Forty-nine million years ago, the Arctic Ocean was nowhere close to the expanse of thick ice sheets and sub-zero waters we see today. It was a lush, steamy basin where crocodiles basked, palm trees swayed and turtles glided through waters as warm as the Gulf of Mexico.
The polar regions were a greenhouse paradise, thriving under a thick blanket of atmospheric carbon dioxide (CO₂), with levels hovering around 3,500 parts per million—almost nine times higher than today. It seemed like the planet was locked into an eternal summer, with no ice in sight.
But nature had other plans.
It would take one tiny plant—not a catastrophe, not a volcanic eruption—to tip the balance and send Earth spiraling into an ice age that would last nearly a million years.
That plant was Azolla—a fern no bigger than a fingernail. And its story may hold the key to fighting climate change today.
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Azolla Is A Fern Like No Other
This genus of tiny, fast-growing aquatic ferns, is sometimes called the “water fern” or “mosquito fern” for its ability to form thick mats over ponds and slow-moving water.
Found in freshwater environments worldwide, these plants have an extraordinary trait—they form a symbiotic relationship with a nitrogen-fixing cyanobacterium (Anabaena azollae) that allows them to thrive even in nutrient-poor waters.
This means Azolla can grow explosively, doubling its biomass in just two to three days under the right conditions.
But what really sets Azolla apart is its capacity to pull carbon and nitrogen from the environment at an unprecedented rate. During photosynthesis, Azolla absorbs massive amounts of CO₂ which it traps within its tissues.
When it dies and sinks to the bottom of water bodies, much of this carbon is effectively buried, removing it from the atmosphere for millennia. Azolla also excels at soaking up nitrates and phosphates, making it a powerful tool for cleaning polluted waters.
In agricultural settings, farmers have long recognized its benefits. In Asian rice paddies, for example, Azolla has been used for over a thousand years as a biofertilizer, improving soil quality and reducing the need for synthetic fertilizers. This humble fern isn’t just a carbon sponge—it’s an environmental multitool.
A pond covered in Azolla, an aquatic fern. It is frequently used in rice production as green manure ... [+] and biofertilizer due to its nitrogen-fixing capabilities.getty
How One Tiny Fern Caused An Ice Age
Roughly 49 million years ago, a perfect storm of geological events created an ideal environment for an Azolla takeover.
The Arctic Ocean, nearly isolated from larger oceans by land bridges, became a vast, shallow, nutrient-rich basin. This stagnant body of water developed a freshwater layer on top, fueled by river inflows and monsoon-like rainfall.
The warm, sunlit conditions were perfect for Azolla, which began to spread uncontrollably across the Arctic’s surface, forming dense mats that covered millions of square kilometers.
For 800,000 years, this Azolla bloom cycle repeated itself. As the plants grew, they absorbed vast amounts of CO₂ from the atmosphere. When they died, their remains sank into the oxygen-poor depths of the Arctic Ocean, where decomposition was limited.
Instead of releasing the stored carbon back into the atmosphere, Azolla effectively locked it away in sediment, preventing it from contributing to the greenhouse effect.
The impact was staggering.
Azolla’s sequestration contributed to lowering atmospheric CO₂ levels from 3,500 ppm to around 500 ppm, according to an August 2000 study published in Nature—a dramatic decline that cooled the planet. Over time, Earth’s climate shifted from a “greenhouse” state to the cooler “icehouse” conditions we know today.
The Arctic eventually froze over, polar ice caps formed and the global climate system changed forever.
This transformation was not an overnight shift, but rather a prolonged process fueled by Azolla’s relentless carbon sequestration. For 800,000 years, the fern continued to bloom and sink, gradually stripping the atmosphere of CO₂ and reinforcing the cooling trend.
The sustained drawdown of greenhouse gases led to a cascading series of climate shifts. With CO₂ concentrations plummeting, global temperatures steadily declined and the planet’s capacity to trap heat continued to drop. The cooling effect altered oceanic circulation patterns, leading to stronger thermohaline currents that further distributed cold water across the globe.
Meanwhile, the Arctic Ocean, once a warm and stratified basin, saw its surface waters cool significantly, eventually allowing sea ice to form.
Azolla Could Cool The Planet Again—But Only Under The Right Conditions
Fast forward to today, and we're facing the opposite problem: rising CO₂ levels, global warming and accelerating climate change. Could Azolla help us reverse course once more?
Theoretically, yes.
The same carbon-sequestering abilities that helped trigger an ice age could be harnessed to mitigate modern climate change. Large-scale cultivation of Azolla in controlled aquatic environments could pull significant amounts of CO₂ from the air.
According to an October 2021 study published in Environmental Science and Pollution Research, strategically growing Azolla in an area equal to a fifth of the Amazon forest could help offset the annual increase in carbon emissions.
However, there’s a lot that needs to be addressed before we can cool the planet again.
Unlike the Eocene Arctic, today’s oceans aren’t ideal for Azolla expansion. The plant thrives in freshwater, and large-scale cultivation would require controlled ponds, wetlands or modified agricultural systems to prevent unwanted spread.
There’s also the question of what to do with the harvested biomass. If Azolla is left to decompose in open water, much of its stored carbon could be re-released into the atmosphere, negating its benefits. Effective carbon sequestration would require burying Azolla biomass or converting it into biochar for long-term storage.
Beyond carbon capture, Azolla’s potential extends to wastewater treatment and sustainable agriculture.
The fern’s ability to absorb nitrates and phosphates makes it an excellent candidate for cleaning polluted rivers and lakes. In agriculture, it could replace synthetic fertilizers, reducing soil degradation and improving crop yields without contributing to greenhouse gas emissions.
Azolla has already shaped Earth's climate once before, transforming a greenhouse world into an icehouse. Now, as humanity struggles with rising temperatures and escalating CO₂ levels, this ancient fern might just offer a way forward.
But as with any climate solution, there’s no silver bullet. Successfully using Azolla to combat climate change requires careful planning, large-scale infrastructure and a strategy to ensure the carbon it captures remains locked away. It also requires balancing its benefits with the risks of uncontrolled growth, particularly in non-native ecosystems.
Still, the story of Azolla serves as a powerful reminder: sometimes, the most profound changes come from the smallest of things. The same tiny fern that froze the planet millions of years ago might just hold the key to cooling it down again.
Azolla’s massive impact exposes the potential hidden across our natural world to heal and rejuvenate itself. Take a 2-minute quiz to find out where you stand on theClimate Change Worry Scale.