Centuries before Europeans arrived, an advanced civilization flourished in Mesoamerica, a region extending from southern Mexico through Central America. The Maya mastered astronomy, developed an elaborate written language, built towering monuments, and left behind exquisite artifacts. According to NASA archaeologist Tom Sever, the Mayan civilization in Mesoamerica was one of the densest populations in human history. Around 800 A.D., after two millennia of steady growth, the Mayan population reached an all-time high. Population density ranged from 500 to 700 people per square mile in the rural areas, and from 1,800 to 2,600 people per square mile near the center of the Mayan Empire (in what is now northern Guatemala). In comparison, Los Angeles County averaged 2,345 people per square mile in 2000. Yet by studying remains of Mayan settlements, Sever found that by 950 A.D., the population had crashed. “Perhaps as many as 90 to 95 percent of the Maya died,” he said.
For Sever, figuring out how the Maya flourished—but ultimately failed—in Mesoamerica is about more than simply solving a 1,200-year-old mystery. Since the 1980s, he has tried to understand the history of the Maya and their natural environment, a story that may hold important lessons for people living there today. Using satellite data and climate models, Sever and his colleagues hope to help governments and citizens throughout Mesoamerica ensure that the region can continue to support the people who live there. By learning from the Maya, modern humans may avoid sharing their fate. Mayan Deforestation Before its collapse, the Mayan empire stretched out from its center in northern Guatemala’s Petén region across the lowlands of the Yucatán Peninsula. Pollen samples collected from columns of soil that archeologists have excavated across the region provide evidence of widespread deforestation approximately 1,200 years ago, when weed pollen almost completely replaced tree pollen. The clearing of rainforest led to heightened erosion and evaporation; the evidence of the erosion appears in thick layers of sediment washed into lakes. “Another piece of evidence,” explained Sever, “is the thickness of the floor stones in the Mayan ruins. They would have needed about 20 trees [to build a fire large and hot enough] to make a plaster floor stone that is about one square meter. In the earliest ruins, these stones were a foot or more thick, but they progressively got thinner. The most recently built ones were only a few inches thick.” Sever’s colleague, atmospheric scientist Bob Oglesby of Marshall Space Flight Center, calls the Mayan deforestation episode “the granddaddy of all deforestation events.” Studies of settlement remains show that this deforestation coincided with a dramatic drop in the Mayan population. “After the Mayan collapse, this area was abandoned and the forest recovered. But as people have returned over the last three decades, the deforestation has returned,” Sever explained. Today, the regenerated forests of the Petén are the largest remaining tropical forests in Central America. While present-day deforestation in the Petén region hasn’t yet occurred on a Mayan scale, today’s technology could easily enable modern residents to surpass the Maya in cutting trees. According to the Food and Agriculture Organization of the United Nations, deforestation in Guatemala averaged 1.7 percent annually between 1990 and 2000. Besides a cautionary tale about what can happen to civilizations when they clear-cut surrounding forests, the long-gone Mayan civilization also offers clues to a more sustainable use of the landscape. Before their catastrophic decline, the Maya thrived in Central America for two millennia. “We want to know how the Maya used this landscape because we don’t know how to use it successfully today,” said Sever. Although the Maya’s secrets for success are harder to discern than their reasons for failure, Sever has at least one idea.
Archaeologist Tom Sever (left) and remote sensing specialist Dan Irwin (right) have pooled their skills to understand the Maya. Populations in densely forested regions often rely on slash-and-burn agriculture. At first glance, this might seem like the approach the Maya used, but Sever doesn’t think so. “In slash-and-burn agriculture, people clear the land to plant corn, for instance,” he said. “They get 100 percent productivity the first year, 60 percent the next year, and something less than that afterwards. So in three to five years, the land is basically useless, and they have to move on.” In a sparsely populated region, slash-and-burn agriculture might work, but Mesoamerica around 800 A.D. was one of the most densely populated areas in the pre-industrial world. “Slash and burn wouldn’t have enabled a population to grow to that size,” he said. Sever believes the Maya took a different approach to farming: effective water management. “The biggest threat we face doing fieldwork in this region is dying of thirst,” Sever explained. Even the rainforest experiences an annual dry season; the trees hang on by tapping groundwater. “The Maya couldn’t use groundwater because it was 500 feet below them, and they had no technology to reach it, so they depended on rainwater.” In the Petén region Sever studies, rainwater accumulates in swamplands, known as bajos, that cover about 40 percent of the landscape. Today, that rainwater evaporates before anyone can use it effectively, but excavations and satellite images have revealed networks of canals among the bajos, apparently dug during the time of the Maya. Sever suspects that the Maya used the canals to redirect and reuse the rainwater. This labor-intensive agriculture, which probably kept farmers working diligently all day, would have barely outpaced demand. If the Maya farmed the bajos, however, they took advantage of an additional 40 percent of the landscape, which would have made a significant contribution to food production. Modern Mesoamericans consider the bajos worthless and ignore them. “We’re trying to understand how to control water and enable this landscape to support current populations, to reduce some of the stress on the economy and environment,” Sever said. Climate Change In the end, Oglesby speculated, the increased productivity the Maya gained by farming the bajos might have made them too successful. “Population pressure might then have led to their having to clear more and more land, both for settlement and for agriculture,” he said. Oglesby has used three-dimensional regional climate models to help visualize the Mayan demise, and what he has found so far is intriguing. “If we completely deforest the area and replace it with grassland, we find that it gets considerably warmer—as much as 5 to 6 degrees Celsius,” Oglesby said. Sunlight that normally evaporates water from the rainforest canopy would instead heat the ground. Although his model paints a more extreme picture than what actually happened (the region was heavily, but probably not completely deforested), Oglesby suspects that deforestation contributed to a drought. Lake sediment cores indicate that the Mayan deforestation appears to have coincided with natural climate variability that was already producing a drought. “Combined with the land-use changes, the drought was a double whammy,” he said. By 950 A.D., the Mayan lowland cities were largely deserted. Learning from the Mayan Legacy Today, population density in Central America is only a fraction of what it was during the Mayan peak. In Belize, for example, population density may be as low as 26 people per square mile (10 people per square kilometer). Yet human pressure on the environment is still significant.
The razor-sharp border between Mexico and Guatemala, as seen in this 1988 Landsat image, shows the impact of high rural population on the rainforest. Guatemala’s sparsely populated Petén district stands in stark contrast to the stripped and tilled landscape of Mexico. This image prompted the leaders of Mexico and Guatemala to set aside long-standing tensions and focus on preserving the rainforest. (Image courtesy of NASA.) The effectiveness of modern tree-cutting technology became clear to Sever in the late 1980s. NASA and the National Geographic Society hired him to study the potential impact of a hydroelectric dam on the Usumacinta River in Guatemala. Sever, who had pioneered the use of remote-sensing data in finding archaeological sites, turned to satellite imagery once again. Using Landsat data, he produced an image showing part of the border between Guatemala and Mexico. Most political borders are invisible in satellite images, but this border was obvious. The rainforest—still intact in Guatemala—stopped abruptly at the Mexican border, where the landscape had been stripped. Sever’s images stunned Guatemala’s president. “There had been tensions along the Mexico-Guatemala border for about 150 years,” said Sever. After seeing the satellite image, however, both nations’ presidents “decided that the environment must unite them.” In a ceremony on a river bridge between the countries, Guatemalan President Vinicio Cerezo and Mexican President Carlos Salinas shook hands and pledged to protect the dwindling rainforest. It marked the beginning of a larger effort to protect the environment in Mesoamerica. |
