This transition has an extensive literature associated with it, and while it is not yet fully understood, a very interesting model has been suggested by: PaillardD., “The timing of Pleistocene glaciations from a simple multiple-state climate model”, Nature391, 378–381 (1998).
KastingJ.F. and GrinspoonD.H., The Faint Young Sun Problem, contained in SonettC.P.GiampapaM.S., and MatthewsM.S., editors, The Sun in Time (University of Arizona Press, Tucson1991), pp. 447–462; GoughD.O., Solar Interior Structure and Luminosity, Solar Physics74, 21–34 (1981); BernerR. A.LasagaA. C., and GarrelsR. M., “The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years”, Am. J. Sci.283, 641–683 (1983).
6.
RoyerD. L., “CO2-forced climate thresholds during the Phanerozoic”, Geochim. Cosmochim. Acta70, 5665–5675 (2006).
7.
See, for example, TuchmanB. W., A Distant Mirror (Ballantine Books, New York1978), Chapt. 2; KerrR. A., “The Little Ice Age—Only the Latest Big Chill”, Science284, 2069 (1999).