Could a little fern reverse global warming?


The genome sequencing of filiculoid azolla conducted by more than forty scientists from around the world has reopened the chimerical dream of the ability of this small aquatic fern to help us counter the increase in greenhouse gas emissions and therefore its effectiveness in the fight against climate change.

Azola It is a genus that includes seven species of aquatic ferns so tiny that at first sight they can be confused with small algae or mosses. They are freshwater pleutophytes that form symbiotic relationships with atmospheric nitrogen-fixing cyanobacteria. Once attached, the fern can assimilate it as a nutrient, allowing them to grow so quickly that they become dangerous invasive plants.

One of the most remarkable abilities of Azola is its impressive capacity to capture CO2: up to nine tonnes per hectare per year. To put this figure in a comparative context, Spanish forests capture about five tons per hectare per year. With such powers, there are those who think that Azola could play an important role in slowing or even reversing climate change.

Fern, grow and die in the arctic

About 49 million years (ma) ago, the planet was a much warmer place and Azola abounded in the Arctic Ocean. Fossil records show that during this Eocene period, huge populations of this fern grew and reproduced floating in a then closed ocean. This organism and its accompanying abundant organic and siliceous freshwater microfossils indicate an episodic rise in Arctic surface waters during an interval of about 800,000 years known as the Azolla event.

Back then, the Arctic was very different. All land masses were clustered around it, there were no polar ice caps, and a mild climate prevailed in a calm, enclosed sea where continental precipitation poured millions of hectometres of nutrient-rich fresh water into a salt water pond. Without agitation, the fresh and salt water did not mix: the denser salt water sank to the bottom while the fresh water remained on top.

Because there was no mixing, the saltwater layer was virtually anoxic, while the surface freshwater layer was highly oxygenated and received months of continuous sunshine. In these warm waters, Azola prospered extraordinarily. This little plant grows fast, reproduces very quickly and dies very quickly. Azola it needs very few nutrients and gets all its nitrogen from the atmosphere through symbiotic cyanobacteria, which means they can thrive and die without overconsuming the nutrients in the water.

A carbon dioxide sink

Every summer there was a great bloom of Azola which covered almost the entire Arctic. Then the expansive mass of the fern quickly disappeared, and its remains were buried in the salt water that accumulated at the bottom. Since there was no oxygen, there were also no bacteria to decompose plant matter: every year thousands of tons of specimens of Azola they accumulated without breaking down on the seabed and thus trapped CO₂ on the seabed.

The effect was so great that during the event Azola These tiny ferns have extracted tens of billions of tons of CO2: 80% of what exists in the atmosphere. The concentration of this greenhouse gas has gone from 3,500 to 650 ppm. This rapid decline caused the poles to freeze over and was one of the catalysts for the ice age that helped cool the planet to a climate similar to that prevailing today.

Fern, the solution to current climate change?

Currently our atmosphere contains about 420 ppm of CO2, a much lower concentration than when ferns dominated. Azola. To reverse human-induced climate change, we need pre-industrial concentrations below 300 ppm. Could we enjoy a sink like the one in Azola tackle the problem of climate change? Let’s do numbers.

On average, the Azolla time domain reduced global CO2 each year by 0.0035625 ppm. This means that it would take around 31,000 years to achieve a drop from our current 410ppm to 300ppm, which paints a sad picture of our destructive capacity: if we could replicate one of the fastest cooling processes in Earth’s history, it would take more than 30,000 years to clean up the atmospheric damage we have caused over the past seventy years.

If we manage to maintain the CO2 concentration around 450 ppm, reproduce the event Azola and have a lot of patience, we could stop climate change. But there’s only one problem: could we really replicate it? After all, the global cooling of Azola he came from an entire ocean turned into a “farm” of this fern.

It would therefore be necessary to reproduce the Arctic and its conditions of 49 million years ago. Sadly, there’s no place in the world quite like it, so we should be a bit more practical. The ancient Arctic Ocean was 4,000,000 km². There are a total of 5,170,000 km² of freshwater lakes in the world.


In an unprecedented madness, we could turn 77% of all these lakes into huge farms of Azola. First we had to kill all the native life, then we could create oxygen dead zones at the bottom and provide the nutrients needed to trigger a fern explosion.

Moreover, with modern farming methods, we could have an even higher rate of CO2 uptake than during the original proliferation of Azola as long as we guarantee the perfect conditions. However, it does not seem very reasonable to destroy the original ecosystems of these lakes to save the planet.

doAzola can save the world as some think? Impossible: it would involve a great sacrifice and a level of commitment never made by humans; we would need to destroy some of the world’s most unique habitats and work for tens of thousands of years just to reverse the last seventy years of human activity. But doing the math serves at least to highlight our ability to self-destruct by accelerating global warming.

Character font: Manuel Peinado Lorca / THE CONVERSATION

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