The Mayan civilization suffered 44 years of drought in its last two centuries.

After centuries of splendor, the Mayan civilization began to decline in the 8th century AD, collapsing completely in the following two. They abandoned their cities, with their pyramids, stadiums, ceremonial plazas... and their fields, sustained by hydraulic engineering as ingenious and technological as that of their contemporaries, the Arabs. This decline has been blamed on neighboring wars, invasions from the north and south, disease, climate change... or a combination of all or several of these factors. But among them all, the impact of climate on the foundation of such agrarian societies stands out. Now, the study of a stalagmite from a cave near Mayan cities like Chichén Itzá has made it possible to clarify its role: according to this analysis, published in Science Advances , at least 44 of the last 200 years of the Classic Period saw extreme droughts.

Like the first agrarian empires of the Near and Middle East , based on the urban exploitation of surplus grain from the countryside, Mayan cities depended on the production of various crops, especially corn. And, like those , drought could have swept them away. What a group of researchers has now discovered is that from 870 to 1100 AD, there were eight extreme drought events in the Yucatán Peninsula, one of the core areas of Mayan civilization. And they define what an extreme drought means: three or more consecutive years with the dry season lengthening by at least three months, or, altogether, with no wet season at all.

The first of these droughts began in 894. It was followed by a year of normal rainfall, followed by five consecutive years with almost no rain. The longest-lasting event occurred in 929, when rainfall was anomalously low for 13 years. It was the longest-lasting drought on record, both pre-Columbian and in subsequent centuries.

Sediments at the bottom of lakes and the study of other stalagmites had already shown the prominent role of drought during the so-called Terminal Classic Maya period. The various speleothems (such as stalactites and stalagmites) grow drop by drop with the minerals present in the filtered water, and they do so year by year, layer by layer. This allows them, like tree rings, to be used as environmental witnesses, particularly of rainfall. The great contribution of this work here is that they have been able to observe the variation in precipitation not layer by layer or year by year, but almost month by month.

“Knowing the average annual rainfall doesn't tell us as much as knowing what each rainy season was like,” recalls Daniel James, a researcher specializing in past climate reconstruction at University College London (UK) and first author of the study. Maize grows throughout the wet season and is harvested at its end. And crop yield depends on the rainfall. “Being able to isolate the rainy season allows us to accurately track the duration of the drought from the rainy season, which is what determines the success or failure of crops,” adds James.

The researchers found that their dating of the drought events corresponded relatively well with those obtained from other speleothems and lake sediments. Overall, they found that the climatic data were consistent with the archaeological data: inscriptions on commemorative stelae, monument construction, and political activity at several important northern Maya sites halted at different times during this period of climatic stress.

Things could have been far worse. The stalagmite segment they studied, from the Tzabnah Grottoes, a few dozen kilometers from Mayan cities like Chichén Itzá and Uxmal, records the period between 870 and 1100. But there is a gap of about 50 years, between 1021 and 1070, during which the speleothem did not grow. James, who conducted this study while researching for the University of Cambridge, recalls in an email that "there are many possible reasons for this, one being that there could have been so little rain that the dripping stopped completely during a severe drought." Or "quite the opposite," says David Domínguez, a geologist from the University of Salamanca. "If it rains a lot, a dissolution occurs that prevents growth," explains the Spanish scientist, an expert in speleothems who was not involved in this research.

Stalagmites form when water drips from the ceiling of a cave and minerals, particularly calcite, precipitate. Isotopes of elements such as oxygen and carbon are trapped inside, helping us understand where the water comes from. In the case of the Mayan cave, they estimated that it took a month for the water falling to the surface to infiltrate. By dating and analyzing the oxygen isotope layers within the stalagmite, researchers were able to detect droughts and their duration. The different layers don't indicate how much rainfall occurred, but, "in years with little rain, the isotopes are heavier," Domínguez explains.

Cities like Uxmal were abandoned at the end of this period. But others show ambivalent signs. Everything indicates that Chichén Viejo declined, but what is now known as Chichén Itzá prospered for a while longer. “The differences between the sites reflect the different social responses to drought,” says James, the first author of the study. “Chichén Itzá had a wide range of trade networks and was highly centralized, which would have allowed for the accumulation or importation of resources in times of scarcity,” he adds.

In 2021, an unrelated study showed how a short drought, lasting no more than a few months or a single wet season, would cause supply problems, but 89% of production would still be achieved. However, in cases of extreme drought, as defined here, it would result in a reduction of harvests to a tenth . However, one of the authors, Scott Fedick, professor emeritus in the Department of Anthropology at the University of California, Riverside, believes the authors overlook the resilience of Maya cities.

“In our [2021] paper, there is a marked difference between the food plants available during moderate and extreme drought. While most annual species will not produce enough, a wide variety of nutritious perennial food plants would,” Fedick writes. He believes the authors of the new study overemphasize the impact of drought on annual species, such as corn, beans, and squash, without acknowledging “the potential contribution to food security of numerous drought-tolerant perennial species that would persist during moderate drought and only gradually decline after extreme drought.”

Historian Rafael Cobos, a professor at the Autonomous University of Yucatán (Mexico) and an expert on Mayan civilization, recalls that the region's cities faced similar pressures. "My research suggests that both Uxmal and Chichén Itzá were contemporaneous, and both pre-Columbian cities completed their development and peak at the end of the 11th century, just as the great drought that affected Yucatán and the rest of the world was at its peak," he says via email. Although he acknowledges that there are those who, based on the archaeological record, defend the survival of centers like Chichén Itzá that flourished even in this context of climatic adversity, ultimately, "the Mayan civilization, with its society that depended on the production of corn fields for food, could not sustain its large population, and a social-political-economic debacle—the collapse—occurred."