms, the retention time in the Atlantic Ocean proved far superior. Based on our initial results, injecting carbon dioxide into the Atlantic Ocean would be more effective than injecting it at the same depth in either the Pacific Ocean or the Indian Ocean."

Future climate change could affect both the uptake of carbon dioxide in the ocean basins and the ocean circulation patterns themselves, Jain said. "As sea-surface temperatures increase, the density of the water decreases and thus slows down the ocean thermohaline circulation, so the ocean's ability to absorb carbon dioxide also decreases. This leaves more carbon dioxide in the atmosphere, exacerbating the problem. At the same time, the reduced ocean circulation will decrease the ocean mixing, which decreases the ventilation to the atmosphere of carbon injected into the deep ocean. Our model results show that this effect is more dominating in the Atlantic Ocean."

Tucking away excess carbon dioxide in Davy Jones's locker is not a permanent solution for reducing the amount of carbon dioxide in the atmosphere. "Sequestering carbon in the deep ocean is, at best, a technique to buy time," Jain said.

"Carbon dioxide dumped in the oceans won't stay there forever. Eventually it will percolate to the surface and into the atmosphere."

To buy as much time as possible, the carbon dioxide must remain trapped for as long as possible. "The big question is in what region of which ocean will future climate change have the least effect," Jain said. "That's where we will want to store the carbon dioxide."

Jain and Cao will present their latest findings at the American Geophysical Union meeting in San Francisco, Dec. 6-10. The U.S. Department of Energy funded the work.

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