CERN's CLOUD Experiment Finds Ocean Plankton Seed Far More Clouds Than Climate Models Predicted

A 50-year hunch, finally confirmed

For almost half a century, scientists have suspected that microscopic marine plankton help build the clouds that drift over Earth's oceans. A new experiment from the CLOUD Collaboration at CERN, led by the University of Helsinki and published this week in the journal Nature, has confirmed that hunch and revealed the effect is far larger than anyone expected.

The study focused on methanesulfonic acid, or MSA, a compound that forms when gases emitted by marine phytoplankton are oxidised in the atmosphere. Inside CERN's CLOUD chamber, researchers recreated the ultra-low concentrations and frigid temperatures of remote marine air, spanning from +9C down to -52C. They found that MSA can trigger the formation and growth of aerosol particles, the tiny seeds that water vapour condenses around to form clouds.

Why MSA changes the picture

For decades, sulphuric acid was considered the primary driver of new particle formation in the atmosphere. The CLOUD results show that once temperatures drop below -10C, with even trace amounts of ammonia present, MSA is just as effective at creating new particle nuclei as sulphuric acid. Crucially, the two acids reinforce one another by forming shared molecular clusters, helping fragile nanometre-sized particles survive long enough to grow into cloud condensation nuclei.

"Since MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that particle nucleation rates might be accelerated up to tenfold and growth rates up to twofold compared with sulphuric acid and ammonia alone," said Jasper Kirkby, spokesperson of the CLOUD Collaboration.

The mechanism starts with plankton releasing dimethylsulphide, the gas responsible for the characteristic smell of the sea. When that gas oxidises in the air, it produces both sulphuric acid and MSA at comparable concentrations.

Filling a gap in climate models

The discovery helps explain the surprisingly high number of particles observed over the Southern Ocean and in the cold marine upper troposphere, regions where current climate models underestimate cloud condensation nuclei by more than half. Because this MSA pathway is not built into today's models, the results should sharpen future climate projections.

The timing matters. As pollution controls drive down fossil-fuel emissions of sulphur dioxide, natural biological sources of cloud seeds from marine plankton may play an increasingly important role in the climate system.

"The CLOUD Collaboration has made an important advance in our understanding of climate," said Gautier Hamel de Monchenault, CERN Director for Research and Computing. "It is crucial to deepen our understanding of aerosols: in this case, increased biogenic CCN will affect estimates of the Earth's climate sensitivity as well as projections of climate warming."