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Would we notice if the Amazon went up in smoke?

I he Amazon rainforest is the largest living reservoir of carbon dioxide on the land surface of Earth. Its trees contain some 77 billion tons of carbon, and its soils perhaps as much again. That is about twenty years' worth of man-made emissions from burning fossil fuels. The rainforest is also an engine of the world's climate system, recycling both heat and moisture. More than half of the raindrops that fall on the forest canopy never reach the ground; instead they evaporate back into the air to produce more rain downwind. The forest needs the rain, but the rain also needs the forest.

But as scientists come to understand the importance of the Amazon for maintaining climate, they are also discovering that it may itself be under threat from climate change. We are familiar enough with the damage done to the world's biggest and lushest jungle by farmers armed with chain saws and firebrands. But, hard as they try, they can destroy the rainforest only slowly. Despite many decades of effort, most of this jungle, the size of western Europe, remains intact. Climate change, on the other hand, could overwhelm it in a few years.

Until recently, many ecologists have thought of the Amazon rainforest much as their glaciologist colleagues conceived of the Greenland ice sheet: as big and extremely stable. The Greenland ice maintained the climate that kept the ice securely frozen, while the Amazon rainforest maintained the rains that watered the forest. But, just as with the Greenland ice sheet, the idea that the Amazon is stable has taken a knock: some researchers believe that it is in reality a very dynamic place, and that the entire ecosystem may be close to a tipping point beyond which it will suffer runaway destruction in an orgy of fire and drought. Nobody is quite sure what would happen if the Amazon rainforest disappeared. It would certainly give an extra kick to climate change by releasing its stores of carbon dioxide. It would most likely diminish rainfall in Brazil. It might also change weather systems right across the Northern Hemisphere.

One man who is trying to find out how unstable the Amazon rainforest might be is Dan Nepstad, a forest ecologist nominally attached to the Woods Hole Research Center, in Massachusetts, but based for more than two decades in the Amazon. He doesn't just watch the forest: he conducts large experiments within it. In 200 1, Nepstad began creating a man-made drought in a small patch of jungle in the Tapajos National Forest, outside the river port of Santarem. Although in most years much of the Amazon has rain virtually every day, Tapajos is on the eastern fringe of the rainforest proper, where weather cycles can shut down the rains for months. The forest here is, to some extent, adapted to drought. But there are limits, and Nepstad has been trying to find out where they lie.

He has covered the 2.5-acre plot with more than 5,000 transparent plastic panels, which let in the sunlight but divert the rain into wooden gutters that drain to canals and a moat. Meanwhile, high above the forest canopy, he has erected gantries linked by catwalks, so that he can study the trees in detail as the artificial drought progresses. The work was all done by hand to avoid damaging the dense forest, and the scientists soon found they were not alone. The canals became "congregating places for every kind of snake you can imagine," says Nepstad. Caimans and jaguars cruised by, just, it seemed, to find out what was going on.

The results were worth the effort. The forest, it turns out, can handle two years of drought without great trouble. The trees extend their roots deeper to find water and slow their metabolism to conserve water. But after that, the trees start dying. Beginning with the tallest, they come crashing down, releasing carbon to the air as they rot, and exposing the forest floor to the drying sun. By the third year, the plot was storing only about 2 tons of carbon, whereas a neighboring control plot, on which rain continued to fall, held close to 8 tons. The "lock was broken" on a corner of one of the planet's great carbon stores. The study shows that the Amazon is "headed in a terrible direction," wrote the ecologist Deborah Clark, of the University of Missouri, discussing the findings in Science. "Given that droughts in the Amazon are projected to increase in several climate models, the implications for these rich ecosystems are grim."

Everywhere in the jungle, drought is followed by fire. So, in early 2005, Nepstad started an even more audacious experiment. He set fire to another stretch of forest with kerosene torches. "We want to know if recurring fire may threaten the very existence of the forest," he says. The initial findings were not good: the fires crept low along the forest floor, and no huge flames burst through the canopy. The fire may even have been invisible to the satellites that keep a constant watch overhead. But many trees died nonetheless, as their bark scorched and the flow of sap from their roots was stanched.

Nepstad's experiments are part of a huge international effort to monitor the health of the Amazon, called the Large-scale Biosphere-Atmosphere Experiment in Amazonia. From planes and satellites and gantries above the jungle, researchers from a dozen countries have been sniffing the forest's breath and assessing its survival strategies. The current estimate is that fires in the forest are releasing some 200 million tons of carbon a year—far more than is absorbed by the growing forest. The Amazon has become a significant source of carbon dioxide, adding to global warming. More worrying still, the experiment is discovering a drying trend across the Amazon that leaves it ever more vulnerable to fires. Nepstad's work suggests that beyond a certain point, the forest will be unable to recover from the fires, and will begin a process of rapid drying that he calls the "savannization" of the Amazon.

And even as he concluded his drought experiment, nature seemed to replicate it. The rains failed across the Amazon through 2005, killing trees, triggering fires, and reducing the ability of the forest to recycle moisture in future—thus increasing the risk of future drought. Nepstad's experiments suggest that the rainforest is close to the edge—to permanent drought, rampant burning, savannization, or worse. In the final weeks of 2005, the rains returned. The forest may recover this time. But if future climate change causes significant drying that lasts from one year to the next, feedbacks in the forest could realize Nepstad's worst fears.

The 2005 drought was caused by extremely warm temperatures in the tropical Atlantic—the same high temperatures that are believed to have caused the record-breaking hurricane season that year. The rising air that triggered the hurricanes eventually came back to earth, suppressing the formation of storm clouds over the

Amazon. And, as I discovered at Britain's Hadley Centre for Climate Prediction, that is precisely what climate modelers are forecasting for future decades.

The Hadley Centre's global climate model is generally regarded as one of the world's top three. And it predicts that business-as-usual increases in industrial carbon dioxide emissions worldwide in the coming decades will generate warmer sea temperatures, subjecting the Amazon to repeated droughts, and thus creating "threshold conditions" beyond which fires will take hold. The Amazon rainforest will be dead before the end of the century. Not partly dead, or sick, but dead and gone. "The region will be able to support only shrubs or grass at most," said a study published by the Hadley Centre in 2005.

Not all models agree about that. But the Hadley model is the best at reproducing the current relationship between ocean temperatures and Amazon rainfall, so it has a good chance of being right about the future, too. Nepstad himself predicts that a "megafire event" will spread across the region. As areas in the more vulnerable eastern rainforest die, they will cease to recycle moisture back into the atmosphere to provide rainfall downwind.

A wave of aridity will travel west, creating the conditions for fire to rip through the heart of the jungle.

With the trees gone, the thin soils will bake in the sun. Rainforest could literally turn to desert. The Hadley forecast includes a graph of the Amazon's forest's future carbon. It predicts that the store of a steady 77 billion tons over the past half century will shrink to 44 billion tons by 2050 and 16.5 billion tons by the end of the century. That, it calculates, would be enough to increase the expected rate of warming worldwide by at least 5° percent.

The Amazon rainforest does not just create rain for itself. By one calculation, approaching 6 trillion tons of water evaporates from the jungle each year, and about half of that moisture is exported from the Amazon basin. Some travels into the Andes, where it creates clouds that swathe some mountains so tightly that their surfaces have never been seen by satellite. Some blows south to water the pampas of Argentina, some east toward South Africa, and some north toward the Caribbean. The forest is a vital rainmaking machine for most of South America. As much as half of Argentina's rain may begin as evaporation from the Amazon.

But the benefits of the great Amazonian hydrological engine extend much further, and are not restricted to rainfall. The moisture also carries energy. A lot of solar energy is used to evaporate moisture from the forest canopy. This is one reason why forests stay cooler than the surrounding plains. And when the moisture condenses to form new clouds, that energy is released into the air. It powers weather systems and high-level winds known as jets far into the Northern Hemisphere. Nicola Gedney and Paul Valdes, two young climate researchers at the University of Reading, have calculated that this process ultimately drives winter storms across the North Atlantic toward Europe. "There is a relatively direct physical link between changes over the deforested region and the climate of the North Atlantic and western Europe," they say. If the rainforest expires, the hydrological engine, too, is likely to falter, and the link will be cut.

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