Oceans emit sulfur and cool the climate more than previously thought

Researchers have quantified for the first time the global emissions of a sulfur gas produced by marine life, revealing it cools the climate more than previously thought, especially over the Southern Ocean.

The study, published in the journal Science Advances, shows that the oceans not only capture and redistribute the sun's heat, but produce gases that make particles with immediate climatic effects, for example through the brightening of clouds that reflect this heat.

It broadens the climatic impact of marine sulfur because it adds a new compound, methanethiol, that had previously gone unnoticed. Researchers only detected the gas recently, because it used to be notoriously hard to measure and earlier work focussed on warmer oceans, whereas the polar oceans are the emission hotspots.

The research was led by a team of scientists from the Institute of Marine Sciences (ICM-CSIC) and the Blas Cabrera Institute of Physical Chemistry (IQF-CSIC) in Spain. They included Dr Charel Wohl, previously at ICM-CSIC and now at the University of East Anglia (UEA) in the UK.

Their findings represent a major advance on one of the most groundbreaking theories proposed 40 years ago about the role of the ocean in regulating the Earth's climate.

This suggested that microscopic plankton living on the surface of the seas produce sulfur in the form of a gas, dimethyl sulphide, that once in the atmosphere, oxidizes and forms small particles called aerosols.

Aerosols reflect part of the solar radiation back into space and therefore reduce the heat retained by the Earth. Their cooling effect is magnified when they become involved in making clouds, with an effect opposite to, but of the same magnitude as, that of the well-known warming greenhouse gases, such as carbon dioxide or methane.

The researchers argue that this new work improves our understanding of how the climate of the planet is regulated by adding a previously overlooked component and illustrates the crucial importance of sulfur aerosols. They also highlight the magnitude of the impact of human activity on the climate and that the planet will continue to warm if no action is taken.

Dr Wohl, of UEA's Centre for Ocean and Atmospheric Sciences and one of the lead authors, said: "This is the climatic element with the greatest cooling capacity, but also the least understood. 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. The work done here partially closes the longstanding knowledge gap between models and observations."

With this discovery, scientists can now represent the climate more accurately in models that are used to make predictions of +1.5 ºC or +2 ºC warming, a huge contribution to policy making.

"Until now we thought that the oceans emitted sulfur into the atmosphere only in the form of dimethyl sulphide, a residue of plankton that is mainly responsible for the evocative smell of shellfish," said Dr Martí Galí, a researcher at the ICM-CSIC and another of the main study authors.

Dr Wohl added: "Today, thanks to the evolution of measurement techniques, we know that plankton also emit methanethiol, and we have found a way to quantify, on a global scale, where, when and in what quantity this emission occurs.

"Knowing the emissions of this compound will help us to more accurately represent clouds over the Southern Ocean and calculate more realistically their cooling effect."

The researchers gathered all the available measurements of methanethiol in seawater, added those they had made in the Southern Ocean and the Mediterranean coast, and statistically related them to seawater temperature, obtained from satellites.

This allowed them to conclude that, annually and on a global average, methanethiol increases known marine sulfur emissions by 25%.

"It may not seem like much, but methanethiol is more efficient at oxidising and forming aerosols than dimethyl sulfide and, therefore, its climate impact is magnified," said co-lead Dr Julián Villamayor, a researcher at IQF-CSIC.

The team also incorporated the marine emissions of methanethiol into a state-of-the-art climate model to assess their effects on the planet's radiation balance.

It showed the impacts are much more 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.

The work was supported by funding from organisations including the European Research Council and Spanish Ministry of Science and Innovation.