According to a recent paper** published in the journal Nature, only a dramatic fall in atmospheric carbon dioxide levels are able to explain the transition from the mostly ice-free Greenland of three million years ago, to the ice-covered Greenland of today.
I am not convinced, but anyway, the paper begs the question, why did the purported elevated atmospheric carbon dioxide concentrations fall to levels similar to the pre-industrial era?
** Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels, Nature 454, 1102-1105 (28 August 2008), by Daniel J. Lunt, Gavin L. Foster, Alan M. Haywood and & Emma J. Stone.
Read the University of Bristol media release here.