The researchers adjusted a global climate model, GENESIS, to the coldest point just before the collapse and used climate outputs of temperature and precipitation to drive a dynamic ice-sheet model. They used paleomagnetic reconstructions of land mass distributions for 750 and 540 million years ago, but, because the locations of major mountain chains are unknown that long ago, they put mountains analogous to the Andes, all around the edges of tropical land masses in their ice-sheet model.
"Ice sheets did form on the tops of these mountains," says Pollard. "However, the ice sheets never flowed down to sea level, where we find glacial deposits. Tropical temperatures were still too warm and melted the ice before it could flow down from the mountains." The researchers conclude that it is unlikely that tropical sea level glacial deposits formed before the collapse into snowball Earth. However, having them form after the oceans freeze also seemed problematic because once the oceans are frozen, the rates of precipitation decrease drastically, to only a few millimeters per year.
"However, in further simulations with the global climate model for full snowball conditions, snowfall did exceed evaporation of snow and ice in some land areas, allowing a slow build up of tropical ice sheets that would eventually flow to the sea," says Pollard. "It would have taken several thousand years to form big ice sheets this way, but since it takes several million years to reverse snowball Earth, there would have been plenty of time for the ice to form."