Dr Martín Medina-Elizalde | Collapse of the Ancient Maya Civilisation: Aligning History with Geological Analysis

Between 800 and 1000 CE, one of the world’s most advanced ancient civilisations underwent a devastating decline. The collapse of ancient Maya society has widely been attributed to a century-long drought; but so far, there have been few efforts to quantify this event, or to equate scientific findings with historical sources. Through new geological and paleoclimatological analyses, Dr Martín Medina-Elizalde at the University of Massachusetts, Amherst has revealed that the climate changes experienced during the drought followed more complex patterns than previously thought. His team’s discoveries could have important implications for predicting our own society’s future.

Societal Collapse

As any historian will tell you, even the greatest civilisations are far from invulnerable from complete collapse. In order to sustain their large populations, virtually every complex society ever to emerge has required access to reliable water supplies, as well as annual weather variations that are predictable enough to grow a steady supply of crops. If unforeseen climate changes occur that throw these patterns too far out of balance, even the most advanced civilisations may not be able to cope for long.

These changes may occur over periods of hundreds of years – but in the end, without easy access to the basic resources they need to survive, complex societal systems will eventually collapse. This story has been observed many times, in regions all around the world.

Today, the global climate changes brought about by our own actions are displaying concerning parallels with these past events. Already, rising temperatures are leading to an increase in extreme, more unpredictable weather – including droughts, floods, and severe storms. In turn, the security of global food and water supplies is coming under increasing threat.

To fully understand the potential societal implications of a heating planet, Dr Martín Medina-Elizalde of the University of Massachusetts, Amherst believes it is critical for us to study the past. ‘Anthropogenic climate change, marine pollution, and water scarcity represent severe threats to the existence of human societies and life on Earth,’ he explains. ‘My research has been focused on understanding human impacts on natural systems, and how societies have responded to and have been affected by climate and environmental change.’

The Fall of the Maya

Dr Medina-Elizalde’s research focuses particularly on the Maya – one of the world’s most advanced ancient civilisations, which arose on the Yucatan Peninsula in modern-day Mexico. With its reliable summer rainfall, and flat, fertile land, this region provided ideal conditions for the Maya, allowing them to thrive for centuries, and even build vast cities, which still stand today.

Yet between 800 and 1000 CE, centuries before the first Europeans arrived, the population of this region plummeted, and the Maya’s historical record steadily faded – a time that historians now call the ‘Terminal Classic Period’. Through a combination of paleoclimate records and archaeological evidence, historians determined that this collapse coincided with a ‘megadrought’ across the Yucatan Peninsula.

Such an extreme event must have caused severe damage to food production and human health, while also triggering wars and societal unrest; and eventually, an unravelling of the entire political system. Yet through more recent geological analysis, some studies have provided evidence for more complex patterns than a single, century-long drought.

‘By understanding the drivers of climate change on long timescales, we can better predict the future, and its impacts on human cultures and ecosystems.’

Quantifying Historical Rainfall

To better understand how the Maya civilisation ended, Dr Medina-Elizalde believes that a detailed, quantitative knowledge of historical rainfall on the Yucatan Peninsula is essential. He has set out a detailed plan for this project, comprising four overarching goals. ‘The project aims firstly to quantify climate variability in the region on ecological and geological time scales; and secondly, to understand the drivers of this variability,’ he says. ‘Thirdly, it aims to determine the role of drought in driving the collapse of the Maya civilisation, and finally; to produce some of the first estimates of the hydrological sensitivity of tropical regions to shifts in climate.’

To achieve these aims, Dr Medina-Elizalde and his colleagues have travelled across the Yucatan Peninsula, gathering a variety of geological samples. Among these are stalagmites – distinctive rock formations that rise up from cave floors, and form on geological timescales, as minerals contained in water dripping from the cave ceiling are deposited layer by layer.

Since the year 2004, the team has been collecting stalagmite specimens from various caves within the Yucatan Peninsula. The first stalagmite they collected was a 45-centimetre-long specimen from a cave near the city of Merida, which they nicknamed ‘Chaac’ after the Maya god of rain. This specimen was able to yield some of the first quantitative measurements of how much it rained during the period when the Maya civilisation collapsed.

Within the limestone of Chaac, the researchers looked for variations in the naturally-occurring isotopes, oxygen-18 (¹⁸O) and oxygen-16 (¹⁶O). They found that the relative composition of these two oxygen isotopes in calcium carbonate molecules within Chaac ultimately reflected the proportion of these same isotopes in water from rainfall. Furthermore, they found that the heavier the rainfall event, the higher the concentration of the lighter oxygen isotope would be in the stalagmite.

As it turned out, rainfall produced from very cold, high-elevation clouds consisted of very few water molecules containing the heavier oxygen isotope ¹⁸O compared to the original ocean water that provided the moisture to begin with. Dr Medina-Elizalde and his colleagues found that heavy rainfall events tended to result from very high-elevation clouds, and the resulting rainfall was the most isotopically ‘light’, having a higher proportion of the light isotope (¹⁶O).

Paleoclimatologists refer to this phenomenon as the ‘amount effect’ – as large amounts of rainfall contain a smaller proportion of molecules containing ¹⁸O, while light rainfalls (not from very high, cold clouds) contain a larger proportion of this heavy isotope. Therefore, carbonate molecules containing very low proportions of the heavy oxygen isotope record times when rainfall amounts were high, whereas at times when rainfall amounts were lower, carbonate molecules contain a relatively greater amount of the heavier oxygen isotope (¹⁸O).

An Uneven Reduction

This effect has now provided Dr Medina-Elizalde’s team with a vital glimpse into the varying climate of the Yucatan Peninsula over the past 1,500 years and beyond. Of particular interest are the changes that occurred around the Terminal Classic Period. ‘We reconstructed precipitation changes by decoding the chemical composition of stalagmite deposits that grew in caves located in locations, right underneath the ground where the ancient Maya were building their cities,’ Dr Medina-Elizalde recounts.

For the Chaac stalagmite, the researchers predicted that prolonged droughts would cause the chemical compositions of any minerals deposited over those years to contain a higher proportion of ¹⁸O, compared with ¹⁶O. Through mass spectrometric analyses of tiny stalagmite samples, drilled along the direction of its growth, they could determine variations in the ¹⁸O/¹⁶O ratio of its different layers, deposited as little as one year apart.

From this analysis, Dr Medina-Elizalde’s team gained the first detailed picture of how levels of annual rainfall varied each year throughout the Terminal Classic Period. During the worst periods of drought, they concluded that the Yucatan Peninsula experienced up to a 40% reduction in annual rainfall.

Parallels with History

Through a series of mathematical simulations, the team aimed to recreate the ¹⁸O isotope levels observed in Chaac and the oxygen composition of carbonate shells found in sediments from two lakes, the latter produced by three close colleagues. Their simulations showed that this reduction was likely associated with a decrease in the frequency and intensity of tropical storms. Yet contrary to previous studies, their measurements suggested that this reduction didn’t occur in a single megadrought.

Instead, the Mayans would have experienced a succession of drought events; the longest and most severe of which would have reduced Lake Chichancanab’s water level by some 30%. According to other geological sources, similarly dry conditions were also emerging at the same time in southern China, tropical Africa, and South America. This would suggest a planetary-scale reorganisation of motions in the oceans and atmosphere at the end of the first millennium CE.

Crucially, Dr Medina-Elizalde discovered that the Yucatan’s long drought was interrupted by brief periods of recovery, where rainfall returned to more plentiful levels. This aligns well with the broad historical consensus that the Maya collapse didn’t occur evenly across the entire region. Indeed, one wetter period lasting from around 860 to 890 CE coincided with a brief revitalisation of some Maya cities in the northwest lowlands – within the Puuc region. In addition, cities with deeper soils and more abundant water resources or those near the coasts may have been more resilient against severe droughts – potentially holding out for several decades after other settlements had been abandoned.

Predicting Our Own Future

Today, there are striking parallels between the successive droughts suffered by the ancient Maya, and the uptick in extreme weather brought about by climate change. For Dr Medina-Elizalde, it has never been more important to understand and quantify these parallels, and use them to gain a better understanding of how our own society could change.

‘It is becoming essential to understand conditions before human interference with the environment, in order to attribute an observed change to our activities,’ he explains. ‘By understanding the drivers of climate change on long timescales, we can better predict the future, and its impacts on human cultures and ecosystems.’

Building on his previous work, Dr Medina-Elizalde now plans to expand the scope of his research to better incorporate our knowledge of the Maya themselves, the natural environment they inhabited, and the cultural transformations that unfolded during the Terminal Classic Period. This will involve a close collaboration with researchers from widely varying fields, including historians, archaeologists, and geographers.

Through these relationships, he ultimately hopes to build a detailed theoretical framework of how their society collapsed – potentially providing crucial insights into the changes we need to make as a society, in order to avoid a similar fate on a global scale.