PARIS (AFP) - The Amazonian rainforest has lost much of its ability to absorb climate-altering greenhouse gases from the atmosphere, scientists reported on Wednesday.
In the 1990s, the great forest was able to store as much as two billion tonnes of carbon dioxide (CO2) each year, they said.
Now, though, the uptake has halved, and for the first time is being outstripped by fossil-fuel emissions from Latin America.
The evidence, reported in the journal Nature, comes from a 30-year survey that brought together nearly a hundred researchers working in eight countries.
They looked at 321 forest plots, measuring 200,000 trees and recording tree deaths and growth and new trees.
The scientists found a worrying surge in the rate of trees dying across the Amazon.
"Tree mortality rates have increased by more than a third since the mid-1980s, and this is affecting the Amazon's capacity to store carbon," said Roel Brienen of the University of Leeds in northern England, who led the probe.
The work runs counter to hopes that higher levels of atmospheric CO2 are good for "sinks" - the technical term for forests that soak up the principal greenhouse gas.
As CO2 is a key ingredient in photosynthesis, more of the gas should spur tree growth, according to this theory.
But what appears to have happened is that faster growth has also translated into faster death.
"With time, the growth stimulation feeds through the system, causing trees to live faster, and so die younger," said Brienen's colleague, Oliver Phillips.
There may be additional causes, the researchers said. Spells of drought and exceptionally high temperatures may have killed off millions of trees.
Counting on forests to be a reliable buffer against global warming is a risky assumption, the scientists said.
"All across the world, even intact forests are changing," said Phillips.
"Forests are doing us a huge favour, but we can't rely on them to solve the carbon problem.
"Instead, deeper cuts in emissions will be required to stabilise our climate."