The direct injection of carbon dioxide deep into the ocean has been suggested as one method to help control rising carbon dioxide levels in the atmosphere and mitigate the effects of global warming. But, because the atmosphere interacts with the oceans, the net uptake of carbon dioxide and the oceans' sequestration capacity could be affected by climate change.
"Through a number of physical and chemical interactive 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. "Where the carbon dioxide is injected turns out to be a very important issue."
Developed by Jain and graduate student Long Cao, the Integrated Science Assessment Model is a coupled climate-ocean-terrestrial biosphere-carbon cycle model that allows extensive exploration of key physical and chemical interactions among individual components of the Earth system, as well as among carbon cycle, climate change and ocean circulation.
"A good understanding of climate change, ocean circulation, the ocean carbon cycle and feedback mechanisms is crucial for a reliable projection of atmospheric carbon dioxide concentration and resultant climate change," Jain said. The model is described in the September issue of the Journal of Geophysical Research -- Oceans.
Using the model, Jain and Cao studied the effectiveness of oceanic carbon sequestration by the direct injection of carbon dioxide at different locations and depths.
They 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. The researchers presented their findings in the May issue of the journal Geophysical Research Letters.
"When we ran the model without the climate feedback mechanisms, the Pacific Ocean held more carbon dioxide for a longer time," Cao said. "When we added the feedback mechanisms, however, the retention time in the Atlantic Ocean proved far superior. 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 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," Jain said. "Our model results show that this effect is more dramatic in the Atlantic Ocean."
Sequestering carbon in the deep ocean is not a permanent solution for reducing the amount of carbon dioxide in the atmosphere, the researchers report. "Carbon dioxide dumped in the oceans won't stay there forever," Jain said. "Eventually it will percolate to the surface and into the atmosphere."