Researchers from the University of Reading are looking into the mechanisms in which Earth's climate would transition from a warm one to a very cold one through the fluctuations in solar radiation.
They believe that the Earth enters a "melancholia" state once every ten million years where the whole of Earth's surface will be blanketed in ice. They looked into two feedback mechanisms which would account for these phenomena. You may see the entire research on Physical Review Letters.
(Image credit: NASA/APS)