A study involving nearly 70 scientists has found that the Amazon rainforest is surprisingly sensitive to drought and even a moderate drought can cause it to release massive amounts of carbon dioxide into the atmosphere.

The findings, published Thursday in the journal Science, is said to be evidence that drought causes massive carbon loss in tropical forests.

Oliver Phillips, lead author of the research and a tropical ecology professor at the University of Leeds, said the findings are of major concern because they show that the carbon sink of Amazon rainforest cannot be taken for granted.

"For years, the Amazon forest has been helping to slow down climate change. But relying on this subsidy from nature is extremely dangerous," he said.

"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 stabilize our climate."

Phillips said the study, which involved data from more than 25 years, focused on a drought in the summer of 2005 caused by exceptional warming of the tropical portion of the North Atlantic Ocean. He said the drought gave the scientists a sense of what could happen in the future if the ocean continues to warm as predicted, causing more intense dry seasons.

The drought reversed decades of carbon absorption.

"What we found was that instead of being a carbon sink in 2005, it was a source. It released carbon dioxide into the atmosphere. Many trees died," he said.

"We cannot take this carbon sink for granted. Now we have direct evidence that drought can reverse that process, possibly quite rapidly. It makes it more likely that forests, at some point, will release carbon to the atmosphere when they dry.

Instead of absorbing, releasing

"It was a window into our possible future."

Every year, the Amazon rainforest absorbs about nearly two billion tonnes of carbon dioxide. Phillips said this particular drought caused a release of more than three billion tonnes.

The total impact of the drought, an extra five billion tonnes of carbon dioxide released into the atmosphere, is greater than the annual emissions of Europe and Japan combined.

Phillips said 68 scientists from 13 countries have monitored the Amazon rainforest for more than 25 years. The scientists are part of RAINFOR, a research network that is dedicated to monitoring the Amazonian forests.

Phillips said records kept by the scientists found that the death rates of trees increased during the drought even though most of the forest did not appear to be affected very much. He said the region is so vast that small ecological effects can add up to have a large impact on the carbon released into the atmosphere.

The scientists have found that, over the years, the Amazon rainforest usually acts as a vast carbon sink. A similar process is said to be occurring in Africa. The scientists have estimated that tropical forests have absorbed one fifth of fossil fuel emissions from around the world.

To calculate changes in carbon storage over the years, they examined more than 100 forest plots across the Amazon's 600 million hectares, identified and measured more than 100,000 trees, and recorded tree deaths as well as new trees. Weather patterns were also measured and mapped.

He said the drought gave them an opportunity to study how such drying would affect the forest. They found it reversed the process of carbon sink, turning into a carbon release, and where the drought was strongest, the most trees died.

The Amazon accounts for more than half of the world's rainforest. Phillips said no other ecosystem on Earth is home to so many species nor exerts such control on the carbon cycle.