BILLIONS of trees died in the record drought that struck the Amazon basin in 2010, raising fears the vast forest is on the verge of a tipping point, where it will stop absorbing greenhouse gas emissions and instead increase them.

The dense forests of the Amazon soak up more than a quarter of the world’s atmospheric carbon, making it a critically important buffer against global warming. But if the Amazon switches from a carbon sink to a carbon source, that prompts further droughts and mass tree deaths, such a feedback loop could cause runaway climate change, with disastrous consequences.

“Put starkly, current emissions pathways risk playing Russian roulette with the world’s largest forest,” said tropical forest expert Dr Simon Lewis, at the University of Leeds, England, who led the research published recently in the journal Science. Lewis was careful to note that significant scientific uncertainties remain and that the 2005 drought — thought then to be of once-a-century severity — and the 2010 drought might yet be explained by natural climate variation.

“We can’t just wait and see because there is no going back,” he said. “We won’t know we have passed the point where the Amazon turns from a sink to a source until afterwards, when it will be too late.”

Dr Alex Bowen, from the London School of Economics Grantham Research Institute on Climate Change, said huge emissions of carbon from the Amazon would make it even harder to keep global greenhouse gases at a low enough level to avoid dangerous climate change. “It therefore makes it even more important for there to be strong and urgent reductions in man-made emissions.”

The revelation of mass tree deaths in the Amazon is a major blow to efforts to reduce the destruction of the world’s forests, one of the biggest sources of global carbon emissions. The recent use of satellite imagery by Brazilian law-enforcement teams has drastically cut deforestation rates and replanting in Asia had slowed the net loss.

The 2010 Amazonian drought resulted in several states-of-emergencies declared and the lowest ever level of the major tributary, the Rio Negro. Lewis, with colleagues in Brazil, examined satellite-derived rainfall measurements and found that the 2010 drought was even worse than the very severe 2005 drought, affecting an area 60 per cent wider with a harsh dry season.

The researchers have 126 one-hectare plots spread across the Amazon, in which every single tree is tagged and monitored. After 2005 they counted how many trees had died and worked out how much carbon would be pumped into the atmosphere as the wood rotted. In addition, the reduced growth of the water-stressed trees means the forest failed to absorb the 1.5bn tonnes of carbon that it would in a normal year.

Applying the same principles to the 2010 drought, they estimated that 8.5bn tonnes of CO2 will be released — more than the 7.7bn tonnes emitted in 2009 by China, the biggest polluting nation in the world. This estimate does not include forest fires, which release carbon and increase in dry years.

“The Amazon is such a big area that even a small shift [in conditions] there can have a global impact,” said Lewis.

He also expects the drought to have an impact on the animals that live in the region, one of the most biodiverse places on Earth.

Lewis said that two such severe droughts in the Amazon within five years was highly unusual, but that a natural variation in climate over decade-long periods could not be ruled out. The driving factor of the annual weather patterns is the warmth of the sea in the Atlantic. Increasing droughts in the Amazon are found in some climate models, he added.

This means the 2005 and 2010 droughts are consistent with the idea that global warming will cause more droughts in future, emit more carbon, and potentially lead to runaway climate change. “The greenhouse gases we have already emitted may mean there are several more droughts in the pipeline,” he said.

— The Guardian, London