Several scientists over the centuries came close to proposing the idea of ice ages. Scottish naturalist James Hutton (1726–1797) observed strangely shaped glacial boulders (erratics) near Geneva, Switzerland. Based on this, he published his theory in 1795 that alpine glaciers were more extensive in the past. In 1824, Jens Esmark (1763–1839) proposed that past glaciation had occurred on a much larger continental scale.

But the most persuasive argument for ice ages came in 1837, when Swiss-American geologist Louis Agassiz (1807–1873) gave his now-famous speech on past widespread ice age conditions. He proposed that nearly all of northern Europe and Britain had once been covered by ice, and he subsequently found evidence for his theory in New England. Others eventually uncovered additional evidence. In 1839, United States geologist Timothy Conrad (1803–1877) discovered evidence of polished rocks, striations, and erratic boulders in western New York, supporting Agassiz’s theory that Ice Age glaciation was worldwide. In 1842, the first attempt to explain the ice ages using an astronomical connection was made by French scientist Joseph Adhemar (1797–1862). He proposed that the ice ages were the result of the 22,000-year precession of the equinox, a natural movement of the Earth’s axis that causes the seasons to switch over thousands of years. In other words, the current summer months would become winter months and vice versa.