From 1550 to 1850, Earth mysteriously got colder. Communities from Greenland to the Alps were swallowed up by glaciers, and bodies of water like the Baltic Sea and Manhattan Harbor froze over. But what caused this Little Ice Age?
A lot of explanations have been put forward to explain this mysterious cooling, including particularly weak solar activity, changes in the Earth's orbit, and just the general variability of climate over time. Indeed, one of the more intriguing proposals is that the massive waves of death that struck the human population - first with the Black Death in the 14th and 15th centuries, and then the widespread death of indigenous Americans after European contact - caused massive amounts of former agricultural land to become forests again, absorbing lots more carbon dioxide and cooling the planet.