Volcanoes may have a stronger cooling effect on the Earth than previously thought, an Open University team says.
Volcanic eruptions release sulphur dioxide gas
In Geophysical Research Letters, the UK scientists say large eruptions can kick off a contest between different types of bacteria in peat bogs and wetlands.
Dust and gas from large eruptions are known to block out sunlight, cooling Earth for two or three years.
New data show bacteria producing the greenhouse gas methane are suppressed by other bugs, further cooling Earth.
The time needed for the methane-producing bacteria to recover to pre-eruption levels is between five and 10 years, say the researchers.
The latest study shows that the impact of acid-rain fallout on methane-producing bacteria can outlive the short-term cooling effect of sulphuric acid in the atmosphere.
Sulphur dioxide in volcanic plumes turns to sulphuric acid in contact with water and falls to Earth as acid rain.
"Our findings show that volcanic eruptions have another, more indirect, effect," said co-author Dr Vincent Gauci of the earth sciences department at the Open University, Milton Keynes.
"Volcanoes have been known to influence climate in the past, but this showed that the potential impact could be extended up to decadal scales and it's all done by little bugs," he told the BBC News website.
Dr Gauci and his team measured methane produced by wetlands
Over much of geological time, natural wetlands have been the major contributor of global methane. Today, natural and man-made wetlands (rice paddies) contribute about 50% of the total methane source.
Dr Gauci and Open University colleagues Nancy Dise and Steve Blake intended to simulate the sulphurous fallout from the Laki Craters volcanic eruption in Iceland during the summer of 1783.
They dosed 20, 2x2m plots of peatland in Scotland's Moidach More with sodium sulphate on a weekly or monthly basis between July 1997 and December 1998.
Some of the plots served as controls, while others were subjected to varying levels of sulphate deposition.
The researchers measured the methane using static gas exchange chambers on each plot. The results show that methane production from the plots had fallen by between 30 and 40% by 1998, the last year of treatment.
"When we went back in 2000, we were surprised to find methane production was still suppressed," Dr Gauci explained.
The researchers think that the abundant emissions after a volcanic eruption allow sulphate-reducing bacteria in the wetlands to out-compete the methane-producing microbes (methanogens).
The methanogens become excluded from exploiting a significant proportion of their energy source, resulting in lower methane production.