The Azolla event is a paleoclimatology scenario hypothesized to have occurred in the middle Eocene epoch, around , when blooms of the carbon-fixing freshwater fern Azolla are thought to have happened in the Arctic Ocean.  As the fern died and sank to the stagnant sea floor, they were incorporated into the sediment over a period of about 800,000 years; the resulting draw-down of carbon dioxide has been speculated to have helped reverse the planet from the "greenhouse Earth" state of the Paleocene-Eocene Thermal Maximum, when the planet was hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.