Viewed from space, the Amazon rainforest doesn’t look like an ecosystem on the brink. Clouds still coalesce from the breath of some 390 billion trees. Rivers snake their way through what appears to be a sea of endless green.

Yet satellite images taken over the past several decades reveal that more than 75 percent of the rainforest is losing resilience, according to a study published Monday in the journal Nature Climate Change. The vegetation is drier and takes longer to regenerate after a disturbance. Even the most densely forested tracts struggle to bounce back.

This widespread weakness offers an early warning sign that the Amazon is nearing its “tipping point,” the study’s authors say. Amid rising temperatures and other human pressures, the ecosystem could suffer sudden and irreversible dieback. More than half of the rainforest could be converted into savanna in a matter of decades – a transition that would imperil biodiversity, shift regional weather patterns and dramatically accelerate climate change.

Historically, the Amazon has been one of Earth’s most important “carbon sinks,” pulling billions of tons of carbon dioxide from the air and storing it in vegetation. Researchers fear that this carbon’s sudden release would put humanity’s most ambitious climate goal – limiting temperature rise to 1.5 degrees Celsius (2.7 degrees Fahrenheit) – out of reach.

“As a scientist, I am not supposed to have anxiety. But after reading this paper, I am very, very anxious,” said Carlos Nobre, a climate scientist at the University of São Paulo’s Institute of Advanced Studies, who was not involved in the new research. “This paper shows we are moving in the completely wrong direction … If we exceed the tipping point, that’s very bad news.”

The Amazon is one of several “tipping elements” in the global climate, scientists say. Rather than steadily worsening as the planet warms, these systems have the potential to abruptly switch from one phase to another – possibly with very little warning.

For the past 50 million years, the Amazon has been in a wet rainforest phase. The trees themselves ensured their continued existence: Water evaporating from leaves created an endless loop of rainfall, while the dense canopy prevented sunlight from drying out the soil. The contours of the forest may have shifted somewhat in response to ice ages, wildfires and rising seas, but it was always able to return to its lush, verdant state.

Yet human-caused warming and deforestation have hijacked this self-reinforcing system. Hotter conditions in the Atlantic Ocean have extended the Amazon’s dry season by several weeks. By felling roughly 17 percent of its trees, people have undercut the forest’s water recycling mechanism. Trees stressed by drought are more vulnerable to wildfires. And the more trees die, the less rain falls, which in turn makes tree die-offs worse.

At a certain point, the ecosystem will lose more trees than it can recover in these hot, dry conditions. The dark, dense, damp tropical rainforest will give way to a more open savanna.

Mathematician Niklas Boers, who contributed to the new paper, compared it to someone leaning back in a chair. If they don’t tilt too far, they can easily return to having all four legs on the floor. But once they pass the tipping point, the whole system comes crashing down. And it is much harder to get up again than it was to fall.

The satellite imagery Boers and his colleagues analyzed suggests that the Amazon is still wobbling on the edge of tipping, the scientists say. Looking at tracts of forest with at least 80 percent broadleaf tree cover – areas that have not been heavily affected by deforestation – researchers found that the vast majority of forest patches recover more slowly after seasonal fluctuations than they did 20 years ago. Tracts in the rainforest’s drier southern reaches, as well as ones that were closer to roads, suffered the most.

“The resilience loss we have observed means we have likely moved closer to that critical point,” said Boers, who studies earth system dynamics at the Technical University of Munich and the Potsdam Institute for Climate Impacts Research. “But it also means we haven’t passed the tipping point, so there’s hope.”

The Nature Climate Change paper does not pinpoint when the Amazon could cross this dangerous threshold. Even once the ecosystem has been completely destabilized, it may persist until an outside force – for example, a megafire or severe drought – pushes it over the edge. The moment of no return might not be obvious until it’s too late to do something, said lead author Chris Boulton, a climate scientist at University of Exeter.

“My friend uses the idea of Wile E. Coyote running off a cliff,” Boulton said. “He looks fine, and he suddenly looks down and realizes he’s gone over the cliff.”

That’s what makes this study – the first empirical assessment of instability across the entire rainforest – so valuable, he added. “If we’re showing that one of these systems is moving toward a tipping point, that might make people wake up,” Boulton said.

For the 10 percent of known species that make their homes in the Amazon, loss of the rainforest could signal a death knell. Catastrophic dieback would imperil millions of people who rely on the ecosystem for food; 70 percent of the rain that falls in northern Argentina, a South American breadbasket, comes from Amazonian trees.

Exceeding the Amazon’s tipping point would also unleash several years’ worth of global greenhouse gas pollution into the atmosphere. Already, studies show some areas of the Amazon are producing about 300 million tons more carbon than they pull out of the air – an amount roughly equal to annual emissions from Japan.

The warming consequences of suddenly losing half the rainforest would be felt thousands of miles away and for centuries into the future, scientists warn. It would mean escalating storms and worsening wildfires, chronic food shortages and nearly a foot of sea level rise inundating coastal communities. It could trigger other tipping points, such as the melting of ice sheets or the disruption of the South American monsoon.

Yet unlike ice sheets and monsoon systems, which respond solely to the amount of heat humans are trapping in Earth’s atmosphere, the Amazon is being pushed toward its tipping point by two forces: deforestation and climate change. This also gives Boers hope because it means humanity has two strategies for protecting the ecosystem.

“If we take one of those factors out of the equation, my intuition would be that the system would be able to cope with it,” he said. “That’s exactly what one should tell the Brazilian, Columbian and Peruvian governments: Stop deforestation today.”