The Eocene is not only known for containing the warmest period during the Cenozoic, but it also marked the decline into an icehouse climate and the rapid expansion of the Antarctic ice sheet. The transition from a warming climate into a cooling climate began at around 49 million years ago. Isotopes of carbon and oxygen indicate a shift to a global cooling climate. The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. One proposed cause of the reduction in carbon dioxide during the warming to cooling transition was the azolla event. The increased warmth at the poles, the isolated Arctic basin during the early Eocene, and the significantly high amounts of carbon dioxide possibly led to azolla blooms across the Arctic Ocean. The isolation of the Arctic Ocean led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments and effectively sequestered the carbon. The ability for the azolla to sequester carbon is exceptional, and the enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate. Cooling after this event continued due to continual decrease in atmospheric carbon dioxide from organic productivity and weathering from mountain building.