The Amazon is a labyrinth of a thousand rivers. They are born at 21,000 feet, with seasonal melts from the Sajama ice cap in Bolivia, and they are born in the dark rock of PeruтАЩs Apacheta cliff, as glacial seepage spraying white from its pores. They are born less than 100 miles from the Pacific Ocean; they are born in the middle of the South American continent, in BrazilтАЩs high plains, savannas and sandstone ridges. Most are just tributaries of tributaries, headwaters for much larger rivers тАФ the Caquet├б, the Madre de Dios, the Iriri, the Tapaj├│s тАФ any of which, on its own, would already be among the largest rivers in the world. Where these tributaries empty, just south of the Equator, they form the aorta of the Amazon proper, more than 10 miles wide at its widest point. From the AmazonтАЩs farthest source to its mouth in the Atlantic, water flows for 4,000 miles, almost as long as the Nile. Measured by the volume it releases into the ocean тАФ the equivalent of a dozen Mississippis, one-fifth of all the fresh water that reaches the worldтАЩs seas тАФ the Amazon is the largest river in the world.
The consensus used to be that ecosystems are merely a product of prevailing weather patterns. But in the 1970s, the Brazilian researcher Eneas Salati proved that the Amazon, with its roughly 400 billion trees, also creates its own weather. On an average day, a single large tree releases more than 100 gallons of water as vapor. This not only lowers the air temperature through evaporative cooling; as Salati discovered by tracking oxygen isotopes in rainwater samples, it also gives rise to тАЬflying riversтАЭ тАФ rain clouds that recycle the forestтАЩs own moisture five or six times, ultimately generating as much as 45 percent of its total precipitation. By creating the conditions for a continental swath of evergreens, this process is crucial to the AmazonтАЩs role as a global тАЬsinkтАЭ for carbon.
Many scientists now fear, however, that this virtuous cycle is breaking down. Just in the past half-century, 17 percent of the Amazon тАФ an area larger than Texas тАФ has been converted to croplands or cattle pasture. Less forest means less recycled rain, less vapor to cool the air, less of a canopy to shield against sunlight. Under drier, hotter conditions, even the lushest of Amazonian trees will shed leaves to save water, inhibiting photosynthesis тАФ a feedback loop that is only exacerbated by global warming. According to the Brazilian Earth system scientist Carlos Nobre, if deforestation reaches 20 to 25 percent of the original area, the flying rivers will weaken enough that a rainforest simply will not be able to survive in most of the Amazon Basin. Instead it will collapse into scrubby savanna, possibly in a matter of decades.
Much of the evidence for this theory тАФ including GattiтАЩs air-тАЛsample studies тАФ emerged thanks to a groundbreaking initiative led by Nobre himself. When Nobre started trying to forecast the impact of deforestation back in 1988, he had to do it at the University of Maryland, because his home country lacked the computing power for serious climate modeling. Brazil was so strapped for resources that foreign researchers even dominated Amazon fieldwork. But Nobre spearheaded a program that, in the words of a Nature editorial, тАЬrevolutionized understanding of the Amazon rainforest and its role in the Earth system.тАЭ Established in 1999 and known as the Large-тАЛScale Biosphere-тАЛAtmosphere Experiment in Amazonia, or L.B.A., it united disciplines that usually did not collaborate, bringing together chemists like Gatti with biologists and meteorologists. While funding mostly came from the United States and Europe, Nobre insisted that South Americans play leading roles, thus giving rise to a whole new generation of┬аBrazilian climate scientists.
Until recently, Nobre was working under the assumption that the Amazon would not become a net source of carbon for at least another few decades. But GattiтАЩs research is not the only sign that, as he put it to me over Skype, тАЬwe are on the eve of this tipping point.тАЭ The rain machine is slowing. Droughts used to come once every couple of decades, with a megadrought every century or two. But just since 1998 there have been five, two of them extreme. The effect is particularly acute in the eastern Amazon, which has already lost a staggering 30 percent of its forest. The dry season there used to be three months long; now it lasts more than four. During the driest months, rainfall has declined by as much as a third in four decades, while average temperatures have risen by as much as 3.1 degrees Celsius тАФ triple the annual increase for the world as a whole in the fossil-fuel era. In some parts, jungles are already being colonized by grasses.
Losing the Amazon, one of the most biodiverse ecosystems on Earth, would be catastrophic for the tens of thousands of species that make their home there. Rising temperatures could also drive millions of people in the region to become climate refugees. And it would represent a more symbolic death, too, as тАЬsaving the rainforestтАЭ has long been a kind of synecdoche for modern environmentalism as a whole. What scientists are most concerned about, though, is the potential for this regional, ecological tipping point to produce knock-on effects in the global climate. Because the AmazonтАЩs flying rivers circulate back over the continent, the impact may already be reaching beyond the rainforest. In 2015, BrazilтАЩs populous southeast was hit by historic water shortages; in 2021, quasi-biblical sandstorms swept the region. If the flying rivers peter out entirely, it could affect atmospheric circulation even beyond South America, possibly influencing the weather as far away as the western United States.
