The destruction of the Amazon rain forest may become irreversible much sooner than expected, according to a new study presented yesterday at the joint conference of the Geological Society of America and the Geological Society of London. Whereas scientists generally agree that the outlook for world rain forests is grim, they debate when the ruination will be complete. Current estimates for the Amazon River Basin rain forests put doomsday at 75 to 100 years from now. But the new model, developed by James Alcock at Penn State University, moves the date ahead to merely 40 to 50 years from now. Moreover, Alcock says the destruction will reach a point of no return in only 10 to 15 years.

"Because of the way tropical rain forests work, they are dependent on trees to return water to the air," Alcock says. "This interdependence of climate and forest means risks to the forests are much closer at hand than we might expect." Rain forests depend on a process called evapotranspiration: water evaporates from the vegetation but then rains off again over the forest so that, in essence, the rain forest holds on to its moisture. Because this cycle is vital to the rain forest's survival, Alcock dismisses the idea of preserving small portions of rain forest. He says that such patches of forest may not be able to sustain the evapotranspiration cycle; less rainfall could lead to more forest fires and, ultimately, destroy even more forest land.

Alcock bases his analysis on a mathematical model he developed to study the impact of man-made deforestation. Unlike earlier studies, he looked at local rather than regional or global impacts of forest destruction. In doing so, he also took note of the threat to many animals that inhabit the Amazon River Basin. "There are already a large number of species that are endangered, because the forest itself is endangered," he says. "We might be able to keep a few animals at the zoos, but we'd surely lose a lot of amphibians, reptiles and insects. We couldn't take them all."