Anthropogenic Methane Emissions in the United States


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Methane is a fuel source. Photo by Alvimann.

Methane is a fuel source – but at high levels, it’s also really bad for the atmosphere. Photo by Alvimann.

A study published in the Proceedings of the National Academy of Science shows that U.S. anthropogenic (man-made) methane (CH4) emissions may be higher than previously thought. The main CH4 sources are agriculture activities and fossil fuel production. These results cause concern, as methane is a greenhouse gas more powerful than carbon dioxide; therefore, we may need to revise greenhouse gas reduction programs.

What is Methane?

Methane (CH4) is a gaseous compound; due to its abundance on Earth and its high calorific value, it is widely used as a fuel.

CH4 is a natural component of the atmosphere; in recent years, however, its concentration increased due to anthropogenic activities. The main anthropogenic CH4 source is livestock, from processes such as enteric fermentation and manure management; other sources include natural gas production and distribution, coal mining and landfills.

CH4 As a Greenhouse Gas

The recent increase in the CH4 atmospheric concentration raised concern, as methane is a greenhouse gas much more powerful than carbon dioxide (CO2). According to the Intergovernmental Panel on Climate Change (IPCC), methane’s global warming potential is about 70 times higher than that of CO2, when considering a 20 year period.

Moreover, methane can also cause an increase in the ground level concentration of ozone (O3); this can create health problems for people (i.e. irritation of respiratory system) and affect the ecosystem of some sensitive plant species.

U.S. Emissions: Recent Studies

Due to the dangers to the environment from methane, researchers performed several studies to determine the exact amount of methane present in the atmosphere, and to establish a clear connection between CH4 concentration and anthropogenic activities.

A new investigation, focused on methane emissions in the US, was published in the Proceedings of the National Academy of Science (PNAS) on the 25th of November 2013. The Harvard School of Engineering and Applied Science (SEAS) in cooperation with other research institutions (Carnegie Institution of Science (US) and the National Oceanic and Atmospheric Administration (NOAA, US)) performed the research.

Emissions from ruminants contribute to methane increase. Photo by Mensatic.

Emissions from ruminants contribute to methane increase. Photo by Mensatic.

Methane Emissions: Top-down Approach

In this study, researchers used the top-down method. This means that they did not consider the methane emitted by a single activity and then “scale-up” the values; on the contrary, they measured CH4 concentrations in the atmosphere in several locations in different parts of the country. From these data, they calculated the overall US emissions.

Scot Miller, PhD student at the Harvard School of Technology and Applied Science and a key scientist in the project, explains to Decoded Science:

“In our study we used atmospheric methane observation of very different locations, from California to Massachusetts, and from Texas to North Dakota. By carefully analyzing the concentration of methane gas in the atmosphere, and the use of appropriate modeling, we could get a reliable estimate of total CH4 gas emissions over large regions and for the whole of the United States. We considered a two-year period, from 2007 to 2008.”

Surprising Results

The results were unexpected. According to Mr. Miller: “Our results were quite surprising, as we found that, on a national scale, the total anthropogenic CH4 emission was 33.4 ± 1.4 TgC/y (teragrams of carbon per year – 10+9 kilograms of carbon per year). This value is 1.5 and 1.7 times higher than those reported by the US Environmental Protection Agency (EPA) and by the Emission Database for Global Atmospheric Research (EDGAR), respectively. Even more surprising, for some areas of the country, such as the south-central United States, our estimates were 2.7 times higher than those previously reported by the official emission inventory.”   

Underestimated Contributions

The results from this study suggests that in previous methane emission evaluations, scientists have underestimated some contributions.

“We think that the emissions from ruminants and manure could be as much as two-times higher than reported in the existing inventories.” Mr. Miller said. “In the south-central United States, on the other hand, the main methane sources are fossil fuel extraction (drilling and processing) and refining; in fact they contribute 45 ± 13 %.

Estimates by the US EPA indicated a decrease in methane emission factors due to fossil fuel extraction and processing; our data, in contrast, indicate the opposite.”

Fossil fuel production causes methane emission. Photo by click.

Fossil fuel production causes methane emission. Photo by click.

Environmental and Policy Implications

Commenting on these findings, Mr. Miller explained to Decoded Science their possible meaning and long-term implications.

“This study gives us a detailed perspective on total methane anthropogenic emissions in different regions of the United States, at a level of detail not previously available. This new information should be considered.

The possible consequences this can have on the environment, for instance, should be evaluated. Moreover, greenhouse gas policies should also take these data into account: any gas reduction policy, either at national or state level, should consider that CH4 emissions may be higher than previously thought; this may mean that more specific and appropriate actions may be needed.”

If methane production is higher than scientists previously thought, models showing the results of man-made emissions on the environment will need re-calculation. What’s the final result for the atmosphere and the environment? Only time will tell.


Miller, S.M. et al. Anthropogenic emissions of methane in the atmosphere(2013). Proceeding of the National Academy of Science. Accessed November 25, 2013.

Intergovernmental Panel on Climate Change. Net global radiative forcing, global warming potentials and patterns of forcing. Accessed November 25, 2013.

Environmental Protection Agency. Ground Level Ozone. Accessed November 25, 2013.

Harvard University. Scot Miller, PhD Student, Harvard University. Accessed November 25, 2013.

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