Azolla Event

The Azolla Event

49 million years ago, the carbon dioxide content of the Earth’s atmosphere was about 3,500 parts per million (0.35%). The temperature of the planet was hot enough for reptiles and warm climate trees to live in both the Arctic and the Antarctic.

Only 800,000 years later, the carbon dioxide level had dropped to 650 parts per million (0.065%).

The Earth’s climate flipped from the hot period it had been experiencing to the present situation where ice can cover most of Antarctica and the Arctic Ocean can freeze. Since then, the planet has been alternating between glacial periods when icecaps can form in many of the continents, and interglacial periods when most of the ice caps disappear and only the poles and high mountains have icecaps or glaciers.

What caused the sudden (by geological standards) drop in the CO2 level?

In the Arctic, cores drilled down reach a layer containing a lot of carbon. This layer is at least 8 metres think and may be as much as twenty meters. 49 million years ago, the Bering Strait and the other entrances to the Arctic Ocean were smaller than at present. The Arctic Ocean had a lot of fresh water coming into it from the numerous rivers.

The Arctic Ocean formed layers. The top layer was warm fresh water. The bottom layer was cooler saltier water with very little dissolved Oxygen in it.

Azolla grew on the surface of the fresh water of the top layer. A hectare of Azolla can remove about 2.5 tonnes of nitrogen, and 15 tonnes of CO2  a year from the atmosphere. The Nitrogen is taken from the air in the form of a gas and fixed into a chemical compound by the blue green algae part of the Azolla in a form that the fern part of the Azolla can use.

As the Azolla died, the plants sank and reached the bottom stagnant layer where there wasn’t enough Oxygen for the Azolla plants to decay. The Arctic islands, as well as the continents surrounding the Arctic Ocean had forests, and the rivers carried both organic matter and inorganic silt etc. that added to the dead Azolla plants.

Over 800,000 years, this formed the carbon rich layer found by the core drilling. This layer probably contains enough Carbon to account for the drop in the CO2  levels discovered in the geological record.

For an article about global temperature drivers, see:
https://www.quantamagazine.org/how-earths-climate-changes-naturally-and-why-things-are-different-now-20200721/

As of November 2024, the carbon dioxide content of the Earth’s atmosphere was 424 parts per million (0.0424%).

As of May the 11th 2025 the carbon dioxide content of the Earth’s atmosphere was 431 parts per million (0.0431%)

As of  November the 15th 2025, the carbon dioxide content of the Earth's atmosphere was 428 parts per million (0.0428%).