By Alister Doyle, Environment Correspondent
OSLO, Oct 19 (Reuters) - Tropical cyclones may be a tiny help in slowing global warming by washing large amounts of vegetation and soil containing greenhouse gases into the sea, scientists said on Sunday.
A study in Taiwan of the LiWu river showed that floods caused by typhoon Mindulle in 2004 swept into the Pacific Ocean an estimated 0.05 percent of carbon stored in leaves, branches, roots and soil on the hillsides being studied. The carbon sank to the seabed.
"Tropical cyclones could have a significant role in the transfer of atmospheric carbon dioxide to long-term deposits in the deep ocean," according to the findings in the journal Nature Geoscience.
Plants soak up carbon dioxide, a natural greenhouse gas also emitted by burning fossil fuels, and store it as carbon as they grow. The carbon usually gets released back to the air when vegetation rots or is burnt.
"50 to 90 million tonnes of carbon a year is thought to enter the oceans from islands of the west Pacific alone," mainly during cyclones, according to the scientists, based in Britain and Taiwan.
But the scientists said the mechanism would not do much to slow warming caused by mankind, led by burning of fossil fuels.
"The current amount of carbon dioxide building up from manmade sources is about 100-1,000 times faster than this carbon (burial) from the interaction between the cyclones, erosion and forests," said Robert Hilton of Cambridge University who was one of the authors.
"In terms of the manmade carbon cycle this is not going to save us. But it illustrates that the earth has natural ways of dealing with carbon dioxide," he said.
And the scientists said more than half of the carbon might be from fossils in rocks washed down rivers by floods, rather than recent vegetation.
Hilton said the findings from Taiwan were likely to be similar to the impact of Atlantic hurricanes on Caribbean islands.
The experts included Meng-Chiang Chen of the Taroko National Park Headquarters, who had the risky job of going out during cyclones, tied to a harness, to gather water from the LiWu river in containers dangling from a pole.
The U.N. Climate Panel predicted last year that tropical cyclones were likely to get more powerful because of global warming that would also cause more heatwaves, droughts, floods and raise world sea levels.
The carbon burial mechanism might fractionally offset the trend to more powerful storms, Hilton said. But more powerful cyclones would have other damaging effects such as washing away more topsoil, threatening farms.
-- For Reuters latest environment blogs click on: blogs.reuters.com/environment/ (Editing by Charles Dick)