A Revised Ocean-Atmosphere Sulfur Cycle Model: Methanethiol (MeSH), a Biogenic Sulfur Gas from Marine Organisms, Helps Cool the Climate, New Research Finds

     Researchers from England and Spain have discovered that a previously overlooked sulfur gas, methanethiol (MeSH), is being emitted by marine organisms at a higher rate than previously realized from Southern oceans. This is now thought to be one explanation for climate modeling predicting higher warming in the Antarctic than has been observed. According to a good summary of the paper and its implications by Study Finds:

“This is the climatic element with the greatest cooling capacity, but also the least understood,” says Dr. Charel Wohl of the University of East Anglia’s Centre for Ocean and Atmospheric Sciences, in a statement. Wohl spearheaded the research with colleagues at the Institute of Marine Sciences in Spain. “We knew methanethiol was coming out of the ocean, but we had no idea about how much and where. We also did not know it had such an impact on climate.”

“Climate models have greatly overestimated the solar radiation actually reaching the Southern Ocean, largely because they are not capable of correctly simulating clouds,” explains Dr. Wohl. “The work done here partially closes the longstanding knowledge gap between models and observations.”

The researchers developed the first global database of ocean MeSH measurements, acquiring data from multiple research cruises spanning from the tropical Atlantic to the cold icy waters near Antarctica, including some areas with sparse life and others with abundant life.

     Researchers have long regarded another sulfur compound, dimethyl sulfide (DMS), which has a shellfish smell, as the ocean’s main contributor to cooling. It is also a biogenic sulfur gas. Plankton release DMS into the atmosphere forming clouds that reflect sunlight back to space. It was the famed Gaia Hypothesis originator, the late James Lovelock, who discovered DMS in the air above oceans. According to Science:

“Using his own instruments, he discovered the biogenic gases methyl iodide and dimethyl sulfide in the remote marine atmosphere. He also discovered chlorofluorocarbons everywhere, providing critical evidence that they threatened the ozone layer.”

MeSH oxidizes, first mostly to SO2, then it further reacts, oxidizing to SO4, or sulfate aerosol particles that exhibit radiative cooling by reflecting sunlight. Even though MeSH was known to be a byproduct of plankton decomposition, it had yet to be quantified and brought into climate models. The new groundbreaking research revises the scientific model of the ocean-atmosphere sulfur cycle, particularly in the Southern oceans.

     Below is a conceptual model of the revised sulfur cycle from the paper in Science Advances.

From Study Finds:

“In colder waters and open ocean areas, MeSH levels were proportionally higher compared to DMS. Meanwhile, in warmer waters and coastal areas, MeSH made up a smaller fraction of the total sulfur emissions.

“It may not seem like much, but methanethiol is more efficient at oxidizing and forming aerosols than dimethyl sulfide and, therefore, its climate impact is magnified,” says co-lead Dr. Julián Villamayor, a researcher at Blas Cabrera Institute of Physical Chemistry.

When the researchers input these new MeSH measurements into sophisticated climate models, they discovered that including MeSH emissions increased the cooling effect of marine sulfur compounds by 30-70% over the Southern Ocean. This enhancement was particularly strong during the Southern Hemisphere summer when marine life is most active and solar radiation is at its peak.

They also found that MeSH increases the overall cooling effect of DMS by making it last longer in the atmosphere as both gases compete for available oxidants in the atmosphere.

From Study Finds:

“The increased aerosol formation was most pronounced over the Southern Ocean, where globally, methanethiol increases known marine sulfur emissions by 25%. The impacts are most visible in the Southern Hemisphere, where there is more ocean and less human activity, and therefore the presence of sulfur from the burning of fossil fuels is lower.

These findings represent a major advance on a groundbreaking theory proposed 40 years ago about the ocean’s role in regulating Earth’s climate. As human-caused sulfur emissions continue to decline due to air quality regulations, understanding natural sulfur sources becomes increasingly important for predicting future climate changes.”

The researchers combined their new measurements with all available MeSH and DMS concentration data, including satellite data. They then integrated the data into geospatial models, global concentration maps. They developed a statistical model based on relationships between MeSH and DMS that may vary under different conditions.

From Study Finds:

“The study found that MeSH emissions account for about 19% of total marine sulfur emissions globally, with higher proportions (up to 37%) in polar regions. When included in climate models, MeSH increased the atmospheric burden of sulfur compounds by 34% globally and 51% over the Southern Ocean. This led to enhanced formation of cooling sulfate aerosols, with the strongest effects during summer months.”

     Study Finds also notes that the new findings help to explain discrepancies in Southern Ocean climate models. Other implications of the study are the revision of the sulfur cycle model and a revised understanding of marine life’s influence on climate through sulfur gases that form sulfate aerosols that enable cooling.

     The abstract of the paper in Science Advances is shown below along with some figures from the paper.

 

The first figure below shows the variability of MeSH and DMS concentrations in open oceans vs. coasts and tropical regions vs. polar regions, the very high relative MeSh concentrations in the Southern Oceans, and the seasonal variations in concentration. 

     The figure below shows the relative concentrations of MeSH and DMS and the high concentrations at Southern Ocean latitudes. 

References:

 

Overlooked ocean gas emerges as powerful force against global warming. StudyFinds Staff. January 21, 2025. Study Finds. Overlooked ocean gas emerges as powerful force against global warming

Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean. Charel Wohl, Julián Villamayor, Martí Galí, Anoop S. Mahajan, Rafael P. Fernández, Carlos A. Cuevas, Adriana Bossolasco, Qinyi Li, Anthony J. Kettle, and Alfonso Saiz-Lopez. Science Advances. November 27, 2024. Vol 10, Issue 48. Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean | Science Advances

James Lovelock (1919–2022). Father of Earth system science. Timothy M. Lenton. Science. 25 Aug 2022. Vol 377, Issue 6609. p. 927. DOI: 10.1126/science.ade26. James Lovelock (1919–2022) | Science