Remote sensing unravels archeological mystery

The northeastern part of Guatemala, known as the Petén, was once inhabited by several million Mayan people before the collapse of the ancient civilization in the 10th and 11th centuries. It is a mystery, however, how the Maya managed to support such a dense population for centuries in an area that with current agricultural practices can support far fewer people. In addition, archeologists continue to search for clues about what destroyed the Maya.

Figure 1. A 1986 satellite image from the Thematic Mapper contrasts the tilled landscape of Mexico with the lush forest of Guatemala. This political boundary can be seen from space.

Tom Sever at NASA's Marshall Space Flight Center (Huntsville, AL), along with colleagues including Pat Culbert at the Univ. of Arizona and Guatemalan archeologist Vilma Fialko, have been using a variety of remote sensing techniques to investigate both ancient artifacts and current environmental data. They suspect the environmental disaster that brought down the Maya may have been due to massive deforestation, followed by drought.

The Petén, which is the largest rainforest left in Central America, is rapidly being deforested (Figure 1). Until 1970, nearly 90 percent of the Peten remained forested. Since then, more than half of the forest has been cut down. According to Sever, estimates suggest that if current rates of cutting continue only 2 percent of the forest will remain ten years from now. If conservation measures are implemented, then perhaps as much as 16 percent of the forest will still exist (Figure 2).

  

In 1970, only about 20,000 people lived in the Petén. In 1990, the population was more than 300,000. The subsistence farmers, commercial farmers, and cattle ranchers who live in the area now are using methods that result in soil loss and regional degradation.

In 1990, the Petén had a human population density of 26 inhabitants per square mile, while archeologists estimate ancient Mayan population densities of 2600 persons per square mile in the center and between 500 and 1300 per square mile in the more rural areas. Given that the current population isn't nearly as dense as the ancient Mayan population -- and the inhabitants are still destroying the area -- how did the Maya manage to maintain their populations over several centuries?

The researchers are using remote sensing and geographic information systems technology to study Mayan archeology as well as monitor the environment. They have identified a variety of built structures in images from several bands of the multispectral imager Thematic Mapper, which had a 30-m resolution when this work was done (Figure 3).

Figure 3. Thematic Mapper satellite image showing ancient Maya causeways (roadways) as straight dark lines. The temples at Mirador can also be seen as white features in the imagery. Image courtesy of NASA.

In addition, the researchers used airborne imaging from color infrared photography and two specialized instruments: the Thermal Infrared Multispectral Scanner (TIMS) and the Calibrated Airborne Multispectral Scanner (CAMS). TIMS measures radiation in 400-nm intervals from 8.2 to 9.4 µm and in 800- to 1000-nm intervals from 9.4 to 12.2 µm. Although originally designed for geological research, it can also be used for vegetation and soil research, as well as archeology. This instrument measures the emissivity of features on the ground. Daytime and nighttime images often show different information. CAMS is a 9-channel instrument that provides coverage from 460 to 1250 nm with 5-m resolution, as well as color IR photography to identify potential canals and fields.

The visible, IR, and microwave images show roadways, canals, reservoirs, even check dams, none of which can usually be seen from the ground. They also show relatively high ground areas that show evidence of habitation.

There are many areas of seasonally flooded swamps, called Bajos, in the Petén, and there has been lively debate about whether the Maya farmed these areas. These areas represent about 40 percent of the land surface in the Petén. The presence of waterworks around the Bajos, and of populated living areas among the Bajos suggested that the Maya did farm these areas, and used more-sustainable agricultural methods than the typical slash-and-burn farming techniques present-day inhabitants use.

But something happened to the ancient Mayans: a cataclysmic collapse reduced the population by two thirds between 830 and 930 AD. "The Maya collapse may well be the greatest demographic disaster in human history," Sever wrote.¹ While the cause of the collapse is probably due to a variety of factors, it may be linked to deforestation. At the time of the collapse there was no tree pollen, and other evidence also suggests the forests of the Petén were nearly destroyed by the Maya. There is evidence of a 200-year-long drought, which may be related to deforestation. Researchers studying climate in other parts of Central America note that without trees, cumulus clouds fail to form and rainfall diminishes.

Current inhabitants are threatening to destroy the Petén forest today in a shorter time period with a lesser population. Locally, chicle tree harvesters say rainfall has recently diminished.

Reference:

1. Tom Sever, Validating Prehistoric And Current Social Phenomena Upon The Landscape Of The Peten, Guatemala, People and Pixels: Linking Remote Sensing and Social Science, Ed. Diana Liverman, Moran, Rinfuss, Stern, National Academy Press, 1998.