Science paper author: Drought could transform Amazon into carbon emitter

March 5, 2009

GAINESVILLE, Fla. — The Amazon has been called the “planet’s lungs” because it takes up the carbon dioxide spewed out by natural decay, cars and power plants, replacing it with the oxygen living creatures rely on.

But severe droughts of the kind predicted to accompany climate change may slow or even reverse this natural process. At best, that could reduce the Amazon’s contribution to ridding the atmosphere of carbon dioxide. At worst, it could transform the rainforest from a carbon sink into a potential carbon source.

So conclude dozens of scientists including a University of Florida doctoral student in a paper set to appear Friday in the journal Science.

“It’s pretty worrying, because most of the climate change models predict a reduction of 20 to 30 percent of rainfall in the eastern Amazon by end of the 21st century, as well as increases in the length of dry seasons,” said Paulo Brando, one of at least 40 authors of the paper and a doctoral student in botany at UF’s School of Natural Resources and Environment.

The scientists studied the Amazon’s response to a severe drought in 2005, the worst to hit the region in at least 100 years. They measured the amount of growth in trees, and how many trees died, in more than 130 test plots spread throughout the huge rainforest over several years. They extrapolated the results to determine how much atmospheric carbon the trees gained or lost in 2005 compared with the other years of more normal rainfall.

“We saw that the drought provided a kind of natural experiment that we could use to evaluate a potential future of the forest,” Brando said.

The scientists’ main conclusion: Most plots that were hit by the drought switched from storing carbon to releasing it, partly because of slowed growth during the drought, but mostly because of the drought-induced death of large trees that would otherwise have taken high amounts of carbon out of the air.

“If you just reduce growth, you can recover it next year if you grow twice as much,” said Brando. “But if you lose big trees, it will take much longer to recover.”

If enough of the Amazon underwent this transformation, it could cease assimilating carbon altogether and instead become a carbon emitter, he said. Deforestation and planting of crops could further impinge the forest’s traditional role as a carbon sink.

One of the control plots the scientists cited – a plot that was not affected by the drought — was maintained by Brando, who is studying the dynamics of forest response to climate change. Brando also analyzed data and contributed other material to the Science paper.

He can be reached for interviews by calling 352-682-4542 or e-mailing paulobrando@gmail.com.