text: With 800,000 years of Azolla bloom episodes and a  basin to cover, even by very conservative estimates more than enough carbon could be sequestered by plant burial to account for the observed 80% drop in CO by this one phenomenon alone. Other factors almost certainly played a role. This drop initiated the switch from a greenhouse to the current icehouse Earth; the Arctic cooled from an average sea-surface temperature of 13 °C to todays &minus;9 °C, and the rest of the globe underwent a similar change. For perhaps the first time in its history, the planet had ice caps at both of its poles.  A geologically rapid decrease in temperature between 49 and , around the Azolla' event, is evident; dropstones (which are taken as evidence for the presence of glaciers) are common in Arctic sediments thereafter. This is set against a backdrop of gradual, long-term cooling; it is not until  that evidence for widespread northern polar freezing is common.