04 Feb 2011
Two severe droughts have hit the Amazon rainforest in the space of five years, raising concerns over its ability to counter greenhouse gas emissions.
In 2005, the Amazon was struck by a ''one-in-100 year'' drought that killed huge numbers of trees.
The event led to the release of an estimated five billion tonnes of carbon dioxide from rotting vegetation and the forest's reduced ability to soak up the greenhouse gas.
But a new analysis of rainfall across 5.3 million square kilometres of Amazonia has now shown that another drought last year may have been even more severe.
The new drought caused the Rio Negro tributary of the Amazon river to fall to its lowest level on record.
Experts fear that if such extreme droughts become more frequent, the Amazon may cease to provide a natural buffer to man-made carbon emissions.
Dr Simon Lewis, from the University of Leeds, who led the research reported today in the journal Science, said: ''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 vast rainforest covers an area around 25 times the size of the UK and in a normal year absorbs an estimated 1.5 billion tonnes of carbon dioxide.
This counter-balances emissions from deforestation, logging and fire across the Amazon and scientists believe it has applied a brake to climate change in recent decades.
Scientists predict the rainforest's carbon absorption rate for both 2010 and 2011 will be cut. A further five billion tonnes of carbon dioxide is expected to be released over the coming years as trees killed by the new drought rot.
Further droughts could eventually turn the Amazon from a carbon sink to a carbon emitter, say experts.
Some global climate simulations suggest that this is a likely scenario.
Dr Lewis added: "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 gasses that could speed it up."
Co-author Dr Paulo Brando, from Brazil's Amazon Environmental Research Institute (IPAM), said: "We will not know exactly how many trees were killed until we can complete forest measurements on the ground.
"It could be that many of the drought susceptible trees were killed off in 2005, which would reduce the number killed last year. On the other hand, the first drought may have weakened a large number of trees so increasing the number dying in the 2010 dry season.
"Our results should be seen as an initial estimate. The emissions estimates do not include those from forest fires, which spread over extensive areas of the Amazon during hot and dry years. These fires release large amounts of carbon to the atmosphere."