According to an article by Vincent Gauci, Nancy Dise, and Stephen Blake, recently published in Geophysical Research Letters, volcanoes may have a stronger cooling effect on the Earth than previously thought. Large eruptions can lead to competition between different types of bacteria in peatlands and other wetlands. Dust and gas from large eruptions are known to block out sunlight, cooling Earth for two or three years. New data show that 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. Sulphur dioxide in volcanic plumes turns to sulphuric acid in contact with water and falls to Earth as acid rain. The impact of acid-rain fallout on methane-producing bacteria can outlive the short-term cooling effect of sulphuric acid in the atmosphere. The potential impact could be extended up to decadal scales.
To simulate the sulphurous fallout from the Laki Craters volcanic eruption in Iceland during the summer of 1783, 20, 2x2m plots of peatland in Scotland’s Moidach More were treated with sodium sulphate on a weekly or monthly basis between July 1997 and December 1998. Results showed that methane production from the plots had fallen by between 30 and 40% by 1998, the last year of treatment. In 2000, methane production was still suppressed. The abundant emissions after a volcanic eruption may 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.
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.
Gauci, V., Dise, N., Blake, S. (2005) Long-term suppression of wetland methane flux following a pulse of simulated acid rain. Geophys. Res. Lett., Vol. 32, No. 12