Researchers have long been trying to determine whether increases in methane gas concentrations in the atmosphere prior to the industrial revolution were the result of natural or human processes. In an attempt to address this issue, Oregon State doctoral student Logan Mitchell, with help from his major professor Edward Brook of the College of Earth, Ocean, and Atmospheric Sciences, and their colleagues assessed the variety of causes behind methane increases. As is often the case with science, the answer appears to be both.
According to the EPA, methane is the second most abundant greenhouse gas emitted from human activities in the United States. It can contribute to global warming at a rate twenty times greater than carbon dioxide. Methane emissions come from a variety of sources including agriculture, livestock, natural gas, oil and coal production and transport, and natural and man-made wetlands.
In this study, released earlier this month in Science, Mitchell sought to explain the opposing views of proponents and opponents of the “early anthropogenic hypothesis” which theorizes that the annual 17% increase in methane production from about 5,000 years ago to the beginning of the industrial revolution was the result of human activities. The theory correlates activities such as rice farming with the increase. Opponents believe natural processes were the culprit.
Mitchell and his colleagues studied ice cores from Antarctica and Greenland, and the differing methane concentrations between the cores. This difference, called Inter-Polar Difference, represents divergent atmospheric methane concentrations between the Northern and Southern Hemispheres, which remained stable for thousands of years according to Mitchell. He adds that if human or natural processes alone caused methane increases, measureable changes in the Inter-Polar Difference would have been observed.
Mitchell developed a model which accounts for methane production from agricultural and livestock cultivation and wetland processes. He compared the results of the model to the data from the ice cores, and found that when both natural and anthropogenic sources were considered, the results closely matched the ice core data. These findings are in line with some theories of present day climate change, natural and anthropogenic forces working in tandem to elevate greenhouse gas emissions.
By Kristen Daly
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