Could a small fern help us reverse global warming?


The genome sequencing of Azolla filiculoides carried out 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 counteract the increase in greenhouse gas emissions and thus its effectiveness in combating climate change.

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

One of the most remarkable abilities of Azolla is its impressive capacity to capture CO₂: up to nine tons per hectare per year. To place 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 Azolla could be an important player in slowing down or even reversing climate change.

massive proliferation of Azolla filiculoides in a river.
Charlie Barnes/Flickr, CC BY-NC

Grow and die in the Arctic

About 49 million years ago (ma), the planet was a much warmer place and Azolla abounded in the Arctic Ocean. Fossil records show that during that 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 approximately 800,000 years known as the Azolla event.

Back then, the Arctic was very different. All the land masses were clustered around it, there were no polar ice caps, and a mild climate prevailed in a calm, closed sea where continental rainfall dumped millions of hectometers of nutrient-rich fresh water into a pool of salt water. Lacking agitation, the fresh and salt waters did not mix: the denser salt water sank to the bottom while the fresh water remained on top.

Schematic paleogeographic reconstruction (early middle Eocene; about 49 my ago) showing the geographic distribution of five species of Azolla in the Arctic Ocean (circled in red) and in the Nordic Seas. During that time the Arctic was an almost closed sea that was only connected to the rest of the seas by a small strait (yellow arrow).
Bark et al. 2012, Palaeogeography, Palaeoclimatology, Palaeoecology

Because there was no mixing, the saltwater layer was virtually anoxic, while the freshwater surface layer was highly oxygenated and received months of continuous sunshine. In those warm waters, Azolla prospered extraordinarily.

This little plant grows quickly, reproduces very quickly, and dies very quickly. Azolla it needs very few nutrients and gets all its nitrogen from the atmosphere thanks to symbiotic cyanobacteria, which means they can flourish and die without overconsuming the nutrients in the water.

A carbon dioxide sink

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

The effect was so great that during the event Azolla These tiny ferns extracted tens of billions of tons of CO₂: 80% of what exists in the atmosphere.

The concentration of this greenhouse gas went from 3,500 to 650 ppm. That rapid decline caused the poles to freeze and was one of the catalysts for the Ice Age that helped cool the planet to a similar climate to the one that prevails today.

Evolution of the concentration of carbon dioxide in the atmosphere in the last 60 million years.
Pearson and Palmer (2000), Nature

The solution to present climate change?

Our atmosphere currently contains about 420 ppm CO₂, a much lower concentration than when ferns ruled Azolla. To reverse man-made climate change we need pre-industrial concentrations of less than 300 ppm. Could we take advantage of a sink like the one in Azolla to combat the problem of climate change? Let’s do numbers.

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

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

We would therefore need to reproduce the Arctic and its conditions 49 million years ago. Unfortunately, there is nowhere in the world that is quite like it, so we would have to be a little 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 unprecedented folly, we could transform 77% of all those lakes into huge farms of Azolla. First of all, we would have to kill off all the native life and then we could engineer oxygen dead zones at the bottom and provide the necessary nutrients to start an explosion of the ferns. What’s more, with modern farming methods, we could have an even higher CO₂ uptake rate than during the original bloom of Azolla as long as we guarantee the perfect conditions. Now, it doesn’t seem very reasonable to destroy the original ecosystems of these lakes to save the planet.

doAzolla 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 at least serves to highlight our ability to self-destruct by accelerating global warming.


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