This is the main conclusion of a study conducted by the ICM-CSIC and IQF-CSIC, which has quantified for the first time the global emission of a sulphur gas produced by marine life and its contribution to forming particles and clouds in the atmosphere.
A research team from the Institut de Ciències del Mar (ICM-CSIC) and the Blas Cabrera Institute of Physical Chemistry (IQF-CSIC) has, for the first time, quantified the global marine emission of methanethiol—a sulphur gas produced by marine life—and its contribution to forming particles and clouds in the atmosphere, which have cooling effects on the climate. The study, published in Science Advances, reveals that the ocean emits sulphur and cools the climate more than expected.
Nearly 40 years ago, one of the most groundbreaking hypotheses about the ocean's role in regulating Earth's climate was proposed. This hypothesis suggested that microscopic plankton living on the ocean's surface produces sulphur in gas form, which, once in the atmosphere, oxidises and forms tiny particles known as aerosols. Aerosols reflect part of the solar radiation back into space, reducing the heat the Earth retains—an effect opposite to that of greenhouse gases. The cooling effect of aerosols is amplified as these particles are essential for the condensation of water droplets and the formation of optically dense clouds. Clouds are the climatic element with the greatest cooling capacity.
A new component of the sea's smell
The new discovery broadens the climate impact of marine sulphur by adding a previously unnoticed compound.
‘Until now we considered that the oceans emitted sulphur into the atmosphere only in the form of dimethyl sulphide, a planktonic residue that is mainly responsible for the evocative smell of seafood,’ says Martí Galí, a researcher at the ICM-CSIC and one of the main authors of the study.
‘Today, thanks to the evolution of measuring instruments, we know that they also emit methanethiol, and we have found a way to quantify, on a global scale, where, when and in what quantity this emission occurs,’ adds Charel Wohl, another of the lead authors, who was working at the ICM-CSIC during the study and is currently at the University of East Anglia, UK (COAS).
To do this, the researchers have gathered all the available measurements of this new compound, added together those they had made in the Southern Ocean and the Mediterranean coast, and statistically related them to satellite temperature data. This has allowed them to conclude that methanethiol increases known marine sulphur emissions by 25% on an annual global average.
‘It may not seem like much, but methanethiol is more efficient at oxidising and forming aerosols than dimethyl sulphide, so its climate impact is magnified,’ says researcher Julián Villamayor.
A new climate actor
The research team has incorporated marine methanethiol emissions into a state-of-the-art climate model to assess their effects on the planetary radiation balance.
‘The impacts are much more visible in the southern hemisphere, where there are fewer continents and less human activity, and less sulphur from burning fossil fuels. This is where the study shows us what the ocean's influence on climate was like before the Industrial Revolution,’ explains Alfonso Saiz-López, from IQF-CSIC, one of the coordinators of the work and responsible for the modelling.
Considering methanethiol in the climate model means increasing the formation of sulphur aerosols over the Southern Ocean by 30% to 70%, which decreases incident solar radiation in summer by between 0.3 and 1.5 W/m2.
‘Current climate models greatly overestimate the solar radiation that we know actually reaches the Southern Ocean, mainly because they are not able to simulate clouds correctly. Incorporating this new sulphur emission will bring the model and reality a little closer together,’ says Rafel Simó, of the ICM-CSIC and the other coordinator of the study.
The study is further evidence that the oceans not only capture and distribute the sun's heat, and take some of the carbon dioxide that humans release into the atmosphere, but also produce gases and particles with immediate climatic effects. Even so, scientists point out, the scale of the impact of human activity is such that the planet is warming and will continue to warm if nothing is done.