Depending on how you look at it, something good can always come out of something bad. That's actually the case in a new study on greenhouse gases by NASA scientists and others. The researchers discovered that acid rain inhibits a swampland bacteria from producing methane, a greenhouse gas.
Animation above: This movie from the U.S. Environmental Protection Agency highlights the science of acid rain, and its effects. Click arrow on bottom right to move to next image. Credit: U.S. EPA
Methane, a gas that contributes to warming our planet, is produced by natural processes and human activities. Increased amounts of methane and other greenhouse gases in our atmosphere are warming the Earth beyond its average temperature.
Carbon, heat and moisture are known to influence methane production by members of the Archaea, single-celled creatures. Under normal conditions, these microbes consume organic carbon in the soil for energy and release methane as a byproduct. Wetlands provide an ideal environment for these microbes. When acid rain drops sulfate onto wetlands, another type of bacteria, ones that reduce sulfate are able to outcompete the Archea, limiting the total production of methane.
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.
Image to right: Inland wetlands are most common on floodplains along rivers and streams. Scientists have discovered that acid rain actually inhibits a bacteria found in swamplands from producing methane, a greenhouse gas. Inland wetlands include marshes and wet meadows dominated by herbaceous plants, swamps dominated by shrubs, and wooded swamps dominated by trees. Credit: U.S. EPA Region 1/Leo Kenney
"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.
Image to left: Coastal wetlands in the United States, as their name suggests, are found along the Atlantic, Pacific, Alaskan, and Gulf coasts. They are closely linked to our nation's estuaries, where sea water mixes with fresh water to form an environment of varying salinities. The salt water and the fluctuating water levels (due to tidal action) combine to create a rather difficult environment for most plants. Credit: U.S. EPA Region 8/Paul McIver
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.
Image to right: Wetlands are among the most productive ecosystems in the world, comparable to rain forests and coral reefs. An immense variety of species of microbes, plants, insects, amphibians, reptiles, birds, fish, and mammals can be part of a wetland ecosystem. Physical and chemical features such as climate, landscape shape (topology), geology, and the movement and abundance of water help to determine the plants and animals that inhabit each wetland. The complex, dynamic relationships among the organisms inhabiting the wetland environment are referred to as food webs. Credit: U.S. EPA/ Mark Sharp
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.
NASA's Science Directorate works to improve the lives of all humans through the exploration and study of Earth's system, the solar system and the Universe.