They’re invisible to the human eye but once helped cool the planet. Now, the very algae that slowed global warming 14,000 years ago are disappearing and their vanishing act could accelerate the climate crisis.

New research led by scientists at Germany’s Alfred Wegener Institute has uncovered how microscopic algae in Antarctic waters pulled enormous amounts of carbon dioxide out of the atmosphere during a critical period of post-Ice Age warming. But those same organisms, once central to Earth’s natural cooling system, are now being lost due to rapidly melting sea ice.

Algae That Helped Trigger a Global Pause

The focus of the study is a specific type of phytoplankton called Phaeocystis. During a period known as the Antarctic Cold Reversal, Phaeocystis blooms exploded in the Southern Ocean following intense spring sea ice melt. These blooms captured and buried CO₂ at scale, playing a major role in slowing Earth’s warming after the last ice age.

This insight has eluded climate science until now. Unlike typical fossilized records, Phaeocystis left behind no visible traces. It was only through a new technique analyzing ancient DNA from deep-sea sediment cores that researchers discovered their presence and impact.

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“We now know these algae contributed to a significant drop in global atmospheric CO₂ during a climatically crucial period,” said Josefine Friederike Weiß, lead author of the study.

Sea Ice and Algal Blooms: A Delicate Balance

The researchers recovered sediment from nearly 2,000 feet beneath the Bransfield Strait near the Antarctic Peninsula. There, genetic fingerprints of Phaeocystis revealed an ecosystem once thriving in a delicate cycle of sea ice growth and melt.

When sea ice expanded further in winter and melted rapidly in spring, the resulting nutrient-rich runoff created perfect conditions for algal growth. The more ice there was, the more algae flourished – and the more carbon was drawn from the atmosphere and sent to the deep ocean floor.

However, in today’s climate, that balance is unraveling.

A System on the Brink

Antarctic sea ice is now retreating at record rates. The ideal conditions that once fueled massive Phaeocystis blooms are disappearing. And with them, we risk losing one of nature’s most effective carbon sinks.

This isn’t just about algae. These blooms form the foundation of the polar marine food web – feeding microscopic animals, fish, seals, and whales. As Phaeocystis fades, so too could the ecosystems that depend on them.

Worse still, these algae also emit dimethyl sulfide (DMS), a compound that helps form clouds. More clouds reflect sunlight away from the planet, cooling the atmosphere. Less DMS means fewer clouds, less cooling and a faster warming feedback loop.

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Learning from the Past to Protect the Future

The study is a reminder that climate isn’t just physics and chemistry, it’s biology. By blending traditional geology with cutting-edge DNA tools, researchers are reconstructing how life itself has regulated Earth’s climate over millennia.

As nations double down on carbon targets, this work adds an important dimension: to forecast future climate change, we must understand the living systems that buffer it. That includes microscopic algae at the poles, which may yet hold the key to stabilizing a rapidly changing world.

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