50 million years ago during the Eocene, the Earth was much hotter than it is now, with levels of greenhouse gases methane and carbon dioxide in the atmosphere far higher than today. There was very little ice on the Earth at that time, and a tiny fresh-water plant called Azolla was able to flourish around the edges of the Arctic Ocean. Many scientists believe that there was so much Azolla drawing carbon dioxide out of the atmosphere that it caused the climate to cool. This so-called “Azolla Event”, along with other factors such as mountain building and changes in ocean currents, led the start of the glacial period in which we are still living.
Azolla is a tiny fern which contains a symbiotic cyanobacterium within its leaves which fixes nitrogen so efficiently that the plant is able to double in mass every 24 hours. This can cause problems in rivers, but some people (for example the Azolla Foundation – see links below) think that it could be used both as a source of food for animals and as a way of sequestering carbon dioxide from today’s atmosphere and so helping to reduce global warming.