Rising global carbon dioxide levels disrupt more than the atmosphere. As more CO2 dissolves into the ocean and lowers the waters' pH, the resulting ocean acidification may alter marine ecosystems. A new study describes another potential consequence of rising sea CO2 concentrations: more cooling clouds condensing over oceans (Environ. Sci. Technol., DOI: 10.1021/es102028k).
The gas dimethylsulfide (DMS), which accounts for about 95% of the sulfur gas wafting up from the ocean, is responsible for some cloud formation. DMS oxidizes in the atmosphere and forms sulfate aerosols that seed cloud condensation.
Zooplankton spark DMS production when they graze on phytoplankton. They lyse the phytoplankton's cells, which release dimethylsulfioniopropionate (DMSP), a precursor of DMS, and an enzyme called DMSP-lyase. When the two chemicals mix, the enzyme converts DMSP into DMS.
Because phytoplankton are photosynthetic organis that depend on CO2, rising ocean CO2 levels might enhance this process. So geochemist Kitack Lee of the Pohang University of Science and Technology in South Korea, and colleagues decided to study how DMS production changed under different ocean conditions.
In Korean coastal waters, the scientists set up nine 2,400-liter enclosures of sea water called mesocos. They used the mesocos, which resembled large, hollow buoys, to test three different ocean condtions.
One condition, the control, reflected contemporary global CO2 levels and temperatures. The other two conditions simulated climate conditions expected in the next century as predicted by an Intergovernmental Panel on Climate Change model: elevated CO2 levels of about 900 ppm and warmer temperatures by 3 degrees. The researchers set up one experimental condition that had both elevated CO2 levels and temperatures, while the other condition only had greater CO2 concentrations.
The researchers then monitored phytoplankton growth and dinoflagellate grazing, while measuring DMS levels by gas chromatography. In the mesocos with elevated CO2 levels alone, DMS levels were 80% greater than in the control enclosures. Meanwhile, in the mesocos with elevated CO2 levels and temperatures, DMS levels were 60% greater than the controls.
The higher CO2 levels created "a chain reaction" that led to more DMS production, Lee says: More CO2 supported larger phytoplankton blooms, which, in turn, increased grazing rates by dinoflagellates.
Lee says that these observations suggest that the CO2-rich conditions of the future may increase DMS production, which could then initiate more cloud formation. And because cloud cover scatters and reflects solar radiation, the process could possibly cool the atmosphere. "It may be a pretty long stretch," Lee says. "But there's an indication." He hopes that researchers will replicate the test in other regions such as North America and Europe to further understand the process.
Marine biologist Ray Sambrotto of Columbia University says this "state-of-the-art" study has broken important ground in linking increasing CO2 levels with greater DMS production. But he says that more research needs to be done before scientists can start predicting more cooling cloud cover. Marine ecosystems differ widely across seasons and geography, Sambrotto explains, and scientists are far from fully grasping how ocean acidification and climate change will affect them.
