Earth scientists have discerned three factors at work globally during the 17th century: increased volcanic eruptions, twice as many El Niño episodes (unusually warm ocean conditions along the tropical west coast of South America), and the virtual disappearance of sunspots, reducing solar output to warm the Earth.

The 17th century saw a proliferation of wars, civil wars and rebellions and more cases of state breakdown around the globe than any previous or subsequent age. Just in the year 1648, rebellions paralyzed both Russia (the largest state in the world) and France (the most populous state in Europe); civil wars broke out in Ukraine, England and Scotland; and irate subjects in Istanbul (Europe’s largest city) strangled Sultan Ibrahim.

Climate alone did not cause all the catastrophes of the 17th century, but it exacerbated many of them. Outbreaks of disease, especially smallpox and plague, tended to be more common when harvests were poor or failed. When an uprising by Irish Catholics on Oct. 23, 1641, drove the Protestant minority from their homes, no one had foreseen a severe cold snap, with heavy frost and snow at a time and in a place that rarely has snow. Thousands of Protestants died of exposure, turning a political protest into a massacre that cried out for vengeance. Oliver Cromwell would later use that episode to justify his brutal campaign to restore Protestant supremacy in Ireland.

But the cold did take a more direct toll. Western Europe experienced the worst harvest of the century in 1648. Rioting broke out in Sicily, Stockholm and elsewhere when bread prices spiked. In the Alps, poor growing seasons became the norm in the 1640s, and records document the disappearance of fields, farmsteads and even whole villages as glaciers advanced to the farthest extent since the last Ice Age. One consequence of crop failures and food shortages stands out in French military records: Soldiers born in the second half of the 1600s were, on average, an inch shorter than those born after 1700, and those born in the famine years were noticeably shorter than the rest.

Few areas of the world survived the 17th century unscathed by extreme weather. In China, a combination of droughts and disastrous harvests, coupled with rising tax demands and cutbacks in government programs, unleashed a wave of banditry and chaos; starving Manchu clansmen from the north undertook a brutal conquest that lasted a generation. North America and West Africa both experienced famines and savage wars. In India, drought followed by floods killed over a million people in Gujarat between 1627 and 1630. In Japan, a mass rebellion broke out on the island of Kyushu following several poor harvests. Five years later, famine, followed by an unusually severe winter, killed perhaps 500,000 Japanese.