Research on a drought last year in the Amazon has raised doubts about the ability of the giant rainforest to absorb greenhouse gas emissions.
Scientists from Britain and Brazil report in the journal Science that the 2010 drought was more widespead than in 2005 - the last big one - with more trees probably lost.
The 2005 drought had been termed a "one in a century" event.
In drought years, the Amazon region changes from being a net absorber of carbon dioxide into a net emitter.
The BBC's science correspondent says the scientists suggest this is further evidence of the Amazon's vulnerability to rising global temperatures.
They also suggest the days of the Amazon forest curbing the impact of rising greenhouse gas emissions may be coming to an end.
The 2010 drought saw the Amazon River at its lowest levels for half a century, with several tributaries completely dry and more than 20 municipalities declaring a state of emergency.
"It's difficult to detect patterns from just two observed droughts, but to have them close together is concerning," Dr Simon Lewis from the University of Leeds told BBC News.
Both droughts were associated with unusually warm seas in the Atlantic Ocean off the Brazilian coast.
"If that turns out to be driven by escalating greenhouse gas concentrations in the atmosphere, it could imply that we'll see more drought years in the near future," said Dr Lewis.
"If events like this do happen more often, the Amazon rainforest would reach a point where it shifts from being a valuable carbon sink slowing climate change to a major source of greenhouse gases."
