BioEd Online

Volcanoes can cool the planet by keeping methane-producing bacteria at bay, researchers say.

Scientists already knew that a major volcanic eruption can cool the Earth for a couple of years, because the particles and chemicals thrown into the air make clouds that reflect sunlight back out into space.

But Vincent Gauci, a geophysicist from the Open University, Milton Keynes, UK, says that the sulphur dioxide in volcanic plumes also affects the climate indirectly. Falling in acid rain, these fumes can feed sulphur-loving bacteria in wetlands, allowing them to out-compete methane-generating bacteria, and so reducing the amount of methane emitted into the atmosphere. About half the world's methane - a greenhouse gas stronger than carbon dioxide - comes from bacteria in peat bogs and rice paddy fields.

Gauci and his colleagues simulated the sulphurous effects of a volcanic eruption by adding the mineral sodium sulphate to a peat bog in northeast Scotland. They fertilized the area in 1998, and found that methane emissions were 40% lower two years later. The bog still contained elevated levels of sulphur; the researchers calculate it would take five to ten years from time of fertilisation for methane emissions to return to normal. Their results will be published in Geophysical Research Letters¹.

Laki dip

The experiment was intended to recreate the acid-rain fallout from the enormous 1783 eruption of the volcano Laki in Iceland. Laki emitted more than 120 million tonnes of sulphur dioxide that year, ten times more than western European industry produces each year.

"It's an interesting idea," says Clive Oppenheimer, a volcanologist from the University of Cambridge, UK, who has studied the Laki eruption. "Iceland has events like this every few hundred years, and there are volcanoes that continuously pump out sulphur for decades."

But Oppenheimer points out that simulating Laki's impact on a small area of Scotland requires a fair amount of guesswork. "We have really good estimates of how much sulphur came out of Laki," he says, "but we don't know exactly where it rained out as acid rain."

Bigger than industry

Gauci thinks that volcanic sulphur might have a bigger climatic impact than industrial emissions, simply because most wetlands are closer to volcanoes than to factories or power plants.

Further research should pin down how much global cooling volcanoes can trigger in this way, says Gauci. But he suggests that in pre-industrial times, when volcanoes were virtually the sole source of acid rain, these sudden fertilization events could have driven very rapid climate shifts.

"Fifty million years ago, the warm greenhouse climate of the day was due, in large part, to methane from the extensive wetlands that covered the Earth," he says. "During that time, large volcanic eruptions could have been real agents of rapid climate change through this mechanism."

Some scientists have suggested that fertilizing paddy fields with sulphur could slow down climate change. Gauci is trying to assess whether such a move is necessary: he suspects that industry and volcanoes may already produce enough acid rain to maximize the bug-boosting effect.