Mayans may have hastened the demise of their civilization by clearing forests, making an already naturally drying climate drier, U.S. scientists say.

Prolonged drought is thought to have contributed to the eventual collapse of Mayan civilization in Mexico and Central America, and forest razing for cities and agriculture may have made matters worse, the researchers said.

"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," lead study author Benjamin Cook, a climate modeler at Columbia University, said.

More than 19 million people were scattered across the Mayan empire at its height, between A.D. 250 and 900.

Using population records and other data, the researchers reconstructed the progressive loss of rainforest across their territory as the civilization grew.

Computer simulations of how lands newly dominated by crops would have affected climate suggest that in the heavily logged Yucatan Peninsula, rainfall would have declined by as much as 15 percent, while in other Mayan lands such as southern Mexico, it would have fallen by 5 percent.

As agricultural crops replace a forest's dark canopy, more sunlight bounces back into space, Cook said.

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," he said.

Overall, the researchers attributed 60 percent of the drying estimated at the time of the Mayans' peak to deforestation, a Columbia release said Tuesday.