Oceans' 'Twilight Zones' Offer Key to Understanding Climate Change

By Rosanne Skirble
Washington DC 03
May 2007

Scientists have warned for years that the carbon dioxide released from burning fossil fuels is building up in the earth's atmosphere, trapping solar heat and contributing to global warming. Fortunately, the world's oceans have been helping to counter this buildup, absorbing almost half the carbon dioxide emitted from auto exhausts and factories. But new research suggests the ocean's role in locking up carbon dioxide is more complex than we knew, and that before CO2 can be safely sequestered, the gas must first find its way to a particular zone in the ocean depths.
The surface of the ocean works a lot like a grassy lawn.

Sunlight reaching plant organisms in the upper layers supports photosynthesis, converting light into energy and fixing carbon in the plant cells. These phytoplankton become food for fish and tiny marine animals or zooplankton. When they decompose, their debris falls like "marine snow" into a dimly lit region between 100 and 1,000 meters that Ken Buesseler, a senior scientist with Woods Hole Oceanographic Institution,  calls "the twilight zone." "Think of it as carbon being speckled down and there is a whole group of animals in these mid-waters that feed on this sinking carbon." If the material gets eaten before it sinks out, which is what happens most of the time, Buesseler says, "then it just gets converted back into inorganic carbon and exchanged back with the atmosphere."

Buesseler headed the VERTIGO expeditions in 2004 and 2005. These National Science Foundation-funded research cruises to the Pacific Ocean tracked how much carbon got beyond the twilight zone to deeper ocean waters. Buesseler says, "In the warmer waters off Hawaii, very little of the carbon reached the deep ocean -- only 20 about percent."

That number shot up to 50 percent in the North Pacific. Buesseler says a variety of factors could explain the difference. "It might have something to do with the temperature, simply by being 10 degrees Celsius colder in the northern waters."
Plants in those colder waters are actually much larger than the ones in Hawaii. These sink much faster and much more efficiently in the colder ocean regions. Buesseler says the study provides new scientific data to support the ocean's ability to mitigate the impact of climate-changing greenhouse gases like carbon dioxide. If oceans become more like Hawaii and less like the north Pacific, "You might have much less efficient carbon transport and more CO2 in the atmosphere," he says, adding that "things would get warmer than predicted in climate models."
Buesseler plans to lead another research cruise, this time to sample waters in Bermuda, a site monitored year-round. He hopes to compare the results with the data gathered in the VERTIGO expeditions in the Pacific. "We'd like to see how much that carbon flux changes at different times of the year in one system and see what is controlling that variability."

Buesseler says the data - difficult to collect - points to the importance of ocean observation. More than 40 biologists, chemists, physical oceanographers and engineers from 14 institutions and seven countries participated in the VERTIGO expeditions. Their findings are reported in the Journal Science.