For years, researchers have expressed concern that rising temperatures, drought, and deforestation are reducing the capacity of the world's largest rainforest to absorb carbon dioxide from the atmosphere, and help offset emissions from fossil-fuel burning. Recent studies have even suggested that some portions of the tropical landscape already may release more carbon than they store.
But the inhaling and exhaling of CO2 is just one way this damp jungle, the most species-rich on Earth, influences the global climate. Activities in the Amazon, both natural and human-caused, can shift the rainforest's contribution in significant ways, warming the air directly or releasing other greenhouse gases that do.
Drying wetlands and soil compaction from logging, for example, can increase emissions of the greenhouse gas nitrous oxide. Land-clearing fires release black carbon, small particles of soot that absorb sunlight and increase warmth. Deforestation can alter rainfall patterns, further drying and heating the forest. Regular flooding and dam-building releases the potent gas methane, as does cattle ranching, one chief reason forests are destroyed. And roughly 3.5 percent of all methane released globally comes naturally from the Amazon's trees.
Yet no team had ever tried to assess the cumulative impact of these processes, even as the region is being rapidly transformed. The research, supported by the National Geographic Society and published today in Frontiers in Forests and Global Change, estimates that atmospheric warming from all of these sources combined now appears to swamp the forest's natural cooling effect.
"Cutting the forest is interfering with its carbon uptake; that's a problem," says lead author Kristofer Covey, a professor of environmental studies at New York's Skidmore College. "But when you start to look at these other factors alongside CO2, it gets really hard to see how the net effect isn't that the Amazon as a whole is really warming global climate."
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