By Jacqueline Ali
BBC News Online
Acid rain might not be entirely harmful: the sulphate it contains might be helping to slow down global warming, scientists have announced.
Acid rain may combat another type of pollution
The greenhouse gas methane is produced in large quantities by microbes found in wetlands such as peats and marshes.
But these microbes are inhibited by a type of bacteria that thrives on the sulphate present in acid rain.
In time, this could return methane to pre-industrial levels, Proceedings of the National Academy of Sciences says.
Methane is a powerful greenhouse gas, but it exists in much smaller quantities than carbon dioxide; it accounts for 22% of human-caused global warming.
But the majority of atmospheric methane comes from nature.
Methanogenic (methane-producing) microbes, called methanogenic archaea (MA), are an integral feature of global wetlands.
They produce 160m out of 500m tonnes of methane, an activity that can be affected by a number of factors.
Carbon dioxide, heat and moisture are known to stimulate methane production in the micro organisms.
But Dr Vincent Gauci, of the Open University, and colleagues, were interested to see whether the low levels of sulphates present in acid rain - as well as in fog and snow containing acid depositions - affected methane emissions in wetland ecosystems.
They dosed a variety of wetlands in the UK, US and Sweden with various amounts of sulphate, all within the range of acid deposition that occurs globally.
The results showed that methane emission was significantly suppressed by sulphate. All of the experimental sites displayed 30-40% suppression of methane at low levels of simulated acid rain.
The reason is due to rivalry between two groups of anaerobic microbes present in watery environments.
Along with the MA, another group of microbes - called sulphate-reducing bacteria (SRB) - are widespread in wetland ecosystems.
The researchers found that when increased sulphate was present, these bacteria dominated over the MA.
MA gain energy by fermenting organic substrates such as acetates, or they can oxidise atmospheric oxygen. Either method results in the production of methane as a by-product.
Dr Gauci speculates that the SRB were able to outcompete the MA because they have a greater affinity for the organic substrates present in the ecosystem.
"When sulphur levels are high, sulphate-reducing bacteria flourish, and take food from methanogens," he told BBC News Online.
The researchers used a computer model of their findings to estimate the global effect of sulphate depositions from 1960 to 2080.
They discovered that by 2030, levels of sulphur pollution may be sufficient to reduce methane emissions by 15% of the total amount of wetland emissions.
"The model shows that acid rain actually reduces emission to below pre-industrial levels," said Dr Gauci. "This shows that at low levels, acid rain is not harming the ecosystem.
"Inadvertently, it could in fact be quite good."
Dr Ralf Toumi, an atmospheric physicist at Imperial College, London, commented: "This is an important paper that highlights the complexity of the acid rain system.
"However biological estimates on a global scale are notoriously uncertain."
Dr Toumi warned against viewing acid rain as posing a reduced threat to the environment.
"This is no license to produce more acid rain," he said.
"The health and economic benefits of acid rain control far outweigh any minor benefits of methane emission reductions."
Although many countries have successfully implemented controls over sulphur emissions from industry, the authors project that acid rain will increase as regions such as Asia continue to expand industrially.
However, some researchers believe acid rain will peak in the mid 21st Century, and the authors predict that methane will then reassert its dominance.