Enlightening Insight
The next IPCC report on the subject of climate change is just around the corner - but the forecasts will still be subject to great uncertainty: Despite intensive research, much of the mechanism of the atmosphere, oceans and continents, including their living environment, is still in the dark. However, researchers have now shed some light on the question of the cloud-promoting release of sulfur from the world's oceans.
"Although upper-ocean dimethyl sulfide dynamics have been the subject of extensive research, no clear conclusions could be drawn about the main factors controlling DMS concentrations." A frustrating statement for climate researchers articulated here by Sergio Vallina and Rafel Simó from the Institute for Marine Research in Barcelona. Because it has been known for decades that the emission of this sulfur compound produced by plankton organisms in the oceans is somehow related to solar radiation, cloud formation and thus albedo. But it was still open whether it would be via a positive, i.e. self-reinforcing, or a negative, self-regulating feedback.
In view of climate change, dimethyl sulphide, or DMS for short, plays an interesting role in the complex events: The resulting sulphate aerosols stimulate cloud formation and have a cooling effect due to the reduced solar radiation associated with this. And since it is also the main natural source of sulfur in the troposphere, it would be welcome as a well-studied cog in the climate wheelwork model. However, experiments had shown that this little screw sits at the center of its own machinery in which a wide variety of biotic and abiotic factors interact: the composition and activity of phytoplankton and bacterial communities, the feeding activities of zooplankton, and the destruction of cells by photolysis. As if that weren't enough, as expected, a clear but not clear influence of solar radiation was observed. Each answer to the experimental question posed only raised new questions.
Vallina and Simó therefore preferred to rely on real-world measurements. In the northwestern Mediterranean Sea, near the coast of Barcelona, they recorded the DMS concentrations in the upper water layers and the average daily solar radiation on the surface on a monthly basis. By accounting for the attenuation of light underwater and the thickness of the mixed layer, they calculated how much radiation any particle in the water would be exposed to.
When they compared the values, the irradiation or the light dose clearly emerged as the decisive factor for the fluctuations in the DMS concentrations: In a regression analysis, they explained 94 percent of the variability. In other words: the DMS content changes almost exclusively in accordance with the irradiation conditions. The researchers were able to substantiate this finding with values from other measuring stations in the Sargasso Sea. And finally, Vallina and Simó confirm results from 2004, which had brought to light a similarly close relationship for UV radiation and DMS concentrations.
The researchers did not discover any connection with chlorophyll measurements, which are used as a measure of phytoplankton biomass, on the contrary: in subtropical and lower temperate latitudes, a maximum of DMS was accompanied by a minimum of biomass - data, which are also confirmed by other studies. In cold regions, on the other hand, the two peaks coincide. Conclusion of the whole: There does not seem to be a global connection between biomass and DMS.
To further confirm this finding, the scientists created a global grid with DMS concentrations from the Global Surface Seawater Database. Since they now lacked the radiation measurements here, they estimated the values using climatological models. Again, the same picture emerged: "DMS tracked solar radiation dose far more closely than plankton biomass or temperature," the authors said. And not only in certain latitudes as with biomass, but simply on the entire planet.
What's behind it? Perhaps the relatively new hypothesis that DMS release by algae is a by-product of a sulfur-based antioxidant mechanism. If high UV radiation means high oxidative stress for the plankton - and thus DMS release - and at the same time inhibits the well-known uptake of DMS by bacteria, the dimethyl sulfide would only accumulate in seawater at high levels of radiation. This also makes it clear how highly variable the DMS emission into the atmosphere is: it does not reflect the seasonal fluctuations in the phytoplankton biomass, but follows the much more detailed ups and downs of solar radiation. And this close connection causes a negative feedback: After phases of high irradiation with DMS accumulation, cloud formation increases, which reduces the oxidative stress for the organisms. Accordingly, less DMS gets into the atmosphere - and cloud formation decreases.
So one question is answered: the feedback is negative, not positive. This is an important finding for climate modellers. If you now understand the other screws, linkages and joints in the gear train DMS, the molecule could actually get a secure place in the factor construction kit.
