Rain forest fails to absorb excess carbon during El Nino

By Joseph B. Verrengia, AP Science writer Copyright 1998 Associated Press December 16, 1998

Instead of inhaling extra carbon dioxide, Brazil's rain forest does the opposite in an El Nino year, exhaling millions of tons of the heat-trapping gas and potentially adding to global warming, scientists say.

The rain forest, under normal conditions, acts as the "lungs" of the planet. Its dense canopy of trees stretches for thousands of miles, releasing oxygen and absorbing as much as 700 million tons of carbon dioxide a year.

But when global climate conditions are scrambled by El Nino and the rain forest becomes parched, scientists from the Woods Hole Research Laboratory in Massachusetts determined the Amazon Basin produces as much as 200 million tons of excess carbon dioxide a year.

The calculations by Hanqin Tian and others are in Thursday's journal Nature.

The study examined three El Nino episodes from 1980 to 1994. It did not include measurements of the record-setting El Nino in 1997-98, but the trend is clear, researchers said.

"In El Nino years, which bring hot and dry weather to much of the Amazon, the ecosystems act as a source of carbon," Tian said.

Scientists say understanding the planet's fluctuating carbon cycle is a key step in accurately predicting - and managing - global warming in the 21st century.

Some studies show global temperatures have been creeping up by as much as one degree as industry, traffic, deforestation and other human activities generate more carbon dioxide and other greenhouse gases - so called because they trap heat from the sun.

Many natural features on the planet act as "carbon sinks" to absorb excess carbon and help to stabilize the planet's climate. The oceans and the rain forests absorb the largest shares, along with tundra and rangeland.

But not always.

During an El Nino, a vast pool of warm water expands in the equatorial Pacific. It upsets weather patterns around the world. In the Amazon, it triggers severe droughts.

Under such severe stress, the forest can't adequately photosynthesize and store carbon dioxide, Tian said.

His results correspond with the results of carbon cycle simulation developed at the Max Planck Institute in Germany, which developed a computer model showing the Amazon acting as a carbon source during three El Nino episodes between 1982-94.

Researchers at Max Planck said the Woods Hole study is based on limited field measurements in a small sampling area, and failed to answer the fundamental question of what is canceling the rain forest's role as a carbon sink.

"Such understanding is crucial for long-term predictions," said Colin Prentice, who with Planck colleague Jon Lloyd reviewed the new study for Nature.

Several groups of scientists are trying to understand the world's carbon cycle.

Last week, researchers launched a three-year study to find the "missing sink" to account for lower-than-expected carbon dioxide levels in the Northern Hemisphere. A likely suspect: trees that have regrown in previously logged forests and farmland.

"Something in the Northern Hemisphere is sucking up the carbon," said climatologist Kevin Gurney of Colorado State University. "The missing sink isn't new, but where it is located and how it works are still unresolved."

Because of the hemisphere's large cities and industrialization, carbon dioxide levels are expected to be higher. But measured carbon emissions have been 2 billion tons lower than expected in recent years.

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