This widespread weakness is an early warning that the Amazon is approaching its “tipping point,” study authors say. Under conditions of rising temperatures and other types of anthropogenic impact, the ecosystem may undergo sudden and irreversible extinction. More than half of the rainforest could turn into savannah in a few decades – a transition that would threaten biodiversity, change regional weather patterns and dramatically accelerate climate change.
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Historically, the Amazon has been one of the most important “carbon sinks” on Earth, extracting billions of tons of carbon dioxide from the air and storing it in vegetation. Researchers fear that the sudden release of this carbon will render humanity’s most ambitious climate goal, limiting temperature rise to 1.5 degrees Celsius (2.7 degrees Fahrenheit), unattainable.
“As a scientist, I don’t have to worry. But after reading this article, I am very, very disturbed,” said Carlos Nobre, a climate scientist at the Institute for Advanced Study at the University of São Paulo, who was not involved in the new study. “This article shows that we are heading in a completely wrong direction… If we get past the tipping point, this is very bad news.”
The Amazon is one of several “tipping points” for the global climate, scientists say. Instead of constantly deteriorating as the planet warms, these systems can switch from one phase to the next abruptly—perhaps with very little warning.
For the past 50 million years, the Amazon has been in the rainforest phase. The trees themselves ensured their continued existence: the water evaporating from the leaves created an endless loop of rain, and the dense crown did not allow sunlight to dry the soil. The contours of the forest may have changed somewhat in response to ice ages, wildfires and rising sea levels, but it could always return to its lush, green state.
However, human-induced warming and deforestation have taken over this self-reinforcing system. Hotter conditions in the Atlantic have extended the dry season in the Amazon by several weeks. By cutting down about 17 percent of its trees, people undermined the forest’s water recycling mechanism. Drought-affected trees are more vulnerable to wildfires. And the more trees that die, the less rain falls, which, in turn, exacerbates the death of trees.
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At some point, the ecosystem will lose more trees than it can regenerate in these hot and dry conditions. Dark, dense, humid rainforest will give way to more open savannah.
Mathematician Niklas Burs, who contributed to the new paper, compared it to a man leaning back in his chair. If they don’t lean too far, they can easily fall back to having all four feet on the floor. But as soon as they pass the tipping point, the whole system collapses. And getting up again is much harder than it was to fall.
Scientists say satellite imagery Burs and colleagues have analyzed shows the Amazon is still teetering on the brink of tipping over. Looking at patches of forest with at least 80 percent broadleaf tree cover — areas that haven’t suffered much from deforestation — the researchers found that the vast majority of forest patches are recovering from seasonal fluctuations more slowly than they did 20 years ago. Areas in the drier southern reaches of the rainforest, as well as those closer to roads, were affected the most.
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“The buckling we are seeing means that we are probably approaching this tipping point,” said Burs, who studies earth system dynamics at the Technical University of Munich and the Potsdam Institute for Climate Impact Research. “But it also means we’re not past the tipping point, so there’s hope.”
The Nature Climate Change document does not indicate when the Amazon might cross this dangerous threshold. Even when an ecosystem is completely destabilized, it can persist until an outside force—such as a megafire or severe drought—push it over the edge. The moment of no return may not be clear until it’s too late to act, says lead author Chris Boulton, a climate scientist at the University of Exeter.
“My friend is using the idea of Wile Coyote running off a cliff,” Boulton said. “He looks normal, but suddenly he looks down and realizes that he fell off a cliff.”
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That’s what makes this study — the first empirical assessment of instability in an entire rainforest — so valuable, he added. “If we show that one of these systems is heading towards a tipping point, it could wake people up,” Boulton said.
For the 10% of known species that inhabit the Amazon, the disappearance of the rainforest could be the death knell. A catastrophic extinction will endanger millions of people who rely on the ecosystem for food; 70% of the rain that falls in northern Argentina, the South American breadbasket, falls on Amazonian trees.
Exceeding the critical point of the Amazon will also lead to the release of global greenhouse gas pollution into the atmosphere in a few years. Already, studies show that some areas of the Amazon produce about 300 million tons more carbon than they extract from the air – an amount roughly equal to Japan’s annual emissions.
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Scientists warn that the warming effects of the sudden loss of half of the rainforest will be felt thousands of miles away and for centuries into the future. This will mean escalating storms and increased wildfires, chronic food shortages and nearly a foot of sea level rise, flooding coastal communities. This could trigger other tipping points, such as melting ice sheets or disruption of the South American monsoon.
However, unlike ice sheets and monsoon systems that respond solely to the amount of heat humans trap in the Earth’s atmosphere, the Amazon is being pushed to breaking point by two forces: deforestation and climate change. It also gives the Boers hope, as it means that humanity has two strategies to protect the ecosystem.
“If we take one of these factors out of the equation, my intuition tells me that the system can handle it,” he said. “That’s exactly what the governments of Brazil, Colombia and Peru need to say: stop deforestation today.”
Terrence McCoy of Rio de Janeiro contributed to this report.