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Trees Won?t Save Us From Greenhouse Gas

U.S. and Brazilian researchers say waterways in the Amazon are releasing far more carbon dioxide into the atmosphere than previously thought. That finding suggests tropical rainforests are not carbon ?sinks? that protect the Earth from excess CO2.

Using satellite radar imagery and streamflow measurements, researchers determined that the amount of carbon dioxide naturally coming off rivers, streams and flooded areas in the vast Amazon basin is three times what they expected to find.

Researcher Jeffrey Richey at the University of Seattle found that normal water processes account for 20 % of the CO2, and the rest comes from decaying forest debris that has been swept into the water, causing it to give off ?river breath.? The carbon-dioxide release into the atmosphere takes several years to complete as trees and plants decay.

Richey estimates that tropical forest waterways worldwide are emitting 2 trillion pounds of carbon dioxide annually. That?s equal to about one-fifth the amount of carbon dioxide generated every year by deforestation, burning fossil fuels and other human activities.

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Forests are less effective at slowing climate change than scientists thought because they mop up less carbon dioxide than expected. William Schlesinger of Duke University discovered this when he conducted a four-year experiment to see how much CO2 trees will absorb from the atmosphere when pollution has raised levels of the gas.

The results mean that the world cannot rely on trees to solve the problem of CO2 emissions. ?It throws doubt on nations such as the U.S. who have carbon sequestration as their only strategy for dealing with the problem,? Schlesinger says.

Global CO2 emissions from sources such as car exhausts and industry are predicted to double between now and 2050. More CO2 means that trees grow faster and lock up more carbon. Some researchers hoped that plants might mop up all the extra gas.

But earlier experiments to find out how much CO2 plants can absorb have taken place in sealed environments such as greenhouses. These conditions don?t reproduce the outdoor climate conditions of temperature, humidity and rain.

Schlesinger and his colleagues monitored the growth of mature trees in Duke Forest, North Carolina. They staked out six plots of trees with rings of 32 vertical pipes. At 3 sites, the pipes pumped out air enriched with CO2 to mimic conditions predicted for 2050. At the other three sites, they pumped out normal turn-of-the-century air. The system monitored CO2 levels within the ring and makes adjustments to maintain the right mix.

The team found that the trees in the 2050 atmosphere did convert more carbon dioxide into plant matter, locking up 27 per cent more carbon than at control sites. However, even if this extra growth occurs in existing temperate forests all over the world in 2050, the trees will only absorb 10 per cent of human-generated CO2. ?They will soak up carbon, but the study contradicts those who say they will soak up large amounts,? says Schlesinger.

Peter Cox at Britain?s Meteorological Office in Bracknell, Berkshire says, ?The effect is not as large as people had expected. Eventually you have to deal with the root cause.?

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