The shore of a sea of nearly 400 billion trees winds through the central Brazilian state of Mato Grosso. Here, the Amazon rainforest rubs up against the Cerrado, the world’s largest savanna.
The two are distinct worlds — one a wet and verdant jungle, the other relatively dry and blanketed in wild grasses, shrubs and small trees. But no clear line demarcates the Amazon and the Cerrado. Instead, there’s a messy transition zone, a continuum of vegetation that grows taller toward the rainforest. Over thousands of years, the boundary ebbs and flows, driven by natural fluctuations in climate.
“But in this formula is a new element,” says ecologist Beatriz Marimon of Mato Grosso State University in Nova Xavantina. Humans, with their ambitions to domesticate the land, she says.
About half a century ago, throngs of people started streaming into the region along new highways, clearing forest for farmland and cattle ranches, she says. Fifty years is a blink in the life span of a forest nearly as old as the dinosaurs, but it’s plenty of time for humans to remodel a landscape.
In 2007, earth system scientist Carlos Nobre, now at the University of São Paulo, and his colleagues suggested that much of the Amazon could transform into a savanna if deforestation exceeded 40 percent of the forest’s original area, which was mostly whole before the 1970s.
About a decade later, after accounting for interactions between climate change, deforestation and fire, Nobre and a colleague offered a more dire warning. If just 20 to 25 percent of the Amazon was deforested and global warming reached about 2.4 degrees Celsius above pre-industrial levels, the ecosystem could collapse and much of the forest could transform into savanna and shrubland, they found. Today, humans have already deforested about 17 percent of the overall Amazon, and damaged much more, estimates suggest.
That the Amazon may transform into something else after reaching a point…
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