Temperature of the earth
Four times, glaciers have covered much of the northern hemisphere. Between the glaciations, interglacial periods have typically been rather warm. After the end of the last glaciation, the Earth was warmer than it is now. This was long called the "climatic optimum," a term now dropped just because it makes it sound like we would be better off with a warmer climate. One cannot argue that global warming would be bad if that were the case.