A skeptical reader asked me recently how it is that climatologists are able to “know” how cold or warm Earth was tens of thousands or even millions of years ago. This is an excellent question.
One way is through ice core samples. Long-standing glaciers in Greenland and Antarctica have been collecting ice in a seasonal pattern and a carefully gathered core will reveal these layers. Inside each layer, microscopic air bubbles contain various isotopes of gasses, some of which are more likely at certain temperatures.
Another way is in the fossil record. Fossils of tropical plants in a certain layer of sediment, indicate that sediment was formed in a warm period.
Another way is lake bottom sediment cores which contain fossilized pollen grains of plants that had sunk to the lake bottom each year. Again, the types of plants can indicate what sort of climate was occurring at that time. These and other methods cannot tell us anything specific about past weather, but they can certainly reveal generalized periods of warm or cold weather over years or centuries.