Wetlands may produce as much as 320 million tons of methane annually but only about half of that, or 160 million tons, is ultimately released to the atmosphere. The other 160 million tons never makes it to the atmosphere because it is destroyed via oxidation as it moves from wet soils below the water table through dry soil to the surface. Despite substantial oxidation, natural wetlands remain the single largest source of methane emission accounting for about one third of the global annual total methane.
"It's a complicated process because multiple factors at microscopic to global scales interact in these processes," said Elaine Matthews, a scientist at NASA's Goddard Institute for Space Studies (GISS), New York. Matthews is co-author of the study on acid rain and methane in wetlands. "The maximum emission of methane from wetlands occurs when conditions are warm and wet, while the biggest reduction in methane emissions is achieved when the location of wetlands, sulfates contained in acid rain, high temperatures and substantial precipitation all come together, to reduce optimal methane emissions from wetlands." These factors vary over time and space.
According to Matthews, by 1960 these counteracting processes probably reduced methane emission from wetlands to pre-industrial levels. However, methane emission is predicted to rise in response to 21st century climate change faster than sulfate suppression increases, meaning that wetland emissions of methane will begin to rise above those occurring before industrial sulfate pollution began.
In order to determine how the acid rain interacts with methane in wetlands, lead author of the study, Dr. Vincent Gauci of Open University, United Kingdom and his colleagues took to the field. In the U.S., Britain and Sweden they attempted to determine if low levels of sulfate, like those in acid rain, affected methane emissions in wetlands. They applied several quantities of sulfate, similar to the amounts found in acid rain, to the wetlands they were studying. The results, acquired over several years, showed that these low doses of sulfate suppressed methane emissions between 30-40 percent. Matthews and climate experts expect methane emissions to increase over the 21st century in response to climate change. They also predict that sulfate levels in rainfall will increase, especially in Asia. The authors have attempted to predict how this ecological balancing act will turn out for the 21st century.
"When we used all the field data with the NASA computer models and applied it to a global scale, it shows that the effect of acid rain from 1960 to 2030 actually reduces methane emissions to below pre-industrial levels," said Gauci. The effect more than compensates for the increase in methane emission that would be expected as wetlands become warmer. In this way, acid rain acts like a temporary lid on the largest methane source.
Gauci is cautious about the image presented by acid rain. "We wouldn't want to give the impression that acid rain is a good thing - it has long been known that acid rain damages natural ecosystems such as forests, grasslands, rivers and lakes. But our findings suggest that small amounts of pollution may also have a positive effect in suppressing this important greenhouse gas. Moreover, they point to how complex the Earth system is," he noted.
Most attention has been given to the negative aspects of pollution but if scientists want to understand all of Earth's complexities and make better predictions of future climate we need to understand interactions among a suite of processes that are not always well understood. "That's not to say that acid rain is a good thing. Rather this study illuminates really well how we have to work to understand relationships among microscopic-to-global processes, at the same time that we attempt to represent them in relatively simple ways," Matthews said.
While sulfate deposition results almost exclusively from human activities, it may serve to delay impacts from the increase of at least one greenhouse gas, methane, in the short term. The study recently appeared in the Proceedings of the National Academy of Sciences.