More CO₂ May Mean More Cooling Cloud Cover

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 organisms 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 mesocosms. They used the mesocosms, 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 mesocosms with elevated CO₂ levels alone, DMS levels were 80% greater than in the control enclosures. Meanwhile, in the mesocosms 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.