Chemical signatures from sediments in lake cores reveal that the centuries-long drought during the fall of Classic Maya civilization was worse than researchers had imagined.

For centuries, the Maya people relied on rain to keep them alive. But then, suddenly, the skies went dry.

At least, that’s what the latest research suggests.

From about 250 to 900 CE, the Maya thrived in what’s known as its Classic period. During this time, the Maya built cities with plazas and multistory temples, devised a complex calendar system, and housed an urban population density that rivals Los Angeles County today.

But then, sometime between the 8th and 9th centuries, many of the bustling Maya cities fell silent. By around 900 CE, a number of the grand cities had been abandoned.

“Rainfall decreased on average by about half and up to 70% during peak drought conditions.” Scholars have many theories about what went wrong. Some speculate that deforestation drove people away; others believe that war and political strife tore cities apart. Still more note that the whole idea of a collapse is too simplistic because not all Maya cities fell, and some were reinhabited. The jury is still out because none of the hypotheses can fully explain what caused a society advanced enough to conceptualize the number zero and potentially predict meteor showers to crumble.

A study unveiled today in Science offers fodder for another answer: a severe drop in rainfall that coincided with the Maya downfall.

At the end of the Classic period in the northern reaches of the Maya civilization, “rainfall decreased on average by about half and up to 70% during peak drought conditions,” lead author Nick Evans, a Ph.D. candidate at the University of Cambridge, told Eos. Given the finding, “our research provides another piece of the puzzle for understanding the Maya collapse,” he said.

A Rain Forest in Drought

The Maya people lived in the lush rain forests throughout what’s mostly now modern-day Guatemala, Belize, and southeastern Mexico. Scholars believe that the Maya relied heavily on rain to fuel their maize fields and fill drinking reservoirs.

But past research indicated that the humid ecosystem of the northern rain forests, near population centers like Chichén Itzá and Uxmal, may have withered. Previously published paleoclimate data gleaned from proxies within lake cores on the Yucatán Peninsula revealed that a drought befell the area during 800–1000 CE.

However, the magnitude of this drought remained unclear, the authors noted in their paper. Was it a mild shift toward less precipitation or an intense dry spell?

To find out more, researchers examined the same lake as in the prior study: Lake Chichancanab, a salty body of water that lies in the northern Yucatán.

Water, Trapped in Crystals

Within Lake Chichancanab, the authors looked at stable isotopes in gypsum, a soft sulfate mineral. Normally, gypsum is dissolved in the lake’s water. But if the lake shrinks—because of, say, a drought—the gypsum reaches saturation and starts raining out as a solid onto the lake bed. The prior paleoclimate data, among other findings, pointed to the mere presence of gypsum in the sediment record as evidence of drought.

Hydrogen and oxygen atoms, the researchers hypothesized, hold the key to understanding the drought. But the new study went a step further. Gypsum is a hydrous mineral, meaning that it has two water molecules bound in its crystalline structure. When gypsum precipitates out from the lake waters, it takes with it several hydrogen and oxygen atoms, capturing in its crystals the characteristics of the lake water from which it formed. These hydrogen and oxygen atoms, the researchers hypothesized, hold the key to understanding the drought.

Oxygen (O) has several types of naturally occurring isotopes. Each isotope has only eight protons in its nucleus but different numbers of neutrons. The lightest of the bunch, 16O, is the most easily evaporated because it’s the lightest load. The heavier oxygen isotopes, 18O and 17O, are more likely to stay behind and not evaporate. The same relationship applies for hydrogen’s stable isotopes.

During a severe dry spell, Lake Chichancanab would have suffered from increased evaporation, meaning that over time, evaporation “enriches the lake in the heavier isotopes of hydrogen and oxygen in water,” the authors wrote. Did this enrichment happen during the previously identified drought?

The Heavy Isotopes’ Tale

The researchers checked the gypsum from Lake Chichancanab sediment cores collected in a prior study and found just that: heavier oxygen and hydrogen isotopes appeared in higher concentrations during times of suspected drought compared to the isotope ratios in modern-day lake waters. From 750 to 1050 CE, intermittent droughts likely plagued the region, specifically around 750–850 CE and 950–1050 CE.

The researchers then simulated how bad the drought would have been to make the isotope ratio seen in the gypsum. The simulations gave a stark number: Precipitation in the area was likely on average 50% of its predrought level. During the worst of the drought, precipitation may have even plummeted by 70% of its predrought level.

“Prior results pointed to more modest reductions in rainfall,” Evans told Eos, “but our new methodology using all stable isotopes in water reveals that the droughts in northern Yucatán were more severe than previously thought.”

Many Questions Remain

How do the new insights shape our understanding of Maya collapse?

“The next step,” said Evans, “is for agronomists to use this information in crop simulation models to predict the impact drought had on Maya agriculture.” He also hopes that archaeologists studying ancient Maya’s collapse will incorporate these results to better explain the cultural transformation that must have occurred when the rains stopped.

“In the Yucatán, where we do have evidence for drought…we do not have permanent abandonments. So clearly other factors also play a role.” But Daniela Triadan, an anthropologist at the University of Arizona who focuses on Maya culture and was not involved in the research, cautioned against applying the results in this study to all of the Maya civilization. For example, she noted that cores from lakes to the south of the Yucatán don’t show signs of drought.

Thus, at more southern Maya sites like Tikal and Seibal, “we have abandonments of settlements with no noticeable [population] returns occurring over a period of about 100 years,” she said. “This is incidentally what people call the Maya ‘collapse.’ And interestingly, in the Yucatán, where we do have evidence for drought—and this paper makes an excellent case—we do not have permanent abandonments. So clearly other factors also play a role.”

She added, “I think we need to be careful not to make this a single factor narrative.”

A Lesson for Today?

If drought did help lead to the Maya downfall, could it be an alarm bell for severe droughts that humans face today? Evans said that there “are no direct parallels” between the Maya drought his team documented and today’s world because of the modern invention of genetically modified organisms (GMOs), drought-resistant plants, and transportation systems to redistribute water and food during shortages.

“Depending on how society copes with environmental changes, outcomes can be very different.” But, he says, “in an abstract sense, there are lessons to be learned about the sensitivity and resilience of society to climate change.”

Takeshi Inomata, an archaeologist at the University of Arizona who focuses on the Maya and was not involved in the new study, agrees. “Climate change does not lead automatically to societal collapse. Depending on how society copes with environmental changes, outcomes can be very different,” he said. “As we try to deal with climate change in the future, [archaeological] studies need to focus more on how past societies like the Maya coped with natural disasters successfully or unsuccessfully.”

—Jenessa Duncombe (@JenessaDuncombe), News Writing and Production Intern