Drought is killing off trees in Brazil's fragile Amazon rainforest and depleting the region's carbon reservoirs - an ecological double-whammy with devastating implications, according to a new study.

The Amazon's lush vegetation in a typical year absorbs nearly two billion tonnes of carbon dioxide.

But a 30-year study published by the journal Science found that the world's largest tropical rain forest is surprisingly sensitive to drought, and that the resulting loss of vegetation will have a greater-than-anticipated effect in causing a sharp spike in greenhouse gases.

The Amazon tree canopy, which absorbs massive amounts of greenhouse gases, often succumbs to the effects of dryness, thereby accelerating global warming by not absorbing CO2, say scientists.

Drought also accelerates the depletion of the region's carbon sinks, natural reservoirs that accumulate and store the chemical compound for an indefinite period.
Massive release

The research from more than 40 institutions around the world was gathered during the particularly harsh 2005 drought, which had a severe impact on the flora of the Amazon.

Researchers say the total impact of the drought was an additional five billion tonnes of carbon dioxide in the atmosphere - more than the combined annual emissions of Europe and Japan.

The drought that year dramatically reversed decades of carbon absorption, the researchers say.

"For years, the Amazon forest has been helping to slow down climate change. But relying on this subsidy from nature is extremely dangerous," says Professor Oliver Phillips of the University of Leeds, the lead author of the study.

"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."

Visually, most of the Amazon showed little effects of the drought. "But our records prove tree death rates accelerated," says Phillips.

"Because the region is so vast, even small ecological effects can scale-up to a large impact on the planet's carbon cycle."
Vicious cycle

Scientists say the Amazon accounts for more than half of the world's rainforest, covering an area almost three-quarters the size of Australia.

The study, which involved 68 scientists from 13 countries, found that various species of tropical palm trees are particularly vulnerable to drought, which highlights the risk to biodiversity caused by climate change.

The findings are especially sobering because climatologists predict the creation of a potentially devastating cycle, in which the Amazon's hotter and more intense dry seasons lead to more greenhouse gas emissions and even more drought.

by:ABC Science