With a bit more warmth and extra fertilizer, a plant can go from limp to lush. But all the good things for flora are meaningless without water. According to recent analyses, large parts of the Amazon rain forest, which sprawls across an area about as large as western Europe, could end up turning into dry savannas--a state from which there may be no turning back even if the climate returned to normal.

This somber prediction comes out of the latest findings of the Large Scale Biosphere/Atmosphere Experiment in Amazonia, the most ambitious field project ever done in a tropical ecosystem. Since 1999 the experiment, sponsored by the Brazilian government, NASA and the European Union, has brought together 800 scientists, who have been probing the six-million-square-kilometer jungle with instrument-laden towers, airplanes and satellites in a quest to understand how the forest works.

The researchers still have a long way to go for a complete picture. But they think they now know enough to begin to assess how the complex Amazon ecosystem will react to global warming, increased carbon dioxide (which acts as fertilizer) and other changes. Two studies in the project suggest a trend toward the formation of savannas at the eastern and southern parts of the forest.

A key factor is deforestation, which alone in the eastern Amazon could tip the ecosystem toward a drier state. Plants in that region, which is naturally drier than other parts of the forest, help to keep moisture in the area by recycling water through evaporation and transpiration. Without the flora, drought sets in. The mean temperature also goes up.

Change in land use may also dry up the forest in a less straightforward way: by altering the physical properties of rain clouds. In a series of experiments carried out with aircraft flying through smoke plumes during forest fires, a team led by physicist Maria Assunção da Silva Dias of the National Institute for Space Research and her colleague Paulo Artaxo of the University of São Paulo has found that smoke aerosols probably inhibit formation of big water drops, producing clouds that are incapable of rain.

Climatologist Carlos A. Nobre, a leading scientist at the Brazilian National Institute for Space Research, has factored the effects of deforestation and global warming into a computer model that couples vegetation changes with standard global circulation models. In one of Nobre's scenarios, by 2100 up to 60 percent of the forest will turn into cerrado, the type of savanna that dominates the landscape in central Brazil. Such a shift in land cover might be hard to reverse. "We are forcing the forest system into a new state of equilibrium," he says.

The effects of warming by itself are harder to elucidate. But atmospheric scientist Steven C. Wofsy of Harvard University and his colleagues have devised a novel way to estimate the impact of climate change on the current state of the forest. The researchers looked at the likelihood of extreme events, such as abnormal droughts, and matched them to field observations of the drought-sensitive vegetation of Tapajós National Forest in central-eastern Amazonia in the Brazilian state of Pará. They then used their model to generate a simulation spanning 2,500 years and found that forest gives way to cerrado when more than 33 years of drought occur in a century. "You end up bisecting the Amazon with savannas," remarks Lucy Hutyra, a member of Wofsy's team.

The take-home message, she says, is that the extremes are more important than the averages. In a global change scenario, the temperature can rise without signs of ecosystem disturbance in the Amazon. But on the scale of centuries, some areas of the forest, such as the Tapajós region, are highly sensitive to variability. "There is a lot of deadwood and fuel for fire," Wofsy explains. "Suppose you have 10 or 20 years in which nothing happens, then one of those droughts strikes and there is a fire. If the fire isn't too severe, the forest recovers. If it comes again in 25 years or 15 years, everything is destroyed."