Researchers found that natural chemicals in the atmosphere west of equatorial Africa destroyed 50 percent more ozone in that region than expected.

This process also reduced concentrations of methane, another powerful greenhouse gas.

It may well apply in oceans around the world and if so, it would pose major questions about how Earth's inventory of global warming gases is calculated, they said.

Ozone, a naturally-occurring molecule composed of three oxygen atoms, can be beneficial or harmful to human life, depending on where it is.

In the stratosphere, 10 to 50 kilometres (17 to 31 miles) above Earth's surface, ozone protects living things by filtering out the Sun's damaging ultraviolet light.

But in the troposphere, or lower atmosphere, ozone is a noxious pollutant and the most important greenhouse gas after carbon dioxide (CO2), methane and water vapour, which thus makes it a driver of global warming.

Scientists led by John Plane at the University of Leeds, northern England, analysed a year of ozone and methane measurements taken at the Cape Verde Atmospheric Observatory on Sao Vicente, an island some 500 kilometres (380 miles) west of Senegal.

Ozone, a short-lived molecule with a lifetime ranging from a few days to a week, was disappearing at a rate far above what theoretical calculations would have forecast.

Using new instruments developed for their study, the researchers gave the credit to two atmospheric chemicals produced by sea spray and emissions from microscopic sea organisms called phytoplankton: bromine and iodine oxide.

These chemicals attack ozone and break it down, said the study, published in the British journal Nature.

They also boost the atmospheric levels of another compound called the hydroxyl radical, which in turn destroys methane.

This unexpected discovery implies that the mathematical model for calculating the various sources of global warming could be flawed, although global warming itself is not being contested, one of its authors said.

"Global models get levels of ozone in the troposphere about right. So if destruction rates are much higher than thought that means it must be coming from somewhere else," University of York scientist Lucy Carpenter told AFP.

Whether the findings are good news or bad is a matter of interpretation, Carpenter said.

"These gases are being destroyed more rapidly than we supposed," she said.

"That is good because if humans produce less methane, then it might settle at a lower level."

At the same time, however, that would expose a worrying gap in knowledge -- a hole in the balance sheet of greenhouse gases that needed to be explained.

Further work is needed to see whether the phenomenon observed in the tropical Atlantic applies elsewhere, and also to find whether bromine and iodine oxide levels are on the increase.

If they are -- and it would take five years to find out -- the world's oceans could play a bigger role in reducing at least two of the greenhouse gases driving climate change, she said.

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