The old Maya and the sea

While Georgetown University geographer Tim Beach was on a field trip to an ancient Maya wetlands site in present-day Belize, his thermometer and humidity meter displayed a heat index of 134 °F (57 °C). His student wanted to know if the working environment was safe.

Sweltering in the wetlands, which “seems like the last place in which anyone would want to live,” he said, Beach found it striking how much evidence of human habitation remained in the inhospitable landscape, with Maya artifacts lurking deep in floodplain sediments far removed from the main city sites. And much as he would have welcomed the summer rains to break the extreme heat, he knew that their arrival would drive him and his team out of the wetlands.

A complex system

Beach, his George Mason University colleague and wife Sheryl Luzzadder-Beach, and their sponsors at NSF and the National Geographic Society stand to lose a season of field work due to early rains in tropical Mesoamerica. But the ancient Maya people stood to lose a lot more when rain patterns and sea levels changed. Those hydrological impacts are candidate explanations for why the Maya’s civilization “collapsed” and their great cities were abandoned in the Terminal Classic period of the ninth and tenth centuries.

Known for their advanced written language, art, architecture, and mathematical systems, the Maya flourished in the Classic period (AD 250–900) in the part of Central America that encompasses Yucatan, Guatemala, Belize, and southern Mexico. Paradoxically, it was the sophistication of Maya civilization that made it vulnerable. Using a conceptual model that included climate, biophysics, and land, Arizona State University’s Billie Turner has determined that the Maya so extensively transformed their environment that they could not maintain their changes in the face of climate change.

But when the Maya chose to stop maintaining cities and agricultural systems, and how they fared as they adapted to higher sea level, a rising water table, and cycles of heavy rain and drought, remain topics of lively debate. To that debate, Beach and Luzzadder-Beach add new models and geomorphological studies to demonstrate that the Maya were able to adapt to climate change, at least early on.

Managing the wetlands

Much of the Maya Lowlands region lies atop the carbonate Yucatan Platform and its karstic (irregular limestone) landforms, with interior areas uplifted into plateaus and basins and with wetland fields for coastal areas.

In many areas, the lowlands had remained dry for part of the year, with fertile soils providing nutrient-enriched surfaces. Pollen evidence suggests that crops grew earlier; only when sea levels began to rise around 350 BC, which led to a rising water table, did these areas become wetlands.

“Before, the [wetlands] could have been used as surface fields,” explains Beach. “Now, [the Maya] had to dig canals to drain these landscapes and reclaim them.” As part of that reclamation process, they could maintain moisture conditions applicable to the requirements of specific crops. The huge effort required to create those fields “makes us question how important they were to sustenance,” says Luzzadder-Beach.

The Beaches’ excavations within the Chan Cahal and Birds of Paradise (BOP) areas in northwestern Belize uncovered detailed evidence of the transformation from a wet lowland and an active floodplain to today’s rectilinear patterned wetlands. Canal sedimentation marks ecological changes and human manipulation of the hydrological landscape.

The Maya burned and cultivated fields on the Chan Cahal site’s slowly evolving soil surface, which now provides information on when changes occurred. When the groundwater table rose, peats formed and a flood deposited carbonate silt and sands. The groundwater table continued to rise after 50 BC and precipitated gypsum which, combined with eroded upland sediment, buried the flood layers and peat. The Maya reclaimed the land by digging canals to drain the fields and grow crops in higher field zones, where naturally low-ion-concentration rainwater could assist in germinating younger plants. Just 10 km away, the BOP active floodplain accumulated gypsum in the soil matrix but at lower concentrations than at Chan Cahal, because low calcium and sulphate runoff diluted flood waters in the wet season. The Maya dug canals here from AD 750 as continued flooding aggraded the area.

But from AD 950, the canals began to fill in as the Maya ceased to maintain them. Abundant charcoal and carbon isotopic ratios are evidence of disuse, and sediments indicate a return to tropical rainforest.

“A lot of this [research] started because of people recognizing [canal] patterns for the first time,” says Beach. “We have to credit the availability of geospatial information [remote sensing], Luzzadder-Beach adds, “which has given researchers the ability to search for features for free.” That technology, along with ground-penetrating radar, provides “higher spec imagery that can indicate more about these fields.” It’s even provided evidence of dams in the canal system for diverting poorer-quality water.

Effects of droughts

Although evidence of historically dry conditions over the Maya Lowlands coincides with significant population losses in many drought-prone areas, the droughts should have had minimal impact in coastal areas. The wetlands, then, are a place in which to test the hypothesis that easy access to the water table allowed populations to persist throughout the Terminal Classic droughts.

But Beach says “the wetlands that we study went down [were no longer maintained] at about the same time as the other upland areas that would more likely have been affected by drought.”

Even in a wet area, drought can create a tipping point in a chemically limited water supply. Wetlands are based on rainwater diluting ion-saturated water and leaching away excessive gypsum. Sulphur shows up in high concentrations during dry periods, and calcium carbonate is the main precipitant from lakes and rivers during wet periods. Additionally, oxygen isotope ratios from arthropod and gastropod shells in the canal sediments may fractionate in very different ways. A lighter isotope preferentially evaporates away during droughts, and heavier isotopes build up. During wet periods, there is no preferential removal of the lighter isotope.

Throughout the whole Maya phase, there were several drought interludes. But in each case, while one or two city-states may have suffered, the Maya recovered and expanded their population.

Norman Hammond of Boston University, who has studied the ancient Maya for more than 40 years, explains that “the drought theory [of Maya collapse] is controversial because it is the most recent and also has the strongest a priori evidence against it.” In the northern Yucatan, one of the driest areas, cities such as Uxmal and Chichén Itzá flourished through the AD 800–1000 time of drought. “Nobody doubts the droughts existed; it’s their impact on the Maya that’s questionable,” he adds.

The Maya against the land

Although drought as for the sole cause of collapse seems unlikely, the Maya “had so transformed their landscape that they were facing problems,” according to Turner, as cultivating lands and increasing rural populations further denuded the landscape.

Turner models the interplay of effects including deforestation, which reduces rainfall through changes in evaporation and transpiration. Phosphorus, the controlling nutrient in the soil system, is captured by the forest canopy and transferred to the soil. That becomes a problem when the forest has less of a canopy. And when the landscape becomes denuded, invasive bracken fern takes over and is difficult to eradicate.

Hammond adds that slash-and-burn cultivation is a highly conservative method that does not destroy the environment but preserves it. “People who think the Maya devastated their environment by cutting down different patches of forest every couple of years are wrong. [They] were in balance with their environment.”

And geography professor William Woods from Kansas University points out that whereas initial human occupation causes erosion by cutting down forests and adapting the land to make it more productive, the Maya had “adapted many different strategies” to deal with microhabitats. Those adaptions, in turn, were “great for productivity purposes, and good for sustainability.”

Perhaps blaming the environment is just too easy, suggests Georgina Endfield, professor of environmental history at the University of Nottingham in the UK. External triggering events such as climate, she says, may appear to be “more conspicuous in the surviving data relative to more complex underlying socioeconomic or political problems, which can be difficult to identify and decipher.” Thus it becomes easy to reach conclusions about environment being a driver of collapse.

Turner returns to the same question: “It’s not that they couldn’t sustain the system: It’s whether it was worth it monetarily. They had reached a threshold point: Are you going to continue to manage the landscape, or abandon it and go some place else?”

Where they went

The elevated interior region, with the highest density of city-states and rural populations, rises 350–450 meters above sea level. Because the local aquifer is extremely deep, it would have been difficult to access its water. For the Maya living there, the ability to capture and store rainwater was therefore critical. That region “depopulates on the order of 90% and never came back,” says Turner.

But there is “no evidence of a massive die-off,” he says; rather, people migrated to the coastal plains. Trade routes that used to cross the interior shifted around the peninsula, a much less efficient route, but communities had lost the economic power to maintain the central landscape.

Indeed, one widespread misconception is that there was a single reason for the Maya collapse, during which everybody vanished. “It took a century!” says Hammond, and did not have a single cause. “Any explanation has to be multifactorial and synergistic.”

That cities were abandoned means that people moved out, probably into the countryside where they formed “scattered agricultural communities which were less archaeologically visible.” They were no longer making temples and pottery, so they “may have just sank beneath the surface of archaeological visibility.” Some people migrated greater distances. We know this via migration legends that survived until the Spanish could record them. Some may have gone north to the Yucatan and “may have been responsible for florescence of Maya civilisation later.”

“It’s the combined forces of social, cultural, political unrest, demographic pressure, as well as environmental changes/ climatic change (including events such as prolonged / successive drought) that combine in particular ways to effect change in social order at particular points in time,” says Endfield.

And while coastal communities show longer continuity of occupation than central cities, both Woods and Beach speculate that diseases brought by Columbian explorers in the 16th century eventually finished them off.

Did they come back?

Once the abandonment took place, it took no longer than 150–250 years for the forest to come back. “Why didn’t the Maya go back? If you’re not asking that question, you’re not coming back to the main questions,” says Turner.

Hydrological problems likely lie at the heart of that issue: Reoccupation of the interior region required a system to capture and store rainwater. After those hydraulic systems were abandoned, reoccupation would have required labor-intensive organization. In a society where failure implicated both gods and rulers, that was no mean feat.

New data provide better chronologies of ecological changes and population migrations in different areas of the Maya Lowlands. Turning conceptual models into empirical models will add rigor to arguments about how the human–environmental system thrived and declined.

The Maya adapted and thrived for centuries under changing and challenging environmental conditions. Their efforts demonstrate “what really clever beings can do,” according to Beach, and could provide important lessons for how our modern human–environmental system should respond to climate change in the near future.

Further reading

• S. Luzzadder-Beach, T. P. Beach, N. P. Dunning, ‘Wetland fields as mirrors of drought and the Maya abandonment,’ Proc. Natl. Acad. Sci. USA 109, 3646 (2012).
• N. P. Dunning, T. P. Beach, S. Luzzadder-Beach, ‘Kax and kol: Collapse and resilience in lowland Maya civilization,’ Proc. Natl. Acad. Sci. USA 109, 3652 (2012).
• G. H. Endfield, ‘The resilience and adaptive capacity of social–environmental systems in colonial Mexico,’ Proc. Natl. Acad. Sci. USA 109, 3676 (2012).
• B. L. Turner, J. A. Sabloff, ‘The classic Maya collapse in the central lowlands: Insights about human–environment interactions for sustainability science,’ Proc. Natl. Acad. Sci. USA, 21 August 2012, doi: 10.1073/pnas.1210106109.