Amazon rainforest nears tipping point that may see it become savannah

More than three-quarters of the world’s largest rainforest has become less resilient to drought since the early 2000s, with areas near humans and with lower rainfall being the worst hit

By Adam Vaughan

The Amazon rainforest is nearing a tipping point that will see it transform into savannah, according to researchers who have found that the biodiversity hotspot has lost resilience in the past two decades.

Previous studies have warned that the world’s largest rainforest, which acts as a vital brake on climate change, is approaching a critical threshold. But most past research has relied on projections using models, not real world observations.

Now, Tim Lenton at the University of Exeter, UK, and his colleagues have used two sets of satellite data covering between 1996 and 2016 to measure the greenness of the Amazon over time, watching for how it recovered after impacts such as drought and fires. They found that since the early 2000s, 76 per cent of the region had become less resilient, or less able to restore itself to a stable state after being affected by events.

Importantly, says Lenton, the signal of this growing instability was picked up without immediately obvious changes such as huge drops in the forest’s biomass or tree cover.

“Why do we care about it? It’s worth reminding ourselves that if it gets to that tipping point and we commit to losing the Amazon rainforest then we get a significant feedback to global climate change,” says Lenton. He says a shift to savannah, a grassy ecosystem with much less biomass, would unlock about 90 billion tonnes of carbon dioxide stored in the trees and soil. Humanity emits about 40 billion a year.

The team looked at vegetation cover using one satellite data set that measures the optical depth of vegetation using microwaves, and a second one that used infrared instead.

The loss of resilience was faster in parts of the rainforest that have received less rainfall and those that are closer to the biggest signs of human activities, such as large farms and major roads, say the researchers.

Lenton says he cannot put a precise date on how far off the rainforest’s tipping point might be, but expects the transition to savannah to be a process that would take decades. Other Amazon experts say the research adds to a growing body of evidence that the rainforest is approaching a tipping point.

The shifts aren’t even across the rainforest. The south-eastern part of the Amazon, dubbed the “arc of deforestation”, has already changed to the point at which it is now a carbon emitter rather than a carbon sink.

Matt Finer at the Amazon Conservation non-profit organisation says the research could offer a guide for which remaining parts of the forest to prioritise for protection. “The western and north-east Amazon appear the most resilient, reinforcing the critical need to protect these areas now and in the future from current and looming threats like mining, new roads and agriculture expansion,” he says.

Carlos Nobre at the University of São Paulo, Brazil, says the research is “concerning” and “solid data-based” work. “It demonstrates that the resilience of the Amazon tropical rainforest has been very rapidly decreasing over the last 20 years,” he says.

Journal reference: Nature Climate Change, DOI: 10.1038/s41558-022-01287-8