The Azolla event was a catastrophic biogeological event which occurred approximately 49 million years ago (the middle Eocene) in the Arctic Ocean, which was warm and free of ice at the time. The Arctic Ocean was locked in on all sides by the continents, as Greenland, Europe, Asia, and North America were all connected in the north. This body of water became stagnant, much like the present-day Black Sea. Copious amounts of rainfall carried nutrients and fresh water into the Arctic Ocean (some might call it the Arctic Sea), creating a thin layer of freshwater at the surface.
This layer of freshwater would ultimately prove to a major impact on the planet's climate. Quickly after formation, it became colonized extensively by the freshwater fern Azolla. This species is sometimes considered a "super-plant" because of its rapid reproduction — it can double its biomass in 2-3 days until optimal conditions — and excellent nitrogen and carbon fixing capabilities, up to a tonne per acre per year of nitrogen and six tonnes per acre per year of carbon. As Azolla began to cover the 4,000,000 square kilometer (1,544,000 sq mi) basin, the Azolla event was starting, as dead ferns began sinking to the bottom of the sea, where anoxic conditions, caused by a lack of water column mixing, meant that the detritus was fossilized without being digested by bacteria.
After 800,000 years of fern formation, carbon and nitrogen fixation, and the sequestration of carbon on the floor of the Arctic Sea, the Azolla event began to have an impact on global carbon dioxide levels. Atmospheric carbon dioxide content dropped from 3500 ppm (parts per million) to just 650 ppm. The carbon dioxide sequestered by the Azolla event caused the Earth's greenhouse effect to break down, and the poles began to cool rapidly. Before the Azolla event, the average surface water temperature in the Arctic Ocean was 13 degrees C (55.4 degrees F), and after it, it was -9 degrees C (15.8 degrees F), which continues to be the average today.
The Azolla event has had a huge impact on climate patterns until the present day. The fundamental "greenhouse Earth" dynamic gave way to an "icebox Earth" dynamic, whereby the formation of glaciers caused the Earth's surface to become more reflective, thereby cooling it down further. The entire continent of Antarctica, previously forested, slowly began to freeze over. The effect culminated in an Ice Age which began approximately 23 million years ago and continues to the present day. Though we are currently in the middle of an Ice Age, the fact that the contemporary period is an interglacial means that continental glaciers are restricted to Greenland and Antarctica. For much of human history, glaciers covered much of North America and Eurasia.