The Fern That Cooled the Planet


Let’s talk about climate change, and I don’t mean the kind that’s happening right now. I mean the massive shift in climate that happened about 50 million years back, when Earth went from toasty warm to ice age. And that huge change may have mostly been caused by … a fern. Alright, so Earth was really hot 50 million years ago. I’m talking like, total greenhouse planet, lots of CO2 in the air, palm trees and alligators living near the poles. That kind of hot. Then something happened. The planet started to slowly cool, and all those poor gators had to relocate as the poles eventually formed ice caps, and the climate eventually shifted into cycles of hundred-thousand-year ice ages with shorter breaks in between them. In 2004, an Arctic Coring Expedition started poking around the North Pole looking for clues about what might have tipped the scales toward that global cooling so long ago.

When they pulled up sediment core samples from under the Arctic Ocean, they found a series of sediment layers that reached back nearly 80 million years. And sure enough, the scientists noticed something unusual right around the 50 million year mark. A column of tiny fossilized ferns that was almost 10 meters deep. That was …. surprising. The ferns were a type of Azolla, a genus of dime-sized, moss-like aquatic ferns that grow floating on the surface of water. Specifically though, fresh water. But if these ferns grow in fresh water, what were they doing in the arctic ocean? Well, you gotta keep in mind that the Earth’s geography was very different back then. The Arctic ocean was essentially landlocked, and researchers think that runoff from rivers formed a layer of fresh water over the saltwater. Which made it a cozy, nutrient-rich environment that Azolla ferns would have loved. Like, really loved.

The little plant flourished for nearly a million years, erupting in blooms that covered millions of square kilometers. Eventually, though, shifting landmasses reopened a connection to other oceans, causing a deadly influx of saltwater. That’s when the Azolla died and sank to the bottom of the ocean, forming the layers of sediment that we’d pull up millions of years later. But what does all this have to do with Earth cooling down? Well, as you probably know, long-term climate cycles have a lot to do with the atmospheric tug of war between various gases. Extra carbon dioxide, methane, and other greenhouse gases, for example, can trap heat and warm the planet. And Azolla may have helped remove a lot of those gases in a few ways. First, there’s the fern’s relationship with a type of cyanobacteria called Anabaena The bacteria pass between ferns through their reproductive spores, and live within their leaves. Anabaena is great at taking in nitrogen from the atmosphere, and using it to provide the fern with fertilizer. This fertilizing process is so effective that under the right conditions, Azolla can double its mass in just a couple of days. It also would have helped absorb lots and lots of nitrogen from the atmosphere.

There’s also the fact that Azolla, like all photosynthesizing plants, is really good at eating up carbon dioxide. In fact, researchers estimate that over the course of those million years or so, Azolla blooms might have gobbled up about half of all atmospheric CO2 — reducing carbon dioxide in the atmosphere from an estimated 2500 – 3500 parts per million down to like 1500 parts per million, and kicking off a cooling trend in the climate. When the Arctic eventually opened up again, those huge blooms sank deep into the ocean, where a lack of oxygen kept them from decaying, effectively keeping all of that carbon dioxide locked up, and out of the atmosphere. Azolla is still around today, and there are at least six known living species, and there’s enough of the stuff that it’s considered a weed in some places. It can be used as fertilizer, food for livestock, and has shown some promise in wastewater treatment. There’s also a crowdfunded research project that’s currently working on expanding our knowledge of the plant’s evolution and ecology by sequencing the Azolla genome.

Because the question on a lot of minds right now is… can Azolla help cool the planet again? With more research, we might just find out. Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, you can go to And don’t forget to go to and subscribe!.