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Cyclones lock away carbon

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Taiwan's LiWu River

Carbon flushing: Taiwan's LiWu River charged with sediment after one day of rain.

Credit: Robert Hilton

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NEW YORK: Tropical cyclones transfer carbon dioxide from land to the deep ocean, where it may get locked-away long term. The effect could act as a feedback mechanism to temper global warming as it increases the frequency and intensity of cyclones and hurricanes.

Mountain streams transfer organic carbon from vegetation and soil to the seas. Tropical cyclones cause these rivers to flood, which may cause up to 90 per cent of the carbon, which is trapped in mud and sediment, to be flushed out and buried in the seafloor, reports a new study in the British journal Nature Geoscience.

Climate-driven burial

"It is of central importance to understand the natural controls on the carbon cycle," said Robert Hilton, co-author of the study at the University of Cambridge in England. "This is crucial to unravelling Earth's climate history throughout the geological past and perhaps how it may vary in the future."

Hilton's research team used the LiWu River, a mountain river in Taiwan, hit annually by cyclones, as a "natural laboratory" to study how cyclones sequester carbon. They analysed the carbon content of sediment samples from the river during cyclone-induced flooding. The results disproved earlier theories that carbon content would decrease during cyclones.

But, according to Hilton, the carbon burial is at least 100 times slower than the rate of CO2 emissions from human activities. It is part of the planet's natural global CO2 cycle and "is therefore at present not capable of reversing any man-made increase of carbon in the atmosphere," he said.

The next step, he said, is to confirm just how much carbon from vegetation and soil is buried in the ocean as a result of the transfer.

Daniel Schrag, a climate scientist at Harvard University in Cambridge, US, questioned the importance of the effect, however.

"There is no doubt that cyclones added to erosion and carbon sequestration in the area studied. But, for global carbon burial, the findings are trivial," he argued.

Schrag said that most organic carbon ocean sequestration occurs in the 'tropical mud belt', due to rivers near the equator like the Amazon or the Congo. Cyclones hit further off the equator, he said, outside the range where most organic carbon burial occurs.