Marine plankton release a gas that helps in forming aerosol particles that can seed clouds over cold ocean regions -far more effectively than previously assumed. An international study led by the University of Helsinki improves our understanding of the atmosphere and the accuracy of climate models.
For nearly fifty years, scientists have suspected that microscopic marine plankton play a role in cloud formation over the oceans. Now, an experiment led by the University of Helsinki has suggested that it may be more important than previously thought.
Marine plankton release dimethylsulfide (DMS) during photosynthesis - the gas responsible for the characteristic smell of the sea. When oxidised in the atmosphere, DMS forms acidic vapors.
One of these is methanesulfonic acid (MSA). has shown that this previously overlooked gas can trigger the formation and growth of aerosol particles over cold, pristine ocean regions. Moisture can condense around these aerosol particles, eventually forming clouds. Since this mechanism is not accounted for in current climate models, the experimental results not only deepen our understanding of the atmosphere, but also improve the accuracy of climate models.
The experiment was conducted under realistic atmospheric conditions in the CERN CLOUD chamber. The chamber's controlled environment made it possible to measure how cloud seeds form and grow under the ultra-low concentrations and cold temperatures typical of remote marine air - from +9 °C down to −52 °C.
As effective as sulfuric acid in cold marine air
When the chamber temperature fell below −10 °C and even a trace of ammonia was present, MSA formed new nuclei just as effectively as sulfuric acid, which is considered the primary driver of atmospheric aerosol formation.
When mixing freely, sulfuric acid and MSA reinforced one another by forming shared molecular clusters. MSA also accelerated rapid particle growth across the entire temperature range studied. This combination helps fragile nanometre-sized particles survive long enough to grow to a size at which they can seed clouds.
"Because MSA and sulfuric acid generally occur at similar concentrations over cold ocean regions, clouds may form there up to ten times faster than previously thought", says one of the lead authors, Dr.
This helps explain the surprisingly high number of particles observed over the Southern Ocean and in the cold marine upper troposphere.
Implications for clouds and climate
Aerosol particles influence the climate by scattering sunlight and by acting as cloud condensation nuclei (CCN), which seed the clouds.
In general, a higher concentration of CCN leads to a cooling effect on the climate system. Accurately predicting CCN concentrations is therefore essential for improving projections of future climate change. However, current climate models underestimate CCN concentrations over the Southern Ocean by more than half. This introduces a warm bias into the models.
The team's global simulations show that incorporating MSA increases particle and CCN concentrations most strongly over the cold, pristine ocean regions surrounding the Arctic and Antarctic - precisely the areas where models currently fall short.
"As emissions of sulfur dioxide from fossil fuels continue to decline, the natural, biological sources of cloud seeds from marine plankton may be more effective in the climate system. Capturing these processes is essential if we are to reliably predict future climate", say Dr and Professor , corresponding authors of the study.
"This study also underlines the importance of continued investment in cutting-edge measurements and modelling to improve future climate predictions", says Professor , Director of the at the University of Helsinki.
was published in Nature 10.1038/s41586-026-10810-2
