Oct. 21, 2008 -- Hurricanes and typhoons, normally seen as looming threats from global warming, are actually helping to remove carbon dioxide from the atmosphere.
Each year humans emit approximately 7.2 billion tons of the greenhouse gas, trapping vast amounts of heat in the air and oceans. Tropical cyclones derive their energy from warm seas, and some scientists believe global warming will spawn more frequent and more intense storms unless drastic effort is undertaken to cut emissions.
But Robert Hilton of the University of Cambridge in the United Kingdom and a team of researchers found that when two powerful storms lashed Taiwan in 2004, rains eroded thousands of tons of carbon-rich plant matter and soil. The material was sent coursing out of the island's steep mountain range down the LiWu River and into the deep sea, where it was buried in sediment.
"Over the last 30 years large storms, which only last a few days, dominated the erosion there," Hilton said. "Between 77 and 92 percent of carbon was eroded by these storms."
Globally, rivers slough vast amounts of carbon off continents and into the oceans. The Amazon River, the largest in the world by volume, dumps an estimated 13 million tons of carbon into the sea each year.
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By contrast, the study published this week in the journal Nature Geoscience, Hilton and colleagues calculated that Typhoon Mindulle, the stronger of the 2004 storms, washed just 5,500 tons of carbon down the LiWu.
But huge rivers like the Amazon don't bury their carbon efficiently; most of it empties onto shallow continental shelf waters where it can be recycled and eventually emitted back out into the atmosphere.
When a steep river like the LiWu comes roaring out of the mountains at flood stage, its waters are dense with sediment and they quickly descend to the sea floor, where up to 90 percent of the carbon can be buried and removed from Earth's carbon cycle.
So-called "steepland" rivers are prevalent in the tropics throughout the western Pacific and Atlantic Oceans, making the region ripe for erosion by tropical cyclones. In the Pacific alone, some 50-90 million tons of carbon are sequestered in this way annually.
Compared to human activity, this isn't going to make much of a dent in global warming, though.
"Dotting an 'i' would be a good way of putting it, I think, in terms of the global carbon cycle," Basil Gomez of Indiana State University said. "But it's important because not a lot is known about these rivers. Many of them are in places that are increasingly impacted by human activity and will be even more impacted in the future."
"This is a cool study that suggests erosion may not be as big a worry for carbon in some areas as we once thought it was," he said.
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