Many scientists have been considering the idea that the Earth may have completely frozen over sometime in the past. Kirschvink <1992> proposed that a runaway ice-albedo effect on continents amassed in low latitudes caused the descent into a global freeze. During a prolonged glacial period when oceans and atmosphere were isolated from each other, volcanic emissions would cause a buildup of atmospheric carbon dioxide (CO2), with no silicate weathering or photosynthesis to act as a sink. Eventually a reverse ice-albedo effect would cause a transient heat wave as the ice cover melted back rapidly.