什么是 Greenhouse and icehouse climate states？

A greenhouse climate state is a long interval, millions to tens of millions of years, without large continental ice sheets, with high atmospheric carbon dioxide and a small temperature difference between the equator and the poles. An icehouse climate state is a long interval with continental ice sheets at one or both poles, lower carbon dioxide and a steep equator-to-pole gradient. The Earth has been in an icehouse state since about $34\,\mathrm{Ma}$.