They are predicting it will result in some eight billion tonnes of carbon dioxide 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 carbon dioxide 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 carbon dioxide 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 carbon dioxide 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.
"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 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 carbon dioxide to an exporter of some five billion tonnes of carbon dioxide, 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 carbon dioxide from dead and decaying organic matter to reach eight 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.
Normally, the cycle of droughts that hit the Amazon affect northern areas of the region and are associated with the natural el Niño phenomenon in the Pacific Ocean. However, the 2005 and 2010 droughts occurred further south and may be linked with higher sea-surface temperatures in the tropical Atlantic Ocean, the scientists said.
"In 2005, the spatial pattern of the drought affecting the south and southwest of the Amazon was very different from the usual droughts that impact the Amazon every five to seven years associated with el Niño events, which tend to affect the north-east. When climatologists investigated why, they associated it with Atlantic sea-surface temperatures," Dr Lewis said.
"In 2010 we see a drought with a very similar spatial pattern, again affecting the south and the south-west of the Amazon basin, and very similar to 2005, and we know that the Atlantic sea-surface temperatures were anomalously high, but the work has not been done yet to say definitively that that is the cause. Our best hypothesis at the present time is that this 2010 drought was associated with Atlantic sea-surface temperatures, but we have to wait until those scientific papers go through the peer-review process before we can say that more concretely."
Peter Cox, of Exeter University, who analysed the 2005 drought, said: "The droughts in Amazonia in 2005 and 2010 were both associated with unusually warm ocean temperatures in the tropical North Atlantic. This tends to draw the region of most intense rainfall further north and delays the wet season in Amazonia."