What caused the Mayan empire, one of the most advanced civilizations of the ancient world, to collapse after six centuries? Two new studies blame human impacts on the environment.

Researchers have long known that drought was one major reason the Mayan civilization could not survive. As rainfall decreased, farmers could no longer grow needed food. A study published this week in Geophysical Research Letters suggests that the drought may have been worse because of deforestation. As Mayans chopped down jungle canopy to plant crops and build cities, that likely caused a significant portion of the drying.

The researchers used climate modeling to examine how the switch from forest to cropland would have affected rainfall. As crops were planted, less water transferred from the soil to the atmosphere. The study found that deforestation may have caused as much as 60 percent of the drought.

A second study, published in Proceedings of the National Academy of Sciences, agreed that clearing the rainforest likely worsened the drought. It added, however, that there were several other reasons that led to the collapse. There is evidence that some animals, like white-tailed deer, were in decline, along with certain species of trees. Changes to the landscape probably caused soil degradation. As trade shifted to the sea, this also likely weakened the power of the inland cities. Eventually, by about 900 A.D., the empire that once stretched from southern Mexico through Belize, Guatamala, El Salvador and Honduras had collapsed.

Image by Flickr user motleypixel