Sulfates that form in the atmosphere after volcanic eruptions and rain down on wetlands may decrease those areas' emissions of methane for several years, thereby cooling the global climate. The effect, now documented in a field study, could extend volcanic sulfates' well-known short-term cooling effect.
Large volcanic eruptions send much sulfur dioxide high into the atmosphere, where the gas forms aerosols of tiny droplets. These reflect sunlight back into space, thereby cooling temperatures at ground level for up to 2 years, says Vincent Gauci of the Open University in Milton Keynes, England. Eventually, the sulfur dioxide combines with oxygen and water vapor in the atmosphere to form sulfate compounds, which return to ground level in acid rain.
Now, Gauci and his colleagues suggest that volcanic sulfates can also play a climate-changing role at ground level by suppressing wetlands' emission of methane, a planet-warming greenhouse gas.
Between July 1997 and December 1998, the researchers at least once a month spread powdered sodium sulfate on 2-meter-by-2-m plots of a peat bog in northeastern Scotland. The largest total amount of sulfate added to any plot during the 18-month interval represented the amount that scientists estimate was deposited on the wetland by the 1783 eruption of Laki, a volcano in Iceland. That eruption lasted for about a year and pumped out about 10 times the sulfate that industrial sources in Western Europe now produce collectively in a single year, says Gauci.
During the experiment, the researchers measured atmospheric concentrations of methane at ground level over the test plots. Most of the gas probably came from soil-dwelling bacteria, says Gauci. Plots dosed with the most sulfate produced only 58 percent as much methane as did plots that didn't get additional sulfate.
Two years after the researchers stopped adding sulfate, methane production in some plots was still 40 percent below normal.
The added chemicals boosted the growth of certain soil bacteria that use sulfates to produce energy, Gauci suggests. That population explosion, in turn, suppressed methane-producing bacteria that thrive under typical bog conditions.
Analysis of methane data collected after the sulfate dosing stopped suggests that methane generation in the treated plots would take 5 to 7 years to fully recover, the researchers say in an upcoming Geophysical Research Letters.
The field tests hint at the complex biological interactions that can affect atmospheric chemistry, says Clive Oppenheimer of the University of Cambridge in England. However, very little is known about where sulfur dioxide from the Laki eruption ended up, he notes.
The temporary suppression of methane-producing bacteria by sulfate-nourished microbes is "an interesting idea," says David Stevenson, an atmospheric chemist at the University of Edinburgh. Global evidence for that scenario might be found in layers of polar and glacial ice laid down after volcanic eruptions, says Stevenson.
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