Whatever combination of factors explains the European acquisition of empires in the Americas, there is no doubting their global significance. Chief among the consequences was the demographic collapse of Native American societies. Although precise figures remain the subject of much debate, scholars generally agree that the pre-Columbian population of the Western Hemisphere was substantial, perhaps 60 to 80 million. The greatest concentrations of people lived in the Mesoamerican and Andean zones, which were dominated by the Aztec and Inca empires. Long isolation from the Afro-Eurasian world and the lack of most domesticated animals meant the absence of acquired immunities to Old World diseases such as smallpox, measles, typhus, influenza, malaria, and, later, yellow fever.

Therefore, when Native American peoples came into contact with these European and African diseases, they died in appalling numbers, in many cases losing up to 90 percent of the population. As one recent historian has noted, “It was as if the suffering these diseases had caused in Eurasia over the past millennia were concentrated into the span of decades.”5 The densely settled peoples of Caribbean islands virtually vanished within fifty years of Columbus’s arrival. Central Mexico, with a population estimated at some 10 to 20 million before the Spanish conquest, declined to about 1 million by 1650. A native Nahuatl (nah-watl) account depicted the social breakdown that accompanied the smallpox pandemic: “A great many died from this plague, and many others died of hunger. They could not get up to search for food, and everyone else was too sick to care for them, so they starved to death in their beds.”6

The situation was similar in Dutch and British territories of North America. A Dutch observer in New Netherland (later New York) reported in 1656 that “the Indians … affirm that before the arrival of the Christians, and before the small pox broke out amongst them, they were ten times as numerous as they are now, and that their population had been melted down by this disease, whereof nine-tenths of them have died.”7 To Governor Bradford of Plymouth colony (in present-day Massachusetts), such conditions represented the “good hand of God” at work, “sweeping away great multitudes of the natives … that he might make room for us.”8 Not until the late seventeenth century did native numbers begin to recuperate somewhat from this catastrophe, and even then, they did not recover everywhere.

As the Great Dying took hold in the Americas, it interacted with another natural phenomenon, this time one of genuinely global proportions. Known as the Little Ice Age, it was a period of unusually cool temperatures that spanned much of the early modern period, most prominently in the Northern Hemisphere. Scholars continue to debate its causes. Some have suggested a low point in sunspot activity, leading to less intense solar irradiation of the earth, while others have argued that the chief cause was volcanic eruptions, whose ash and gases blocked the sun’s warming energy in the upper atmosphere. More recently, some scientists have linked the Little Ice Age to the demographic collapse in the Americas. The Great Dying, they argue, resulted in the desertion of large areas of Native American farmland and ended the traditional practices of forest management through burning in many regions. These changes sparked a resurgence of plant life, which in turn took large amounts of carbon dioxide, a greenhouse gas, out of the atmosphere, contributing to global cooling. Whatever the causes, shorter growing seasons and less hospitable weather conditions adversely affected food production in regions across the globe.

While the onset, duration, and effects of the Little Ice Age varied from region to region, the impact of a cooler climate reached its peak in many regions in the mid-seventeenth century, helping to spark what scholars term the General Crisis. Much of China, Europe, and North America experienced record or near-record cold winters during this period. Regions near the equator in the tropics and Southern Hemisphere also experienced extreme conditions and irregular rainfall, resulting, for instance, in the growth of the Sahara Desert. Wet, cold summers reduced harvests dramatically in Europe, while severe droughts ruined crops in many other regions, especially China, which suffered its worst years of drought in the previous five centuries between 1637 and 1641. Difficult weather conditions accentuated other stresses in societies, leading to widespread famines, epidemics, uprisings, and wars in which millions perished. Eurasia did not escape lightly from these stresses: the collapse of the Ming dynasty in China, nearly constant warfare in Europe, and civil war in Mughal India all occurred in the context of the General Crisis, which only fully subsided when more favorable weather patterns returned in the eighteenth century.

Nor were the Americas, already devastated by the Great Dying, spared the suffering that accompanied the Little Ice Age and the General Crisis of the seventeenth century. In central Mexico, heartland of the Aztec Empire and the center of Spanish colonial rule in the area, severe drought in the five years after 1639 sent the price of maize skyrocketing, left granaries empty and many people without water, and prompted an unsuccessful plot to declare Mexico’s independence from Spain. Continuing drought years in the decades that followed witnessed repeated public processions of the statue of Our Lady of Guadalupe, who had gained a reputation for producing rain. The Caribbean region during the 1640s experienced the opposite condition—torrential rains that accompanied more frequent El Niño weather patterns—which provided ideal conditions for the breeding of mosquitoes that carried both yellow fever and malaria. A Maya chronicle for 1648 noted, “There was bloody vomit and we began to die.”9

Like the Great Dying, the General Crisis reminds us that climate often plays an important role in shaping human history. But it also reminds us that human activity—the importation of deadly diseases to the Americas, in this case—may also help shape the climate, and that this has been true long before the twenty-first century.