The authors argue that the Europeans accidentally triggered the Little Ice Age by marching into Mexico and South America with guns, germs, and steel — especially germs. Diseases like smallpox and measles travelled on Indigenous trade routes far ahead of the Spanish and Portuguese. In the century after the conquest of Mexico, Lewis and Maslin claim, 50 million Indigenous people died of disease or slavery. In that truly genocidal century, farming civilizations collapsed from Brazil to North America. The survivors left the cities and temples they’d built and reverted to village-level farming. Forest and jungle took over the old empires before the Iberians rode in to take over the villages. Lewis and Maslin argue that as forests reoccupied the farmlands that had fed 50 million people, they absorbed enough carbon dioxide to trigger the Little Ice Age. Antarctic ice cores show a drop in global carbon dioxide starting in 1520 that did not begin to rise again until 1610.