The ancient Maya civilization flourished for an impressive six centuries, with well over a hundred city-states spread out across what is now southern Mexico and northern Central America.
The collapse of several cities in present day Guatemala marked the start of the Classic Maya’s slow decline, which is estimated to have begun in A.D. 695.
Prolonged drought is believed to have played a role, but a recent study adds a new twist – the Maya may have made the droughts worse by clearing away forests for cities and crops, making a naturally drying climate even drier.
“We’re not saying deforestation explains the entire drought, but it does explain a substantial portion of the overall drying that is thought to have occurred,” said Benjamin Cook, a climate modeler at Columbia University’s Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies.
More than 19 million people were scattered across the Maya empire at its height, between A.D. 250 and A.D. 900. Using population records and other data, the researchers managed to painstakingly reconstruct the progressive loss of rainforest as the civilization steadily grew. The researchers ran computer simulations to analyze how lands newly dominated by crops would have affected climate.
In the heavily logged Yucatan peninsula, they determined that rainfall would have declined by as much as 15 percent while in other Maya lands, such as southern Mexico, it would have fallen by 5 percent. Overall, the researchers attributed 60 percent of the drying estimated at the time of the Maya’s peak to deforestation.
“As crops like corn replace a forest’s dark canopy, more sunlight bounces back into space,” Cook explained.
”With the ground absorbing less energy from the sun, less water evaporates from the surface, releasing less moisture into the air to form rain-making clouds. You basically slow things down—the ability to form clouds and precipitation.”
The idea that the Maya changed the climate by clearing away jungle, partly causing their demise, was initially popularized by historian Jared Diamond in his 2005 book Collapse. In the first study to test the hypothesis, climate modeler Robert Oglesby and his colleagues ran a computer simulation of what total deforestation of Maya lands would do to climate.
Their results, published in 2010 in the Journal of Geophysical Research, showed that wet season rainfall could fall 15 to 30 percent if all Maya lands were completely cleared of trees.
Oglesby, who was not involved in the latest Cook study, said that Cook’s estimate of a 5 to 15 percent reduction in rainfall, though lower than his own, makes sense since Cook’s simulation used a realistic tree-clearing scenario.
As noted above, archeologists attribute a variety of factors to the collapse of the Classic Maya, whose ancestors are still living today in parts of Mexico, Guatemala, Belize, El Salvador and Honduras. In addition to a drying climate in several regions, the city-states struggled with overpopulation, changing trade routes, war and peasant revolts.
The Maya cleared the forests to grow corn and other crops, but they also needed the trees for cooking large amounts of lime plaster used in constructing their elaborate cities.
Thomas Sever, an archeologist at the University of Alabama, Huntsville, confirmed it would have taken 20 trees to produce a single square meter of cityscape.
“When you look at these cities and see all the lime and lime plaster, you understand why they needed to cut down the trees to keep their society going,” he said.
Unfortunately, the Maya also lacked the technology to tap the groundwater several hundred feet beneath them. Their reservoirs and canals were able to store and distribute water when rain plentiful, but when the rain failed, they had nowhere to turn.
“By the time of the collapse, every square mile of soil had been turned over,” said Sever.
Scientists know from studying climate records held in cave formations and lake sediments that the Maya suffered through a series of droughts – yet continue to debate their severity.
In a paper earlier this year in Science, researchers Martín Medina-Elizalde and Eelco Rohling of Mexico’s Yucatan Center for Scientific Research found that annual rainfall may have fallen as little as 25 percent during the Maya’s decline, from about A.D. 800 to A.D. 950. Most of the reduction in rainfall, however, may have occurred during the summer growing season when rain would have been most needed for cultivation and replenishing freshwater storage systems.
Today, many of the Maya’s abandoned cities are overgrown with jungle, especially on the Yucatan peninsula. However, satellite images seem to indicate that deforestation is happening rapidly elsewhere, including in other regions the Maya once occupied.
