This Fern is the Best Analog We Have for Stopping Climate Change 

December 21, 2022

By studying geological history, we can extrapolate solutions for the present.

We’ve already seen large scale planetary removal of carbon dioxide from the atmosphere. 49 million years ago, carbon dioxide levels plummeted 80% over a relatively brief moment in geological time. After stumping researchers for years, one fern, named Azolla, has emerged as an unlikely climate hero. By proliferating and sinking to the bottom of the Arctic, it helped create the conditions that gave birth to life as we know it today. 

In this post we’ll dive into the latest research on the Azolla Event, look at how the biosphere and atmosphere interact at scale, and discuss how we can leverage these learnings to address our current climate instability.

The Azolla Event

The Azolla event, discovered only in the last 20 years, gives an explanation for how we got from one of the hottest periods on Earth to our current (and tenuous) icehouse equilibrium. 

Life in the Eocene

In the early Eocene, global surface temperatures were 8℃ higher than today. Tropical forests extended almost to the poles, and atmospheric CO2 concentration was 3500 ppm in contrast to today’s 400 ppm! Then, 49 million years ago, the global climate cooled 6℃, with a simultaneous drop of atmospheric CO2 to 650 ppm, beginning the process that led to the development of the Antarctic ice sheet, and the annual freezing of the surface of the Arctic Ocean.

The Azolla Hypothesis

This dramatic drop stumped researchers until a convincing hypothesis was proposed in 2004. At this time, the Arctic Coring Expedition took advantage of reduced ice cover to take core samples from the Lomonosov Ridge, in the Northern Arctic Ocean. To their surprise, they discovered huge deposits of the freshwater aquatic fern Azolla locked under the seabed. 

Drill core from the Lomonosov Ridge composed of compacted Eocene Azolla

These deposits, up to 20 meters deep, were laid down 49 million years ago, sustained over 800,000 years, and exactly coinciding with the observed drop in temperature and CO2. Noting this correlation, the researchers proposed a causal link, now widely accepted, between the growth of Azolla and the massive drawdown in atmospheric carbon dioxide, accounting for the sudden global cooling in the late Eocene.

A Highly Improbable Event

Azolla is incapable of growth in salt water, and was only able to grow in huge volumes in the Arctic Ocean due to a set of contingent and unusual factors. These factors also ensured that the carbon absorbed by the fern through photosynthesis was sequestered on the seabed rather than being returned to the atmosphere when the plants decomposed. 

During the Eocene, the Arctic Ocean had limited water exchange with the rest of the world ocean and received the flow of a number of large rivers. The lower density freshwater from these rivers pooled on the surface, causing a low salinity layer to form. This layer prevented mixing of the water column, and caused the seabed to become highly anoxic, while also enabling Azolla to proliferate across the surface. As a floating plant with leaves above the water, Azolla had direct access to atmospheric CO2 and only needed a little over an inch of freshwater under it to grow.

Azolla is a fern that floats freely on the surface of calm freshwater bodies, and it is one of the fastest growing plants on the planet, doubling its biomass in as little as two days.   (Photo cred)

Additionally, and unusually among ferns, Azolla is able to fix nitrogen directly from the atmosphere with the help of Anabaena, a blue-green alga (cyanobacterium) related to Spirulina, removing a further limitation to growth. Decomposing organisms require oxygen in order to burn carbon as fuel for their metabolism, and this was severely depleted on the seabed. Therefore, when the ferns died and sank, they were preserved intact until buried by sediment, and the carbon they had absorbed in life was locked into the sedimentary rock under the sea. 

Azolla and the blue-green alga Anabaena azollae maintain a symbiotic relationship: the alga provides nitrogen to the fern, and the fern provides a habitat for the alga. (Photo cred)

Partnering with plants aligns us with the planetary leaders in carbon removal

We must learn to remove carbon faster than ever before, and here the geological record is clear: partnering with plants aligns us with the planetary leaders in carbon removal. Previous episodes of warming over geological history have only ended when specific events in the biosphere caused a mass drawdown of CO2. The cessation of emissions alone has never been enough to reduce atmospheric CO2 on a sufficiently short timescale to be relevant to human civilization. The Azolla event is particularly informative as it shows how specific conditions in the physical environment can cause a self-sustaining biological process with a favorable ratio of carbon extraction to energy investment.  We can adapt the same principles of the Azolla Event, which occurred over a consecutive 800,000 years, to a human effort, which would need to be effective within a human lifetime.

That is our mission at Living Carbon, where we are working with billions of years of evolution to develop living solutions to our climate instability. The Azolla event was so impactful because, one, there was a high rate of photosynthesis, and, two, there was no release of CO2 back into the atmosphere via decomposition. That’s why we’ve created a photosynthesis-enhancement trait, to allow plants to store more carbon on less land, and are developing a trait to slow decomposition. We’re also investigating forms of more permanent biological carbon storage, and the partnerships that different plants have with nitrogen-fixing organisms

The mighty Azolla fern and its partner Anabaena are worthy heroes for our climate story. Give them an inch of freshwater and they can change the world. Imagine what humans could do if we put our creative capacity towards caring for the rare biospherical conditions that give us life.

This post concludes of our Deep Time Series.

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