The vast Amazon rainforest is on the brink of being turned into desert, with catastrophic consequences for the world's climate, alarming research suggests.
And the process, which would be irreversible, could begin as early asnext year.
Studies by the blue-chip Woods Hole Research Centre, carried out in Amazonia, have concluded that the forest cannot withstand more than two consecutive years of drought without breaking down.
Scientists say that this would spread drought into the northern hemisphere, including Britain, and could massively accelerate global warming with incalculable consequences.
The alarming news comes in the midst of a heatwave gripping Britain and much of Europe and the United States.
Temperatures in the south of England reached a July record 36.3C on Tuesday.
And it comes hard on the heels of a warning by an international group of experts, led by the Eastern Orthodox "pope" Bartholomew, last week that the forest is rapidly approaching a "tipping point".
The research - carried out by the Massachusetts-based centre in Santarem on the Amazon river - has taken even the scientists conducting it by surprise.
When Dr Dan Nepstead started the experiment in 2002 - by covering a chunk of rainforest the size of a football pitch with plastic panels to see how it would cope without rain - he surrounded it with sophisticated sensors, expecting to record only minor changes.
The trees managed the first year of drought without difficulty.
In the second year, they sunk their roots deeper to find moisture, but survived.
But in year three, they started dying.
Beginning with the tallest the trees started to come crashing down, exposing the forest floor to the drying sun.
By the end of the year the trees had released more than two-thirds of the carbon dioxide they have stored during their lives, helping to act as a break on global warming.
Instead they began accelerating the climate change.
The Amazon now appears to be entering its second successive year of drought, raising the possibility that it could start dying next year.
The immense forest contains 90 billion tons of carbon, enough in itself to increase the rate of global warming by 50 per cent.
Dr Nepstead expects "mega-fires" rapidly to sweep across the drying jungle.
With the trees gone, the soil will bake in the sun and the rainforest could become desert.
Dr Deborah Clark from the University of Missouri, one of the world's top forest ecologists, says the research shows that "the lock has broken" on the Amazon ecosystem.
She adds: the Amazon is "headed in a terrible direction".