Methane (CH4), is the main component of natural gas. Methane is colorless, odorless, and flammable. It is also a greenhouse gas. Methane is an organic compound, meaning it contains carbon, and is considered to be one of the simplest organic compounds. Methane is the second most common greenhouse gas that humans emit, and accounted for 9 percent of all U.S. human-created greenhouse gas emissions in 2011. Humans emit methane during livestock cultivation, from natural gas pipelines, and other oil and gas extraction activities. Methane can also be naturally emitted from sources such as wetlands, oceans, sediments, wildfires and volcanoes.
Sources of methane emissions
The U.S. Environmental Protection Agency (EPA) identifies three main areas of methane emissions. The chart to the left shows methane emissions in the U.S. by source in 2012.
- Industry: The largest source of methane emissions are the natural gas and petroleum industries. It is during the "production, processing, storage, transmission, and distribution" of petroleum products that methane can be released. The EPA states the emissions from industry can be lowered by upgrading equipment used in petroleum production.
- Agriculture: Livestock including cows, sheep, goats and buffalo, produce large amounts of methane. Methane is produced from livestock during their digestive process and through their manure. These methane emissions are considered by the EPA to be human created. The EPA states that emission from agriculture can be lowered by altering how livestock are fed and how manure is managed.
- Home and business waste: Landfills and waste water treatment also generate methane, 20 percent in 2012 according to the chart. In the U.S. landfills are the third largest source of methane emissions. The EPA states that emissions can be lowered by implementing methane capturing systems at landfills.
Natural sources of methane emissions
The EPA estimates that natural sources of methane produce 37 percent of total flux in emissions each year. While the EPA attempts to quantify how much each of the following sources contributes to methane emissions, they qualify that their findings might not be exact. This difficulty arises from the interconnectedness between the following methane emitters.
Wetlands are the largest producer of natural methane emissions and cover about 5 percent of the earth's surface. The EPA estimates that wetlands emit an average of 48 Tg CH4 per year for high latitude wetlands. Tropical wetlands are estimated to release an average of 128 Tg CH4 per year. Wetlands are areas where the soil is largely saturated with water, affecting soil development and plant and animal life. Wetlands include swamps, some meadows, bogs, fens, and prairie potholes, among others. The processes by which methane is released from wetlands has been studied for many years and is thought to be well-understood. Methane is released from wetlands by bacteria that thrive in anaerobic (or oxygen-free) and moist environments. These bacteria survive by decomposing dead plant material. It is during this process that methane is released.
Oil and natural gas industry methane emissions
There have been several studies attempting to quantify the oil and gas industry's methane emissions. Measuring methane emissions is a fairly new discipline, and as such consensus has not been reached on how to best measure these emissions. Studies in this area measure methane two different ways, the first is a top-down measurement. Top-down readings are "downwind ambient concentration" measures, or measures of methane in the atmosphere taking from above the ground. These emission measurements can be taken directly or calculated using data from satellites. Then, using meteorological data and assumptions regarding the emissions of different sources, a final measurement can be calculated. According to the National Renewable Energy Laboratory (NREL) top-down studies have the benefit of not being subject to sampling bias, but they do rely on meteorology. Top-down studies tend to find higher emission figures than bottom-up studies.
The second method used to measure methane emissions is the bottom-up approach. This method evaluates each emission source, either through direct measurements or through estimates. These measurements are then summed. The bottom-up approach is used by the U.S. Environmental Protection Agency and is the standard practice for calculating other emissions. According to the NREL bottom-up studies have the advantage of being more precise, but bottom-up studies could have sampling bias. The following chart summarizes the advantages and limitations of both techniques.
|Advantages and limitations of methane emission techniques|
|Hot spots can be identified||Measurements are "snapshots" of emissions at a certain time|
|Trends can be assessed without needing multiple measurements||Difficult to get detailed historical emission rates|
|Easier to obtain emissions from large regions||Difficult to disaggregate methane emission sources|
|Basis for historical estimates||Outliers are not identified|
|Individual sources are characterized||Unknown leak sources are not included|
|Direct measurements can be used to verify calculations|
|Spatial emission variability can be addressed|
|Source:Oak Ridge National Laboratory|
It is important to understand how these measures are taken before comparing the various studies that estimate methane emission levels. Environmentalists maintain that a leakage rage of 3 percent or higher of natural gas could erase the climate benefits that come with increased natural gas use.
Potential health, environmental and safety concerns
The increase in natural gas production across the U.S. because of fracking has lead to concerns over the potential effects of methane on public health and safety, and the environment. There are concerns that methane could be contaminating both drinking water and the air. Because the EPA considers methane to be nontoxic, there are currently no federal standards for methane levels in drinking water. Some states however have regulations affecting the amount of methane that can be released during industrial activities.
Concerns over methane contaminating drinking water have arisen from videos showing people who live near natural gas extraction sites setting faucets or hoses on fire. Because methane is flammable, levels of 5 percent or more can combust. Methane can move from trapped locations under the earth to water and the air through methane migration. This migration occurs because methane moves from points of low pressure to high pressure by naturally traveling along the easiest pathway possible. People have tracked methane's natural migration to the earth's surface since the 1800s, but there are concerns that natural gas drilling is increasing this migration. If oil and gas wells are not drilled properly, or lined with the proper steel casings and cements, then methane may migrate because drillers could open up pathways that allow methane migration to occur.
One study of wells in Pennsylvania found that residents living within one kilometer of a natural gas well have higher levels of methane in their drinking water. According to calculations done by Real Clear Science (RCS), these higher levels of methane are not cause for concern. RSC argues that that on average homes in this area with the highest methane levels detected could expect to have .035 percent of their air be composed of methane, from methane migrating from water use in the home to the air. This level is much less than the 5 percent level at which methane becomes explosive, and thus a hazard.
Human health can be harmed by methane when its presence leads to decreased levels of oxygen in the air. Exposure to oxygen levels of less than 15 percent, due to the presence of large amounts of methane, can lead to tiredness, dizziness, and headaches. There are no concerns methane causes cancer or has long term effects on organ health. Little is known though about the effects of methane on reproductive systems.
Compared to carbon dioxide--the most common greenhouse gas emitted by humans--methane has a much shorter lifetime, which is one reason advocates of natural gas push that fuel as more environmental friendly than other fossil fuels. Methane does, however, trap more radiation then carbon dioxide, and the EPA warns that over a 100 year period, pound for pound methane has 20 times larger impact on climate change than carbon dioxide.
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- U.S. Energy Information Administration, “Glossary, M” accessed January 28, 2014
- U.S. Environmental Protection Agency, "Overview of Greenhouse Gases," accessed April 9, 2014
- U.S. Environmental Protection Agency, "Draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012," February 21, 2014
- U.S. Environmental Protection Agency, "Methane and Nitrous Oxide Emissions From Natural Sources," April 2010
- U.S. Environmental Protection Agency, "Wetlands-Wetland Types," July 16, 2013
- Washington Examiner, "Methane leaks from fracking lower than EPA estimate:studies," December 9, 2014
- Oak Ridge National Laboratory, "Greenhouse Gas Emission from Natural Gas: Overview of Recent Developments," accessed December 9, 2014
- National Renewable Energy Laboratory, "U.S. Natural Gas System Methane Emissions: State of knowledge from LCAs, Inventories, and Atmospheric Measurements," April 7, 2014
- Wisconsin Department of Health Services, "Methane," November 16, 2012
- National Public Radio, "Tap Water Torches: How Faulty Gas Drilling Can Lead to Methane Migration," accessed April 157, 2014
- Proceedings of the National Academy of Sciences of the United States of America, "Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction," June 3, 2013
- Real Clear Science, "There's Methane in Your Drinking Water. So What?," June 29, 2013