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We're studying collapsed civilizations so that ours can endure climate change

Paleoclimatologists are digging into the connections between the collapse of Maya Civilization and extreme droughts

Over 1,000 years ago, droughts plagued the Yucatan peninsula. The Yucatan was home to the Classic Lowland Maya Civilization, of pyramids and the number zero fame. Droughts occurred intermittently for centuries, from 200 to 1100 CE. This is an era of Mayan history typically split in two – the Classic (200-800 CE) and Terminal Classic (800-1100 CE) Periods. The droughts coincided with the widespread collapse of the Maya Civilization around 1100 CE.

The first scientific evidence of these droughts was discovered by Dr. David Hodell and other researchers at the University of Florida in 1995, using ancient sediments from Lake Chinchancanab. Since then, the droughts have been a popular example of how extreme climate fluctuations can impact society. However, the magnitude of the droughts has remained an elusive and difficult question to answer. Further, archaeological research has revealed a much more complex history of the Classic Maya reorganization than originally thought, suggesting the droughts were not the only factor destabilizing the Classic Maya. Archaeologists have found evidence that suggest the Maya Civilization experienced social changes including class conflicts, warfare, invasion, and ideological change. 20 years after the discovery of drought evidence in the Yucatan, researchers returned to Lake Chinchancanab to investigate a seemingly simple question: just how dry was it?

In the pioneering Yucatán drought research, Hoddell and his team sampled sediment cores from the bottom of Lake Chinchancanab that were thousands of years old. In the core, they found layers of gypsum, a white chalky mineral often used in plaster or cement. Because gypsum can only form in a lake setting when a large amount of evaporation has occurred, presence of gypsum in lake sediments is evidence of periods in the past where lake levels dropped significantly—signs of past drought events.

A photo of a stepped Mayan pyramid with a weathered stone sculpture of a serpent's head in the foreground.

Photo by Marv Watson on Unsplash

Interestingly, archaeological records show that these periods of droughts coincided with sociopolitical unrest in the region, including increased warfare and internal violence of the Lowland Maya. This sediment core from Lake Chinchancanab was the first quantitative link between climate and instability of the Classic Maya.

Hoddell is now based at the University of Cambridge, but he and his group are still interested in human-climate connections. In this new study, his team went back to Lake Chinchancanab. The researchers are still focusing on the lake’s gypsum, but now they are looking to the ancient lake water that has been trapped in the gypsum since the droughts. The researchers developed new chemistry and modeling techniques to assess how extreme the Classic and Terminal Classic droughts were. By measuring the chemistry of the trapped ancient lake water, they established ideas for what the chemistry and depth of the lake would have looked like during the droughts. With these constraints, the researchers developed a theoretical model of a lake. They tested different climate scenarios to see how the lake chemistry would respond, until the modeled lake chemistry matched the ancient water in the gypsum. It's the scientific equivalent of turning light switches on and off until you figure out which one is the light you actually want.

The researchers ultimately found that rainfall decreased by 50 percent on average compared to today, and as much as 70 percent during the most intense drought conditions. Humidity decreased by two to seven percent. That decrease in rainfall is the equivalent of Seattle becoming as dry as Tucson. "We don't really know what changes in relative humidity might be in that past, because we never really had a tool to constrain it before," says Thomas Bauska, a co-author of the study and researcher at Cambridge University. "[The results] do tell us that the Yucatan was experiencing dry season conditions during a much longer period of the year."

These new results and techniques can open many doors. Much of paleoclimate research relies on qualitatively studying past climate records rather than measuring past climate. It's useful, but this approach prevents us from asking questions precise quantitative data can raise. It's the difference between saying "wow, it was really dry in 1100 CE" and saying "there was a 50-70 percent decrease in rainfall compared to today."

For example, we can learn a lot from the ancient Maya about human resilience and adaptation to climatic extremes. Previous studies in this region showed that during the first Classic Period droughts, the Maya adapted their agricultural practices by rotating their crops to maize (corn) varieties that required less water, but were unsuccessful at adapting in later droughts. But these results were based on qualitative paleoclimate records; hopefully providing more exact estimates of drought intensity will lead to a better understanding of how the Classic Maya reacted in the face of extreme climatic change.

This civilization flourished in the not too distant past: the demise of the Classic Maya occurred around 900 - 1200 CE (though this collapse doesn't mean the Maya disappeared – the remaining population reorganized and formed new communities). Cambridge University, where this research was done, was founded in 1209 CE. But there's still so much that scientists don't know about Mayan history. More concretely understanding the past climate changes of this region is one monumental step toward understanding how the Maya interacted with their environment

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Devang Mehta

Genomics

University of Alberta

I really like the idea of studying how past civilizations coped with changing climates to inform our own response!
Were the scientists able to determine how long these drought periods were? And when you write that “The researchers ultimately found that rainfall decreased by 50 percent on average compared to today, and as much as 70 percent during the most intense drought conditions.”, are those numbers also the same for comparisons with pre-drought conditions in 200CE? and how long did it take for this very large decrease in humidity, i.e. how steep was the decline and did it occur in one long stretch, or in waves? Or is this information that’s not yet been determined?

Brittany Ward responds:

The age models for the lake cores aren’t good enough for them to determine precisely the beginning and end of the drought periods. It’s more accurate to say they found high probabilities of droughts during certain time periods, one from ~750-850 CE and another from ~950-1050 CE. This is where I would really like to see more research. They didn’t mention comparison to pre-drought conditions in their paper. But, in their model they used a period of 1994-2010 climate reanalysis data as a baseline to compare the past simulations to, so I feel this limits them to only comparing against that modern baseline. It wasn’t just a long stretch of continuous low precipitation and humidity, but rather pules or waves (I like that word to describe it!).

Julie Hollis

Geology

Government of Greenland

Fascinating stuff. It actually ties in very closely with a current pitch I have in progress, on climate-related effects on archaeological sites in Greenland. In Greenland, we see both the impacts of current climate change on those sites, and the past record of climate-related environmental impacts on the people whose lives are recorded in those sites. The demise of the Maya corresponds with the disappearance of the Norse from Greenland, after a period of changing their lifestyles to adjust to their changing environment.

Darcy Shapiro

Evolutionary Anthropology

Rutgers University

I really appreciate that you noted at the end of the piece that the end of this particular period of Maya civilization does not mean the Maya disappeared. Popular coverage of archaeology often doesn’t do justice to the fact that past people were very capable of solving problems and adapting to changing conditions - they always just “mysterious disappear.” Maybe their “disappearances” aren’t so mysterious after all, we just weren’t using the right methods before!

Brittany Ward responds:

I’m so happy you appreciate that! This was very frustrating to me when this publication first made headlines.