More than a quarter of the Amazon basin is now releasing more carbon than it absorbs, according to a comprehensive study.
Brazilian researchers flew an aircraft over the rainforest every two weeks for nine years, taking air samples from just above the canopy all the way up to 4.5km.
They found that the eastern side of the Amazon, which accounts for around 28% of the total area, is losing more carbon as a result of deforestation than is being removed from the atmosphere by the growth of trees.
Some of the carbon is lost through fires, deliberately started to clear the forest for agriculture.
But the knock-on effect of an absence of trees is local climate change, with rising temperatures and reduced rainfall accelerating the decline of surrounding areas of forest. Parts of the Amazon have flipped from being a carbon sink to a carbon source.
Mark Wright, director of science for conservation charity WWF, told Sky News that the research showed the Amazon is at a tipping point, where great swathes of forest could be destroyed by self-perpetuating dieback.
“We’re no longer talking about some dystopian future, this is stuff we can see on the ground, these changes are happening here and now,” he said.
“It’s a warning of what is still come to come.
“We know we are moving towards that inextricable situation where the forest will slowly transform into a more grass-like savannah ecosystem and as a result will push more carbon into the atmosphere.”
The world’s plants have absorbed 25% of fossil fuel emissions since 1960, helping to reduce global warming.
The Amazon rainforest has taken up a significant proportion, storing an estimated 123 billion tonnes in the trees and other vegetation.
But the new research suggests it can’t be relied on in future to mop up carbon dioxide from the atmosphere because human activity is disrupting the delicate ecosystem.
The researchers, led by National Institute for Space Research in Brazil, found that on the lush western side of the Amazon basin slightly more carbon is being absorbed through photosynthesis than is being released by dead trees and human impact on the forest.
But it was a significantly different story on the eastern side, where 27% of the forest has been lost, more than twice the rate in the west.
Results published in the journal Nature show that the area has switched from being a carbon sink to a net source during the nine years of the study, with local climate change destabilising the delicate ecosystem.
The researchers say that in the drier months of August to October the temperature in the eastern Amazon has increased by between 1.9C and 2.5C over 40 years. Rainfall has decreased by between 24% and 34%.
The researchers say there is a direct link between the changing climate and tree loss.
The Amazon receives an average of more than 2m of rain a year, with between a quarter and a third of it resulting from moisture released by trees.
With a shrinking forest in the east the atmosphere is drier, stunting the growth of remaining trees and reducing the amount of carbon they absorb.
Some scientists have predicted that if the Amazon reaches a tipping point it will retreat to cover only a relatively small area in the west, with a devastating impact on biodiversity and atmospheric carbon.
But Mark Wright said: “The future is potentially very, very bleak, but it’s not too late.
“If we follow the science, we can clearly see there is scope to do really good agricultural development in Brazil, in a way that will boost their economy, in a way that does not require further degradation.
“If we can concentrate on restoring those lands there is still hope for preventing that kind of runaway process.
“But we have to act now, we can’t keep pushing this off.”
Sky News has launched the first daily prime time news show dedicated to climate change.
The Daily Climate Show is broadcast at 6.30pm and 9.30pm Monday to Friday on Sky News, the Sky News website and app, on YouTube and Twitter.
Hosted by Anna Jones, it follows Sky News correspondents as they investigate how global warming is changing our landscape and how we all live our lives.
The show also highlights solutions to the crisis and how small changes can make a big difference.