Wetland ecosystems are the biggest source of methane greenhouse gas. © Big Picture CD-ROM

Scientists have known for some time that volcanic aerosol particles reflect the Sun's rays back out to space and also create more clouds which have the same effect. It all helps to cool the planet for a year or two. But the new findings show that volcanic eruptions have another, more indirect, effect. Dr Vincent Gauci, lead author of the research appearing in the American Geophysical Union's Geophysical Research Letters, says the resulting sulphuric acid from the volcano helps to reduce – through biological processes – a major source of atmospheric greenhouse gases.

“At the extreme, this effect could cause significant cooling for up to ten years or more,” he concludes. To come up with this conclusion, Gauci and co-authors Drs Nancy Dise and Steve Blake simulated the volcanic acid rain from one of Europe's largest historical eruptions, the Icelandic Laki eruption of 1783 which caused widespread crop damage and deaths around Europe.

“The amount of sulphur dioxide put out by Laki in nine months was ten times more than the amount that now comes from all of western European industrial sources in a year,” says Blake, “[which] would have caused a major natural pollution event.”  Such eruptions create a microbial battleground in wetlands, with sulphate-reducing bacteria suppressing the microbes that normally produce the powerful greenhouse gas methane. This has a cooling effect on the planet, the researchers suggest.

Laki-like pollution events
“We did the simulation on a peat bog in Moray, northeast Scotland, an area we know was affected by the volcanic fallout from the Laki eruption,” says Gauci. There, the scientists found that the reduced methane emission lasts several years beyond the end of the acid rain. “Our calculations show that the emissions would take many years to recover – far longer than volcanoes are currently understood to impact on the atmosphere.”
The researchers now think that volcanoes may exert a more powerful influence over Earth's atmosphere than previously understood. They may even be a more important regulator of wetland greenhouse gases than modern industrial sources of acid rain. Wetland ecosystems are the biggest source of methane and are mostly located in remote areas of the world, away from industrial activity.

Indeed, many of Earth's wetlands seem to be located in volcanically active regions, explain the researchers, such as Indonesia, Patagonia, Kamchatka and Alaska. “Even some wetlands that are quite far away from volcanoes, such as those in Scandinavia or Siberia, will be regularly affected by Laki-like pollution events from Icelandic eruptions,” says Gauci.

There was a period of Earth's pre-history when this effect may have created important climate changes. “This interaction may have been particularly important 50 million years ago when the warm greenhouse climate of the day was due, in large part, to methane from the extensive wetlands that covered the Earth at that time,” Gauci confirms. During that time, large volcanic eruptions could have been real agents of rapid climate change due to what the scientists have described.
The research also points to a long recovery period for wetland ecosystems exposed to industrially-derived acid rain. “We've been getting on top of the sulphur pollution problem in Europe and the USA for a long time now. Our findings show, however, that the effects of acid rain can still linger for a long time.”