Mysterious climate-influencing gas with foul odour discovered in Baltic Sea

A foul-smelling gas called methanethiol, which may affect the climate, has been detected in the Baltic Sea, with its origins remaining unknown, reports Finnish public broadcaster Yle. The gas is most prevalent during cyanobacterial blooms, and researchers at the Utö atmospheric and marine research station are investigating its formation.

Gas linked to algae blooms but source unclear

Methanethiol, which smells like rotten eggs, is believed to contribute to the occasional stench along Baltic Sea shores, particularly in sheltered bays. The gas is most concentrated during cyanobacterial blooms, but its exact source remains uncertain, according to researchers.

Hanna Kovanen, a tourism entrepreneur living on the outer archipelago island of Utö, describes the seasonal odours: “After winter, oil and algae wash up on the shores. In summer, cyanobacteria drift into the bays, and that’s when the smell is strongest.”

While methanethiol peaks during cyanobacterial blooms, its presence in spring—before blooms occur—suggests other sources, such as macroalgae, microscopic phytoplankton, or associated bacteria, says Kaisa Kraft, a marine biologist at the Finnish Environment Institute.

Potential cooling effect on climate

Beyond its foul odour, methanethiol may play a role in climate regulation. When oxidised, sulphur compounds like methanethiol form particles that act as condensation nuclei for clouds, explains Toni Tykkä, a research assistant at the Finnish Meteorological Institute.

“This means more clouds form over the sea, increasing the reflection of sunlight back into space,” Tykkä says. With stricter regulations reducing sulphur emissions from ships, natural sources like methanethiol are becoming more significant in atmospheric processes.

Utö research station ideal for study

The investigation is being conducted at the Utö atmospheric and marine research station, funded by the Academy of Finland. The station’s location—on Finland’s southernmost inhabited island, roughly 90 km from Hanko, Turku, and Mariehamn—makes it uniquely suited for such studies.

“It’s rare to have a continuous monitoring station so close to open sea in Finland,” says Jukka Seppälä, group manager at the Finnish Environment Institute. The station has previously examined the Baltic Sea’s carbon balance and detected fuel leaks from liquefied natural gas ships.

Reducing nutrient runoff into the Baltic Sea could decrease both cyanobacterial blooms and associated odours, researchers note. Improved water quality has already led to clearer bladdertang seaweed washing ashore, according to local observations.