Amazon growth burst puts brake on global warming

February 12, 2004

The growth rate of trees in the Amazon Basin's pristine rainforests has nearly doubled in recent decades, which may have helped slow global warming, but the forests are also dying off faster, scientists say.

Scientists from countries including Britain, Italy, Germany, Brazil and the US have reported the Amazon findings in the British Royal Society's journal Philosophical Transactions B.

Several scientists said the death rate of the forests was slower than the growth rate, causing an increase in the mass of living vegetation.

Yadvinder Malhi, of the University of Edinburgh, said this increased material in the pristine areas - which comprise more than half of the Amazon rainforests - may have stemmed global warming because it helped clean carbon dioxide from the air and slow its build-up in the atmosphere.

But Oliver Phillip, of the University of Leeds, warned that computer simulations suggested the benefits could not be taken for granted. "The process could be reversed in as short a space of time as the next two decades by the combined effects of deforestation and global warming."

The scientists said tropical forests globally had warmed by half a degree in the past 20 years, with forecasts of a further increase of three to eight degrees by the end of the century.

Dr Malhi said it was not known how much heat the trees could stand. If they died off the brake on global warming would go, too.

Several of the scientists suggested causes of the growth changes in the forests, the most likely being increases in the atmospheric concentration of carbon dioxide and surface air temperatures, and possible continent-wide changes in sunshine.

While the pristine Amazon rainforests are increasing their mass, others appear to be breaking up under climatic and human pressure. Selective logging punctures the forest canopy and allows sunlight to penetrate to the forest floor, Dr Malhi said, drying it out and making it much more vulnerable to fire.

Ultimately, saving the world's remaining rainforests also requires a committed effort to move away from burning fossil fuels, the scientists said.

"In the 21st century, we are moving into a human-made atmospheric and climatic situation that has not been experienced on Earth for at least 20 million years," Dr Malhi said.