SAN FRANCISCO--Charts of the seafloor soon may have strange new features: lakes of liquid carbon dioxide. Climate-change scientists are considering the deep ocean as a place to store some of society's excess CO2 emissions. The idea looks feasible, according to several reports presented here on 18 December at a meeting of the American Geophysical Union. However, it's not clear whether the scheme would harm sea life in the long run.
Many researchers are exploring ways to remove CO2--the main culprit behind the planet's intensifying greenhouse effect--from the atmosphere. Options include planting forests, injecting CO2 into geologic formations, and fertilizing the ocean to spawn algal growth (ScienceNOW, 11 October 2000 ). The deep sea is tantalizing, because oceanographers have suspected that CO2 sequestered there would remain trapped.
That hope appears borne out by new simulations from seven international groups. One team, climate modelers Michael Wickett and Ken Caldeira of Lawrence Livermore National Laboratory in Livermore, California, used a 3D ocean circulation model to show that if gaseous CO2 is pumped 3 kilometers deep, virtually none of it escapes. Indeed, "the ocean retains most of the carbon for several centuries" because deep seawater takes so long to circulate back to the surface, Caldeira says. Shallower injections--at 800 meters, for instance--allow some CO2 to diffuse back into the atmosphere within decades, due to currents that exchange water between midwater depths and the surface.
The 3-kilometer figure also proved magical during a cruise off central California in March, directed by oceanographer Peter Brewer of the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing. By releasing bubbles of gaseous CO2 and tracking their motions with a remotely operated submersible, Brewer's team determined that the bubbles start sinking at 2675 meters. At the cold temperatures and extreme pressures of 3 kilometers deep, a slow injection of CO2 creates liquid blobs that "roll around the ocean floor like tumbleweeds," he says. Water molecules encase the CO2 to form a thin layer of "hydrates," an icelike skin that prevents the liquid from mixing with seawater or penetrating the sediments underneath.
Still, the CO2 would slowly dissolve into the ocean, Brewer notes, with unknown effects on nearby marine organisms. Brewer's colleague at MBARI, marine biologist Mario Tamburri, conducted field tests in 1998 and spring 2000 to gauge animal reactions to CO2 at depths of 625 meters and 3 kilometers. His group found that fish were curious about the CO2 hydrates and swam up to them with no observable ill effects--save for a persistent hagfish that entered the beaker and passed out three times. However, seafloor organisms were asphyxiated if CO2 covered them.
Biological uncertainties may be a hurdle, especially with a skeptical public. "There are concerns about environmental consequences," says geochemist James Bishop of Lawrence Berkeley National Laboratory in Berkeley, California. "The question is, can we learn enough to not mess up the job?"