How tiny sugars in the Southern Ocean help clouds freeze

Tiny bits of organic matter bobbing at the ocean’s surface appear to help clouds freeze in some of the most pristine air on Earth.

New research shows that complex sugar molecules shed by marine microbes can trigger ice formation in cloud droplets across the vast Southern Ocean. This happens right in the temperature range that governs how long bright, cooling clouds survive and how much sunlight they bounce back to space.

The work draws on years of field campaigns, lab tests, and computer modeling led by scientists at the Leibniz Institute for Tropospheric Research (TROPOS) and its partners.

It helps explain why climate models have struggled to match observed cloud brightness over remote southern seas. In these regions, dust is scarce and human-made pollution is low.

Clouds need help to freeze

Clouds do not freeze on their own; they need ice-nucleating particles (INPs). These microscopic seeds coax liquid droplets into ice. The kind of particle that dominates changes with temperature.

In the cleaner air around Antarctica and the Southern Ocean, INP levels are tiny. Even small shifts can strongly affect cloud reflectivity, precipitation, and lifetime.

Getting the mix right in climate models is essential for predicting global warming.

A sugar spike reveals a clue

Scientists have long suspected marine biology as a source of INPs, but the actual substances remained mysterious. A turning point came during the 2017 Polarstern expedition PS106.

“During the Polarstern expedition PS106 in 2017, we observed increased glucose concentrations in Arctic samples and concluded this glucose could be an indicator of ice nuclei in seawater,” said senior author Sebastian Zeppenfeld from TROPOS.

“The monosaccharide glucose is a degradation product of polysaccharides. It was therefore obvious to us that polysaccharides could be the missing piece of the puzzle.”

Ocean sugars and freezing clouds

To test the idea, researchers collected material from the ocean’s surface microlayer. This living film teems with bacteria, algae, diatoms, fungi, protists, viruses, and more. Researchers had largely overlooked marine fungi as possible ice starters.

Study co-author Susan Hartmann from TROPOS examined ice nucleation in the laboratory using the INDA (Ice Nucleation Droplet Array) droplet freezing test.

“We investigated the ice nucleation of marine polysaccharides derived from marine fungi and protist, as well as commercially available standard polysaccharides,” she said.

Those droplet-freezing experiments produced the first temperature‑resolved data showing how many ice nuclei these marine polysaccharides generate.

The results fill a key gap: between roughly -15°C (5°F) and -20°C (-4°F), the ocean sugars could account for nearly all biologically driven ice formation in cloud droplets.

Cloud models get missing ice link

Earlier work had shown that proteins tend to initiate freezing in relatively “warm” clouds (above about 28°F), while mineral dust dominates in very cold clouds (below -4°F).

But the Southern Hemisphere offers limited dust sources. Many mixed-phase clouds linger in the middle range – exactly where the new study shows marine polysaccharides are most potent.

Study lead author Roland Schrödner from TROPOS analyzed the data using the TM5 global atmospheric chemistry transport model.

“In our simulations, we were able to show that at -15 to -16 degrees Celsius (5 to 3°F), the polysaccharides over the gigantic areas of the oceans in the clean Southern Hemisphere are probably the most important ice nuclei,” said Schrödner.

“They contribute more to ice formation than mineral dust emitted from the deserts, which is the main type of ice nuclei in climate models. This is a new and important finding for climate models.”

Years of data point to sugars

The study knits together aerosol microphysics, atmospheric chemistry, and global modeling groups at TROPOS.

Polysaccharide concentrations had been sampled during multiple expeditions: the Spanish Antarctic PI‑ICE mission, the German Arctic PASCAL/PS106 cruises, the tropical Atlantic MarParCloud campaign, and long‑term measurements on Spitsbergen.

Only by combining those datasets with lab freezing tests and model simulations could the team quantify the climatic punch of marine sugars.

Next steps for cloud ice research

If nations succeed in cutting man‑made emissions, natural aerosol particles will matter even more for cloud behavior. Clean‑air regions respond sharply to even small aerosol changes, making the Southern Hemisphere a prime natural laboratory.

Beginning July 2025, the HALO‑South aircraft mission – led by TROPOS – will probe clouds, aerosols, and radiation over the Southern Ocean near New Zealand.

A coordinated ground campaign, goSouth‑2, will deploy advanced remote‑sensing gear near Invercargill from September 2025 to March 2027. The goal is to watch clouds evolve in this pristine environment.

Life at the ocean’s surface shapes cloud brightness, revealing deep links between biology and the atmosphere.

The new evidence that marine polysaccharides help seed ice over huge swaths of the Southern Ocean is a vivid reminder: what grows in the water below can change the sky above – and, through clouds, influence Earth’s climate itself.

The study was presented at the EGU General Assembly 2025 in Vienna, Austria. An abstract of the article can be found here.

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