Climate Change May Have Caused Collapse of Cahokia, America's First City

Cahokia mound
File photo: A Cahokia mound. iStock

America's first city—a cosmopolitan metropolis built on the banks of the Mississippi River around 600 A.D.—was once home to tens of thousands of people. It spanned roughly 6 square miles and featured a town center, public plazas and around 120 huge, man-made earthen mounds, some of which were used for human sacrifice and mass burials.

The city, named Cahokia, grew at a steady pace for the first 400 years—but between 1050 and 1100 A.D., the population exploded, reaching an estimated peak of 20,000 people. Two centuries later, the population started to decline, and the city was completely abandoned by 1400.

What caused the collapse of this civilization is unknown, and a number of ideas have been hypothesized. Some researchers have suggested the land was over-cultivated and eventually could not provide enough crops to feed the growing population. Other ideas include disease, as well as drought, flooding, and political and social unrest.

Now, a study published in PNAS suggests that climate change could have been one of the driving factors behind the abandonment of Cahokia.

The team of researchers, including lead author A.J. White, examined the relationship in Cahokia over the years between population size and climate change.

To estimate population size, they analyzed human fecal samples found in a nearby lake. While mostly the people of Cahokia would have defecated on land, some feces would have ended up in bodies of water, where it becomes trapped in layers of sediment. This allowed the researchers to estimate how many people were living in the region at certain points in history.

As expected from previous archaeological evidence in earlier studies, the stool concentrations showed that human occupation increased from 600 A.D., reaching its population peak in 1100. By 1200, numbers started to decline, and by 1400 all the inhabitants had left.

Researchers then compared this information with environmental studies that point to the climate in the region over this period. Lake cores showed evidence of droughts and floods, with significantly reduced rainfall around the time that Cahokia started to decline. The researchers suggest this would have impacted the population's ability to grow maize, impacting food production as a result.

Around 1150 A.D., there is evidence of a huge flood along the Mississippi River. At this point, the team found a significant shift in the number and density of houses being built, as well as a change to craft production. This all suggests there was some sort of social or political stress leading to "a reorganization of some sort."

The researchers say the droughts and flood caused huge stress for the Cahokia's residents, and this was then compounded during the shift from a period of warming, known as the Medieval Climatic Anomaly, to the Little Ice Age, which began around 1300. They say climate change can have a huge influence on societies, posing "significant challenges" that can lead to socio-political change—and this still has relevance in modern times.

"Cultures can be very resilient in face of climate change but resilience doesn't necessarily mean there is no change," said study co-author Sissel Schroeder, a professor of anthropology at the University of Wisconsin–Madison, in a statement. "There can be cultural reorganization or decisions to relocate or migrate. We may see similar pressures today but fewer options to move."