Vast ice sheets (similar to the ones that now cover Greenland and Antarctica) once blanketed a significant portion of the North American continent. During the last million years, roughly the time during which humans have existed as a species, Earth experienced about a dozen glaciations. During the greatest of them, about 650,000 years ago, the Laurentide Continental Ice Sheet, with its northernmost reaches up near Hudson Bay, covered Chicago and points south in ice perhaps a mile thick. So much water was bound up in ice that the level of the seas was about 400 feet lower than it is currently.

Glacial periods tended to last much longer than the interglacial periods, with the last glacial period in human experience peaking about 20,000 years ago, with most of the ice melting during the period 17,500-12,500 years ago. Much of what we call human civilization, therefore, including the practice of agriculture, emerged only during the most recent interglacial period.

During the last million years or so (the Pleistocene Epoch), glacial periods have come at fairly regular intervals of about 100,000 years. This periodicity is partly explained by the “Milankovitch hypothesis,” which states that regular wobbles and tilts in Earth’s axis of spin, as well as stretches in its orbit, cause changes in how warm the planet is, especially in the Northern Hemisphere.