How Marine Plankton Influence Cloud Formation Through Methanesulfonic Acid (MSA)

For nearly 50 years, scientists have debated whether tiny marine organisms could help regulate the climate. A landmark experiment at CERN now provides the strongest evidence yet that they can—and that current climate models have been missing a key piece of the puzzle.

In June 2026, the CLOUD (Cosmics Leaving Outdoor Droplets) Collaboration published results in Nature demonstrating that methanesulfonic acid (MSA), a compound produced from plankton emissions, is a far more important driver of cloud seed formation than previously recognized . The finding has immediate implications for climate model accuracy and projections of future warming.

The Plankton-to-Cloud Pathway

The chain begins with marine phytoplankton. During photosynthesis, these microscopic organisms release dimethyl sulfide (DMS)—the gas responsible for the familiar smell of the sea . In the atmosphere, DMS oxidizes via reactions with hydroxyl radicals, producing both sulfuric acid and methanesulfonic acid (MSA) .

What the CLOUD experiment revealed is that MSA is not a minor byproduct. Under cold atmospheric conditions—typical of the marine upper troposphere and polar regions—MSA acts as an efficient driver of new particle formation and growth, exhibiting ultra-low volatility comparable to sulfuric acid . These particles grow into cloud condensation nuclei (CCN), the seeds around which cloud droplets form .

This mechanism is especially effective in cold, clean marine air where sulfuric acid alone cannot efficiently nucleate particles .

Why This Is a Scientific Shift

For decades, the "CLAW hypothesis" (named after its proposers Charlson, Lovelock, Andreae, and Warren) proposed that plankton DMS emissions could regulate climate through cloud formation . But the mechanism was considered weak or uncertain . The CLOUD experiments now demonstrate that the MSA-driven pathway is a major, previously overlooked route—particularly in the climate-critical regions of the Southern Ocean and Arctic .

"The marine biosphere may be better able to compensate for future reductions of anthropogenic aerosols than previously thought," the CLOUD Collaboration stated .

Climate Model Accuracy: A 50% Gap in CCN Concentrations

Most global climate models have not included MSA-driven new particle formation. When the CLOUD data were incorporated into the EMAC global aerosol-climate model, the results were striking: including MSA-driven particle formation and growth led to an increase of at least 50% in cloud condensation nuclei concentrations over the Southern Ocean and polar regions .

This is a large effect in one of the most climatically important regions on Earth. Observational studies further support the impact: over phytoplankton blooms, cloud droplet number concentration can double, and cloud droplet radius can shrink by 14%, producing a short-wave radiative forcing effect of up to -15 W/m² at the top of the atmosphere—comparable to the aerosol indirect effect over highly polluted regions .

As human-made aerosol pollution decreases (due to clean-air policies), natural plankton-derived aerosols could take over some of the cloud-seeding role, altering projections of how clouds change in a cleaner future .

Implications for Future Warming Projections

The findings suggest the biosphere's DMS-cloud feedback may be stronger than assumed in current IPCC-class models. This could mean a natural negative feedback that partly counteracts warming:

• Strongest cooling in polar regions: Modeling studies indicate that as DMS emissions increase globally, the strongest cooling effect occurs over the Arctic, associated with changes in sea-ice albedo feedbacks .

• Potential moderation of Arctic amplification: If the plankton-cloud feedback strengthens under warming (as warmer oceans could increase biological activity and DMS emissions), this could dampen projected warming rates in the Arctic .

• Larger uncertainty in climate sensitivity: Because the MSA pathway is absent from most current models, the actual climate sensitivity—how much warming occurs for a given CO₂ increase—may be affected.

Important Caveats

The strength of this feedback remains uncertain. Some earlier studies found low sensitivity of CCN to DMS emission changes on a global scale, and the CLAW hypothesis has been controversial . The CLOUD findings revive and strengthen the case, but fully integrating MSA chemistry into Earth system models and validating against observations is still underway . The results are very recent (published June 24–25, 2026) and have not yet been assessed by the broader climate modeling community.

What Comes Next

The CLOUD experiment continues to provide mechanistic understanding of aerosol particle formation that can be parameterized into climate models . The key next steps include: incorporating MSA chemistry into IPCC-class Earth system models, validating the modeled effects against field observations over the Southern Ocean and Arctic, and assessing how the feedback might change under different warming scenarios.

What is already clear: the ocean's biology may have a larger say in future climate than models have given it credit for.