Has the Amazon Reached Its ‘Tipping Point’?

Some Brazilian scientists fear that the Amazon may become a grassy savanna — with profound effects on the climate worldwide.

Illustration by Max Guther

One of the first times Luciana Vanni Gatti tried to collect Amazonian air she got so woozy that she couldn’t even operate the controls. An atmospheric chemist, she wanted to measure the concentration of carbon high above the rainforest. To obtain her samples she had to train bush pilots at obscure air-taxi businesses. The discomfort began as she waited on the tarmac, holding one door open against the wind to keep the tiny cockpit from turning into an oven in the equatorial sun. When at last they took off, they rose precipitously, and every time they plunged into a cloud, the plane seemed to be, in Gatti’s words, sambando — dancing the samba. Then the air temperature dipped below freezing, and her sweat turned cold.

Not that it was all bad. As the frenetic port of Manaus receded, the canopy spread out below like a shaggy carpet, immaculate green except for the pink and yellow blooms of ipê trees, and it was one of those moments — increasingly rare in Gatti’s experience — when you could pretend that nature had no final border, and the Amazon looked like what it somehow still was, the world’s largest rainforest.

The Amazon has been called “the lungs of the earth” because of the amount of carbon dioxide it absorbs — according to most estimates, around half a billion tons per year. The problem, scientifically speaking, is that these estimates have always depended on a series of extrapolations. Some researchers use satellites to detect changes that indicate the presence of greenhouse gases. But the method is indirect, and clouds can contaminate the results. Others start with individual tree measurements in plots scattered across the region, which allows them to calculate the so-called biomass in each trunk, which, in turn, allows them to work out how much carbon is being stored by the ecosystem as a whole. But it’s hard to know how representative small study areas are, because the Amazon is almost as large as the contiguous United States, with regional differences in rainfall, temperature, flora and the extent of logging and agriculture. (One study even warned of the risk of “majestic-forest selection bias.”)

Gatti’s solution was to measure the carbon in the air directly. Which led to the least pleasant part of the flight. The pilot had removed the plane’s back seats to make up for the weight of a special silver “suitcase” donated by the U.S. National Oceanic and Atmospheric Administration. Inside, a thick layer of foam cradled 17 glass flasks with valves that opened and closed at the flick of a switch. Each one was supposed to capture a liter and a half of air from a different altitude, starting at 14,500 feet and going down to 1,000. To ensure that collection always took place above the same point on the map, the pilot had to descend in tight spirals, banking so hard that the horizon went near-vertical.

In a healthy rainforest, the concentration of carbon should decline as you approach the canopy from above, because trees are drawing the element out of the atmosphere and turning it into wood through photosynthesis. In 2010, when Gatti started running two flights a month at each of four different spots in the Brazilian Amazon, she expected to confirm this. But her samples showed the opposite: At lower altitudes, the ratio of carbon increased. This suggested that emissions from the slashing and burning of trees — the preferred method for clearing fields in the Amazon — were actually exceeding the forest’s capacity to absorb carbon. At first Gatti was sure it was an anomaly caused by a passing drought. But the trend not only persisted into wetter years; it intensified.

For a while Gatti simply refused to believe her own data. She even became depressed. She had always felt a deep connection to nature. As a kid in a distant town called Cafelândia, she would climb a tree in front of her house, spending hours in a formation of branches that seemed custom-made to cradle her arms, legs and head. In later years, no matter how many times she flew over the Amazon, she never got used to the sight of freshly paved highways, new dirt roads always branching off them, forming a fish-bone pattern. Sometimes she soared past columns of beige smoke that rose all the way to the stratosphere…

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