A widespread drought in the Amazon rainforest last year caused the "lungs of the world" to produce more carbon dioxide than they absorbed, potentially leading to a dangerous acceleration of global warming.

Scientists have calculated that the 2010 drought was more intense than the "one-in-100-year" drought of 2005.

They are predicting it will result in around eight billion tonnes of CO2 being expelled from the Amazon rainforest, which is more than the total annual carbon emissions of the United States.

For the second time in less than a decade, the earth's greatest rainforest released more CO2 than it absorbed because many of its trees dried out and died.

Scientists believe that the highly unusual nature of the two droughts, which occurred in the space of just five years, may be the result of higher sea-surface temperatures in the tropical Atlantic, which could also be influenced by global warming caused by the release of man-made emissions of CO2 and other greenhouse gases.

The Anglo-Brazilian team of researchers has emphasised that there is as yet no proof that the two highly unusual droughts in the Amazon are the direct result of rising CO2 levels in the atmosphere, but the scientists have warned that the world is gambling with its future if it fails to curb fossil fuel emissions.

Simon Lewis of Leeds University, the lead author of the study, said: "If greenhouse gas emissions contribute to Amazon droughts that in turn cause forests to release carbon, this feedback loop would be extremely concerning. Put more starkly, current emissions pathways risk playing Russian roulette with the world's largest rainforest.

"Two unusual and extreme droughts occurring within a decade may largely offset the carbon absorbed by intact Amazon forests during that time. If events like this 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 that could speed it up.

"Having two events of this magnitude in such close succession is extremely unusual, but is unfortunately consistent with those climate models that project a grim future for Amazonia."

The study, published in the journal Science, analysed satellite data on rainfall across two million square miles (5.2 million sq km) of rainforest during the 2010 dry season. The scientists were able to make a direct comparison with an earlier study of the 2005 drought, which also looked at the effect of the low rainfall on the growth of trees.

In the 2005 drought, the scientists estimated that the rainforest turned from a net absorber of about two billion tonnes of CO2 to an exporter of around 5 billion tonnes of CO2, which is almost as much as the 5.4 billion tonnes emitted annually by the US.

However, the drought last year was more widespread and more intense than the earlier drought, with a far bigger impact on the growth and death of trees, which is why the scientists expect the overall release of CO2 from dead and decaying organic matter to reach 8 billion tonnes.

"The extent of the 2010 drought was much larger than in 2005.

"In 2010, the Rio Negro River, which is the biggest tributary to the Amazon, was at its lowest level since records began at the start of the 20th century, so we have independent evidence of these droughts," Dr Lewis said.

- INDEPENDENT