It covers an area 25 times bigger than Britain, is home to a bewildering concentration of flora and fauna and is often described as the "lungs of the world" for its ability to absorb vast amounts of carbon dioxide through its immense photosynthetic network of trees and leaves.
The Amazon rainforest is one of the biggest and most important living stores of carbon on the planet through its ability to convert atmospheric carbon dioxide into solid carbon, kept locked in the trunks of rainforest trees for centuries.
But this massive natural "sink" for carbon cannot be relied on to continue absorbing carbon dioxide in perpetuity, a study shows. Researchers have found that, for a period in 2005, the Amazon rainforest actually slipped into reverse gear and started to emit more carbon than it absorbed.
Four years ago, a sudden and intense drought in the Amazonian dry season created the sort of conditions that give climate scientists nightmares. Instead of being a net absorber of about two billion tons of carbon dioxide, the forest became a net producer of the greenhouse gas, to the tune of about three billion tons.
The additional quantity of carbon dioxide left in the atmosphere after the drought – some five billion tons – exceeded the annual man-made emissions of Europe and Japan combined. What happened in the dry season of 2005 was a stark reminder of how quickly the factors affecting global warming can change.
"For years, the Amazon forest has been helping to slow down climate change," said Professor Oliver Phillips, from the University of Leeds and the lead author of the study in the journal Science. "But relying on this subsidy from nature is extremely dangerous. The emission of five billion tons of carbon dioxide was huge. It meant that a major part of the biosphere had switched from one function to another, from a carbon sink to a carbon source.
"It shows what could happen if droughts become more frequent, and climate models suggest that Amazonia will get warmer and so put more water stress on vegetation. If the Earth's carbon sinks slow or go into reverse, as our results show is possible, carbon dioxide levels will rise even faster. Deeper cuts in emissions will be required to stabilise our climate."
The study, which involved nearly 70 scientists from 13 countries, examined more than 100,000 trees in 100 forest plots. The scientists had been monitoring changes to the girth of each tree over a period of between 20 and 30 years, so were able to calculate with some precision the effect of the 2005 drought on tree growth.
The drought itself was unusual. Normally, droughts in the Amazon are the result of changes caused by El Niño, the warm Pacific Ocean current, but the one in 2005 was a result of higher-than-average temperatures at the sea surface of the tropical North Atlantic.
"The pattern of the drought was shorter but sharper and more intense than usual," Professor Phillips said. "It affected the southern two-thirds of Amazonia and especially the south-west through reduced rainfall and higher-than-average temperatures. It was the kind of drought we expect to see in a globally warming world. On the ground, it was hard to see because you had to detect by measuring lots of trees over a larger area of land. There was not a massive die-off of trees."
The researchers found that the drought sharply reversed the decades-long growth of the trees. The normal die-off rate of the trees, about 1 per cent per year, doubled to 2 per cent, and the continued expansion of tree girths effectively stopped.
"Visually, most of the forest appeared little affected, but our records prove tree death rates accelerated," Professor Phillips went on. "Because the region is so vast, even small ecological effects can scale-up to a large impact on the planet's carbon cycle."
Humans worldwide are estimated emit about 32 billion tons of carbon dioxide each year but just less than half of this, about 15 billion tons, remains in the atmosphere. The rest is absorbed by natural carbon sinks in the ocean and on land.
Scientists have calculated that the world's tropical forests collectively absorb about 4.8 billion tons of carbon dioxide every year, with the Amazon being the single biggest rainforest sink. Amazonia alone is estimated to store about 100 billion tons of carbon locked up in its trees.
This is why the climate change negotiations in Copenhagen later this year will focus heavily on what can be done to save rainforests to ameliorate the effects of man-made emissions of carbon dioxide.
Lee White, the chief climate change scientist for the government of Gabon, said: "To get an idea of the value of the sink, the removal of nearly five billion tons of carbon dioxide from the atmosphere by intact tropical forests, based on realistic prices for a ton of carbon, should be valued at about £13bn a year. This is a compelling argument for conserving tropical forests." Dr White was a co-author of another study last month on the role played by African tropical forests in processing carbon dioxide.
Professor Phillips added. "It's surprising to see how sensitive the system appears to be. This is the first time anyone has tried to measure the impact of a big tropical drought on the ground. Now we've quantified it and, yes, there's a specificity there and it wouldn't take a huge change to shut down this thing and switch it to an overall source of carbon dioxide."
The Amazon: Facts and figures
* The Amazon rainforest covers an area of some 600 million hectares (2.3 million sq miles), an area of land 25 times bigger than Britain. It is the biggest rainforest on Earth, responsible for about 40 per cent of the world's rainforest absorption of carbon dioxide.
* Satellite surveys indicate that about 5,800 sq miles of the Amazon rainforest is burnt or cleared each year to make way for cattle ranching, farming or other kinds of development.
* More than half of the world's estimated 10 million species of plants, animals and insects live in tropical rainforests. One-fifth of the world's fresh water moves through the Amazon basin.
* Scientists estimate that there are at least 100 billion tons of carbon stored in the trees of the Amazon rainforest and each year the Amazon absorbs about 2 billion tons of carbon dioxide from the atmosphere.
* During the extreme drought of 2005, the Amazon became a net producer of carbon dioxide, releasing an estimated 3 billion tons of the greenhouse gas into the atmosphere – a net increase of 5 billion tons.