The oceans are cooling the climate more than previously thought

Scientists have long considered dimethyl sulfide (DMS) to be the main form of volatile sulfur emitted by the oceans that contributes to aerosol formation. However, recent research has shown that metathiol (MeSH), another volatile degradation product of dimethylsulfoniopropionate (DMSP), plays a much more important role in this process than previously thought.

In fact, up to 85% of DMSP consumed by marine microbes is not directed to DMS but to the demethylation pathway, the final product of which is MeSH. Subsequent measurements in seawater, air, and sea-air flows confirm that MeSH constitutes a substantial part of volatile methylated sulfur (VMS) emissions.

However, previous climate models neglected MeSH emissions, which led to an underestimation of their impact on atmospheric chemistry and the radiative action of aerosols. A new study published in the journal Science Advances aimed to quantify this impact.

The researchers compiled a database of MeSH concentrations in seawater and identified statistical predictors that allowed monthly fields of global MeSH emissions to be created. These data were implemented in a global chemical-climate model, which showed that emissions of MeSH increase the concentration of sulfate aerosols by 30 to 70% over the Southern Ocean and enhance their cooling effect.

Increased MeSH emissions have several important consequences:

• Increasing the concentration of VMS in the atmosphere: Adding MeSH emissions to the model increased the annual global mean atmospheric VMS concentration by 34%, with the largest increase (by 51%) occurring over the Southern Ocean.
• Extending the life of DMS: MeSH effectively competes with DMS in reactions with atmospheric oxidants, which extends the lifetime of DMS and enables its transport over longer distances.
• Increase in SO2 and SO4²⁻ production: Oxidation of MeSH contributes to the increased production of sulfur dioxide (SO2) and subsequently also sulfate aerosols (SO4²⁻).
• Strengthening the cooling effect of aerosols: The increased amount of sulfate aerosols in the atmosphere amplifies their radiation action and contributes to the cooling of the Earth.
• Reducing the radiative bias of climate models: Accounting for MeSH emissions helps reduce the radiative bias of current climate models in the Southern Ocean region, which is crucial for global climate.

The study's conclusions emphasize the importance of MeSH for atmospheric chemistry and the Earth's radiation balance. MeSH should be included in global climate models to more accurately assess the atmospheric oxidizing capacity and radiation balance, both in the reconstruction of pre-industrial scenarios and in global warming projections.

In addition, the study highlights the need for further research into the distribution of MeSH, its biological and chemical cycle in the surface ocean, its interaction with solar radiation and temperature, and its atmospheric chemistry. You can download the entire document here