Amazon forest unlikely to rise from ashes.

Amazon forest unlikely to rise from ashes

Adding extra urgency to the topic of tropical deforestation, simulations with a new breed of computer models suggest that the Amazonian rain forest, once destroyed, probably would not regrow.

Cutting the entire forest would severely alter the climate in the Amazon basin, causing temperatures to rise and precipitation levels to fall -- a shift that would severely hinder development of a new rain forest, report Jagadish Shukla and Piers J. Sellers of the University of Maryland in College Park and Carlos Nobre of the Brazilian Space Research Institute in Sao Jose dos Campos. "These results suggest that a complete and rapid destruction of the Amazon tropical forest could be irreversible," they write in the March 16 SCIENCE. At the present rate of deforestation, the forest might disappear in 50 to 100 years, they say.

Shukla and his colleagues tested the climate effects of deforestation through simulations on a computer model that couples a high-resolution model of the global atmosphere with a biosphere model accounting for vegetation and soil effects. Many scientists in the past have simulated tropical deforestation, but only within the last few years have researchers designed realistic biosphere models that mimic the effect of trees, Shukla says.

The investigators ran a pair of simulations covering one year: a control case using a forest-covered Amazon, and a deforestation case that replaced the rain forest with pasture. The tests predicted the deforested Amazon would have surface and soil temperatures about 1 degrees C to 3 degrees C higher than the forested. On average, precipitation dropped by 26 percent in the deforested Amazon and evaporation decreased by 30 percent. These results match the findings of a similar study, reported in the Nov. 23, 1989 NATURE, that examined a pair of three-year-long simulations on a coupled atmosphere-biosphere model with coarser resolution.

Scientists have long assumed that cutting the rain forest would decrease local evaporation, which provides about half the rainwater in the Amazon basin. But without computer simulations, they could not predict how deforestation would affect the atmospheric circulation patterns that bring in the remaining half of the Amazon's rainwater from outlying regions. The new results indicate complete deforestation would lower by 18 percent the net amount of moisture entering the basin from outside, says Shukla.

The precipitation drop would lengthen the Amazon's dry season -- an effect likely to prevent rain forest regrowth, says Shukla. The model results do not indicate how much climate change would follow a partial deforestation, nor do they apply to other rain forests.

Robert E. Dickinson of the National Center for Atmospheric Research in Boulder, Colo., who ran earlier computer simulations of deforestation, says the two new studies appear to establish strong links between rainfall and the forest. However, he adds, these models will achieve real credibility only when future studies prove they can simulate the large year-to-year shifts in Amazonian rainfall that result from El Ninos and other ocean changes.