Remember that ice cover has a strong, positive feedback loop. If we melted the ice caps, it would cause a large decrease in the albedo of the polar regions which would cause them to warm significantly. Another influence on the climate was the thermohaline circulation of the oceans. Unfortunately, nobody has yet demonstrated that this mechanism could work. Also, it has been thought that the tropical Hadley circulation extended further poleward than it does today.
Hadley cells are very efficient at transporting heat.
High levels of atmosphere may also have been caused by other factors than continental drift. One theory is that there have been variations in the amount of heat radiated from the sun. Another is that huge quantities of dust, put into the atmosphere during periods of intense volcanic activity, have decreased the amount of solar radiation reaching the Earth. In a different aspect of warming and cooling, the addition of carbon dioxide to the atmosphere as a result of burning more and more fossil fuels may cause global warming of atmosphere and oceans and extreme changes in climate, a problem we are dealing with today. Plate tectonics is the theory that the Earth is made up of thin, rigid plates that move relative to each other.