What Ballooning Carbon Emissions Will Do to Trees

A year before I met Hubau, I was in the Amazon, slogging up the steps of a rickety latticework tower. The aluminum lookout was at a site called ZF2, a study area operated by Brazil’s National Institute of Amazonian Research (INPA). Hardly visible in the dim dawn, David Lapola, a biology professor at Brazil’s University of Campinas, beckoned me upward. The thwap of our feet against metal treads drowned out the growls of distant howler monkeys. At sunrise, we reached an observation deck, overlooking the treetops from 10 stories above the ground.

The horizon glowed a fiery orange, but Lapola paid little heed. He was staring straight down, at a disk of experimental forest below us. If only he could get a big enough grant, he lamented, the trees down there could help him forecast the future of the tropical carbon sink.

Not long before, a large team led by a plant ecologist named Roel Brienen had published an ominous paper in the journal Nature. Over decades, the researchers had repeatedly measured the carbon content of 321 plots across the Amazon, using the same method that would later try Hubau’s patience in the Congo. They found that the amount of carbon dioxide absorbed by intact Amazon forest had declined by 30 percent between the 1990s and the 2000s. In a recent phone call, Brienen said that if the trend continues, “in two or three decades the carbon sink could be shut down.” Brienen’s paper speculated that increased CO₂ might actually be causing trees to grow faster and die younger, like a campfire doused in gasoline.

In 2016, Lapola was one of two Brazilian scientists overseeing plans for what Richard Betts, a scientist at the U.K.’s government climate lab, called “one of the most exciting experiments on the planet.” The study is a free-air carbon enrichment (FACE) experiment, in which an open-air plot is treated with extra CO₂. One of the earliest FACE studies in a forest, which began in 1996, boosted the CO₂ concentration in a Tennessee sweetgum plantation by 50 percent—roughly matching the air we’re on track to breathe in 2050. At first, trees grew faster. But they soon stagnated. The soil ran out of nitrogen and the trees couldn’t benefit from extra CO₂.

Richard Norby, who ran the sweetgum experiment, says it’s hard to predict in any given case whether carbon fertilization will make a difference. Scientists have run FACE experiments in temperate forests in the U.S., Europe, and Australia, and sometimes the extra CO₂ helps. Other times, a dearth of water, too much heat, or some other factor gets in the way. “That’s why we need more experiments,” he says.