The drought last year in the Amazon basin was even more widespread and intense than the supposedly once-in-a-century dry spell in 2005.
The second extreme drought to have hit the region in five years raises concerns that world’s largest rain forest – a major sink for atmospheric carbon dioxide – could be approaching a tipping point.
“If drought events continue, the era of intact Amazon forests buffering the increase in atmospheric carbon dioxide may have passed,” a team led by Simon Lewis of Leeds University in the UK writes in tomorrow’s issue of Science.
The team compared rainfall anomalies and water stress to trees across 5.3 million square kilometres of the Amazon basin during 2005 and 2010. Last year’s drought affected a larger area than the oen in 2005, and was most pronounced in southwestern Amazonia, north-central Bolivia and in the state of Mato Grosso in Brasil, they report.
Drought conditions during the dry season last year have led to numerous forest fires and extremely low water levels in many rivers, including the Rio Negro, which in October reached a record low at Manaus, the capital of Brazil’s Amazonas state.
Amazon droughts are often associated with unusually high sea surface temperatures in the tropical Pacific and/or Atlantic Ocean. Climate models suggest that drought conditions will become more frequent in the region (and around the tropics), but whether the recent events can be attributed to global warming is unclear.
The world’s rain forests absorb as much as 1.3 billion tonnes of carbon per year, almost 20% of carbon emissions from fossil-fuel burning. But scientists believe that the 2005 and 2010 Amazon droughts have turned large areas of the Amazon from a carbon sink to a carbon source, possibly offsetting the net gains of up to ten normal years.
How many trees died last year is stillunclear. “We will not know exactly until we can complete forest measurements on the ground,” says Paulo Brando from Brazil’s Amazon Environmental Research Institute in Belém.
“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,” he says.
The magnitude of the impact of drought on rainforests and terrestrial carbon cycles is under intense debate, as is the response of trees to rising atmospheric carbon dioxide. Some scientists think that increased CO2 concentrations will boost plant growth and carbon uptake, possibly offsetting projected reductions in rainfall.
Image: Fires burning in Mato Grosso, Brazil, on August 22, 2010. NASA Earth Observatory.