The beginnings of humanity's modern understanding of the climate took shape in Switzerland in the 1830s. Three men played critical roles: a wild goat hunter who started to wonder about why the boulders near his home were shaped the way they were, a civil engineer who started to take note of sediment patterns in the river valley he was meant to manage, and a scientist who talked a lot with both of them and began to connect their dots with some of his own. Louis Agassiz was the scientist, and his idea was that the Swiss glaciers were part of something that had once been much bigger, a vast sheet of ice that had carved valleys and left behind lakes and ravines. He published his idea in 1840, that there had been an age of ice long before our current memory, and people started seeing other pieces of evidence all over the place. Evidence not just in Switzerland, but also in North America, where Agassiz later moved to become a professor at Harvard University. The moraines and kettle ponds he found in Massachusetts were linked to glaciers that had covered Europe and suggested that changes in climate had taken place at a global scale.

Until Agassiz, people had not seriously questioned whether the climate had been much different in the past than it is today. What Agassiz and then others showed was that it had. The last Ice Age ended about 12,000 years ago, and this has been only the latest part of a recurring cycle going back 2.7 million years and covering a period of time known as the Pleistocene Epoch, which has been marked most of the time by Ice Age conditions. Roughly every 100,000 years, there has been a respite from the ice of about 15,000–20,000 years, periods known a interglacials.