Trump and the Azolla Event
About 50 million years ago, at a time known as the Paleocene-Eocene Thermal Maximum (PETM), atmospheric carbon dioxide levels were much higher than today, probably mostly because there had been a lot of volcanoes erupting over periods of many thousands of years. Consequently, average global temperatures were much higher; there was no ice at either pole and tropical vegetation spread over what is now the Arctic.
Then the CO2 levels dropped and temperatures declined.
Where did the carbon go?
The Arctic Ocean was all but cut off from the rest of the world's oceans as the gap between Norway and Greenland had not opened up. With large rivers and a much higher rainfall adding fresh water to the Arctic Ocean it seems likely that the water became stratified, the denser salt-water below and a layer of fresh water floating above. Over an area of perhaps four million square kilometers, a tiny little floating fern called Azolla could grow and spread across the surface.
Azolla can grow very fast, drawing carbon dioxide from the atmosphere. As the plants died their remains sank to the bottom into an anoxic environment where the carbon remained and was buried. Over a period of almost a million years countless trillions of Azolla plants sequestered enough carbon to reduce the atmospheric CO2 content by perhaps 80%, ending the intense greenhouse effect and allowing a cooler climate to return.
Azolla forms a symbiotic relationship with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen, giving the plant access to this essential nutrient. Given sufficient phosphates, it romps away, doubling its mass every few days. In south-east Asia it has been traditionally cultivated in rice paddies, leaving a nitrogen-rich mulch when the fields are drained.