How a Marine Molecule Could Cool the Planet

The ocean’s chemistry is subtly shifting in response to a warming planet. One of the clearest signals comes in the form of dimethyl sulphide (DMS), a compound released by phytoplankton into the atmosphere, where it helps form clouds, reflect sunlight, and influence the Earth’s energy balance. A new study by the Indian Institute of Tropical Meteorology (IITM), Pune, suggests that as the climate warms, DMS emissions are set to rise globally—changing not only cloud dynamics but also how climate models account for natural aerosol feedbacks.

For a molecule that smells faintly of rotting cabbage, DMS has enjoyed an improbable rise to climate celebrity. First proposed as a climate-relevant molecule in the 1980s, it has since formed the basis of the so-called CLAW hypothesis: that life in the ocean, by producing DMS, might help regulate planetary temperature through cloud formation. Essentially, DMS cools the Earth by generating aerosols that form brighter clouds, which reflect more sunlight and lower surface temperatures. The decline in ship and industrial sulphur emissions—especially post-2020 regulations—has left natural sources like DMS to play a more visible role in cloud dynamics.

The new study estimates an increase in atmospheric DMS emissions of 1.6 to 3.7 percent by 2100, despite a projected decline in seawater DMS concentrations. Says Anoop Mahajan of IITM, “The response is due to changes in the seawater biogeochemistry, which is driven by changes in ocean temperatures. The increase in emissions is a result of an increase in wind speed, which is due to an increase in atmospheric temperatures.”

To track this change more accurately, the IITM team used a machine learning technique called Gaussian Process Regression (GPR). This helped different climate models—specifically those from the CMIP6 ensemble, which are among the most widely used and trusted—reach a consensus on future DMS trends. The result is a far more coherent forecast of a molecule once considered too volatile, too biologically moody, to predict.

Mahajan is quick to swat away any techno-optimist illusions of salvation by plankton. This is no planetary failsafe. “Indeed, it will offset some global warming, but it will not negate it,” he says. “CO₂ reductions will still stay as the only way to ‘save us’.”

That said, DMS is very likely to influence Earth’s radiative balance in the future. “We still do not have an exact number on how much the radiative impact will change, but it will be substantial, especially in regions where there is low air pollution,” Mahajan says. These include vast areas of the Southern Ocean and the polar regions, where aerosol-cloud interactions are more pronounced due to the cleaner air and absence of industrial pollutants.

One of the biggest wildcards is the Arctic, where warming and sea ice melt may actually increase DMS in both seawater and the atmosphere. “The reduction in sea ice will have a massive impact but needs to be understood better,” Mahajan says. Other key regions include the Southern Ocean, which already exhibits high natural DMS emissions, and the Indian Ocean, now warming faster than any other basin except the Arctic. That rate of warming makes the Indian Ocean especially relevant to South Asian monsoon behaviour. “Considering monsoon is driven by land-ocean contrasts, and the fact the DMS will cool the oceanic region more, its effect on monsoon is something we have started looking at,” Mahajan says.

Behind DMS’s atmospheric role lies the metabolic activity of phytoplankton, tiny marine organisms sensitive to changing pH, temperature, and nutrient levels. Ocean acidification could alter not just their population size but their species composition. “Ocean acidification can change phytoplankton concentrations and diversity,” Mahajan says. “In our study, we have accounted for phytoplankton concentrations, but the change in diversity is something we need to study to predict further changes in DMS emissions.”

Understanding how DMS behaves in a warming world won’t replace the need for cutting emissions, but it may help us build better climate models, and make smarter decisions in the years ahead.

climatedimethyl sulphideecologyNews Briefoceanoceanic phytoplanktonWeb Exclusive