In the 11th century, Cahokia was the biggest city in the Americas north of Mexico. Moreover, according to some estimates, it housed more people than London at the time, and no city within what is now the United States matched its population until after the War of Independence. Although many theories have tried to explain the decline and abandonment of such a significant site, new evidence from the feces the inhabitants left behind indicates that changes in climate were a big part of the story.
Cahokia sat on the eastern side of the Mississippi River, opposite modern-day St Louis. Estimates of its peak population vary but go as high as 40,000 people. Why such an important site would be abandoned has troubled archaeologists, but University of Wisconsin-Madison graduate student AJ White has found a possible answer in the human manure left at the bottom of nearby Horseshoe Lake.
White found molecules called fecal stanols, which are uniquely produced in the human gut. Much of Cahokia's sewage ended up in nearby water bodies (similar to many more recent cities), so the team used the ratio of human stanols to those from soil bacteria in each sediment layer to indicate the number of people living in the area at the time. The same sediments also indicate climatic conditions, since water containing lighter oxygen atoms evaporates more easily than water with heavier molecules.
In Proceedings of the National Academy of Sciences, White reports that the start of Cahokia's sustained population decline matches a reduction in summer rain, presumably affecting local maize production. Rainfall only decreased in the warm season, not the whole year, and in around the year 1150, the area experienced a flood so drastic that it appears to have induced a major population reduction, from which the city never recovered.
The work doesn't prove climate was the only reason Cahokia declined, but it does provide powerful evidence that it was a factor. Other research has suggested that the associated pollution in nearby water bodies was a factor in its fall.
By 1150, archaeological digs found signs that Cahokia was under stress, indicated both by falling housing density and a decline in the production of tools. At its peak, Cahokia was a manufacturing center, making quartz farming implements that were traded across much of North America.
Understanding how ancient cultures have responded to a changing climate has taken on new urgency recently, and co-author Dr Sissel Schroeder said in a statement: "Cultures can be very resilient in face of climate change but resilience doesn't necessarily mean there is no change. There can be cultural reorganization or decisions to relocate or migrate,"
Unfortunately, Schroeder warns that "we may see similar pressures today but fewer options to move."