At normal atmospheric pressure and temperature, methane is a colorless, odorless gas composed of carbon and hydrogen (chemical formula CH4). It can burn with oxygen to form carbon dioxide (CO2) and water (H2O). Methane is the primary ingredient of natural gas, which is found underground in large deposits and is used as a fuel. It is also produced by some anaerobic bacteria, that is, bacteria that thrive in the absence of oxygen. Large deposits of methane exist in the Arctic permafrost and the ocean floors as methane hydrate ice (also called methane clathrate); there may be more carbon in these ices than in all coal, natural gas, and petroleum deposits on Earth.
Although there is far less methane in the atmosphere than CO2, it is about 20 times as powerful a greenhouse gasasCO2, molecule for molecule, and so is a significant cause of greenhouse warming. Both human-caused and natural releases of methane are important to climate change.
Historical Background and Scientific Foundations
Methane is created by microorganisms called methanogens, meaning methane-makers, and possibly by green plants as well (though this is disputed). Most of the methane found in natural gas deposits and in methane hydrate ices on the sea floors was formed by ancient methanogens; methanogens in wetlands, insects, animals, and oceans are important producers of methane today. Human activities that release methane, usually by encouraging the growth of methanogens, are waste disposal in landfills, energy production from natural gas and coal, raising cattle and sheep, growing rice, and burning fossil fuels or wood and other biomass.
Once in the atmosphere, methane (CH4) molecules gradually combine with hydroxyl radicals (OH molecules). This reaction produces water and CH3, which is not a greenhouse gas. About 90% of all methane removal from the atmosphere is by this reaction. Another 7% is by uptake by microbes living in soil, and a little less than 2% is due to reaction of methane with chlorine atoms in the ocean.
The average CH4 molecule remains in the atmosphere for 8.4 years before it is removed by one means or another. While it is there, it causes about 20 times more greenhouse warming than a molecule of carbon dioxide. Although there is much less CH4 than CO2 in the atmosphere—less than one two-hundredth the number of molecules—methane produces over one-fourth the radiative forcing that carbon dioxide does. The radiative forcing of a greenhouse gas is the amount of heat that Earth retains per square meter of its surface as a result of that gas being in the atmosphere. In 2005, the radiative forcing of carbon dioxide was 1.66 watts per square
meter and the radiative forcing of methane was .48 watts per square meter.
Atmospheric concentrations of both methane and carbon dioxide today are much greater than they have been at any time for at least 650,000 years, as shown by tiny air bubbles trapped in ancient ice in Antarctica and Greenland. In 2005, the concentration of CH4 was about 1,774 parts per billion, more than double the pre-industrial (pre-1760, approximately) concentration. Over the last 10,000 years, CH4 concentrations varied slowly between 580 and 730 parts per billion, but in the last 200 years they have risen by about 1,000 parts per billion. Not only is the concentration high, but the rise has been the fastest for at least 80,000 years. Several lines of evidence show that the recent rise in the concentrations of CH4 and CO2 is anthropogenic (human-caused), not natural.
Unlike carbon dioxide, the rate of increase of atmospheric CH4 has recently been slowing. Methane increased most rapidly in the late 1970s and early 1980s, more than 1% per year, but in the early 1990s slowed its increase and was practically unchanging from 1999 to 2005. The reason is methane's relatively short lifetime in the atmosphere, about a tenth of carbon dioxide's. Methane builds up in the atmosphere when annual emissions are greater than annual removals. The fact that emissions and removals have evened out implies that emissions have become nearly constant. This is in contrast to carbon dioxide emissions, which continue to increase.
Levels would be stable if emissions and removals increased together, but removals of methane are known to be approximately constant because the main removal mechanism is reaction with the hydroxyl ion, and the amount of hydroxyl ions in the atmosphere has remained steady. In short, anthropogenic methane emissions and natural removal of methane from the atmosphere are, for the moment, in balance. If emissions were to increase again, the methane concentration would increase also until a new balance was struck—one that contributed more to greenhouse warming. Conversely, if methane emissions were to decrease while removals remained steady, methane would contribute less to greenhouse warming.
Impacts and Issues
Methane Hydrate Ice
Although petroleum engineers discovered the existence of methane hydrate ices in the 1930s, when it was found that they were clogging gas pipelines, scientists did not discover until 1970 that vast quantities of methane hydrate are embedded in the ocean floor. This is a white compound made of methane molecules trapped inside porous water ice. The methane is produced by anaerobic, methanogenic bacteria digesting the bodies of other tiny organisms that rain down out of the upper layers of the ocean.
In nature, methane hydrate ice is stable only at temperatures near freezing and under the pressures found at depths of 1,640 ft (500 m) or more; it is found mostly near lower edges of the continental slopes, where the continental shelf drops off to the deep part of the ocean. Methane hydrate ice has been found all over the world, and although there is much uncertainty in estimating its total quantity, most assessments agree that there is probably more carbon locked up in these ices than in all fossil-fuel deposits put together, including coal. Many schemes have been suggested for harvesting the methane from methane hydrate ices to burn as a fuel, but none so far have proved affordable compared to other fuel sources.
WORDS TO KNOW
BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.
METHANE CLATHRATE: Form of water ice containing methane in its crystal structure; also called methane ice or methane hydrate. Methane clathrates form in sediments in many areas of the deep ocean floor. Estimates of total methane clathrate deposits have varied widely; such deposits are now thought to contain some hundreds of billion of tons of carbon, more than all natural gas resources put together. Methane clathrates, however, exist as scattered lumps mixed into sediment on the ocean floor and would be extremely difficult to harvest in large quantities.
METHANOGEN: Bacterium that produces methane as a waste product.
PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F(0°C) for several years.
RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).
Methane hydrate ice may have played a major role in ancient climate change. There is much geological evidence that a major period of global warming 55 million years ago—the Paleocene-Eocene Thermal Maximum, which caused the extinction of many species and triggered the evolution of modern mammals—was produced by a giant methane release from the ocean floor. Evidence from marine sediments shows that some 15 trillion tons of methane hydrate may have formed beneath the world's oceans, then escaped rapidly into the atmosphere when the oceans warmed (for some asyet unknown reason). A sudden burst of greenhouse warming would have followed the methane burp.
A similar event may be caused by anthropogenic greenhouse warming. About 400 billion tons of ethane hydrate ices are locked in Arctic permafrost. The Arctic is already experiencing some of the most extreme greenhouse warming on Earth, about twice the planetary average, with widespread melting of permafrost. If enough methane is released by this process, and possibly also from marine methane ices, greenhouse warming may be increased by 10-15% over what it would be without the methane. This remains a source of uncertainty in predictions of climate change.
Methane Production by Green Plants
In 2005, researchers claimed to have discovered that green plants produce methane, a basic fact that had escaped the notice of generations of earlier researchers. This source of methane is so large and so recently identified that its role in the global methane budget is not yet well-understood, but as of 2006 green plants were estimated to produce between 70 million and 270 million tons of methane per year, about as much as wetlands, the largest previously known single source of methane. According to the scientists announcing the discovery, green plants produce more methane under warmer conditions, so global warming may cause forests to release more methane, accelerating global warming. However, forests absorb carbon dioxide as they grow, and the researchers' calculations also showed that the greenhouse benefits of planting forests (or refraining from cutting them down), achieved through their absorption of carbon dioxide, would greatly outweigh any possible greenhouse contribution from methane release.
Yet the claim that green plants produce methane has been challenged by other scientists, who in July 2007 announced that their experiments showed that green plants produce almost no methane at all—at most 0.3% of the amount claimed previously. Meanwhile, still other scientists have asserted that satellite measurements showing higher methane concentrations over tropical rain-forests confirm vegetation as a methane source. As of this writing, in early 2008, this important point remains undecided, with competing claims and no scientific consensus as yet.
Concentrations May Rise Again
Although methane concentrations were stable in the atmosphere in the early 2000s, this leveling-off may have occurred only because of an unusual drying of the world's wetlands. If this trend is reversed, methane emissions will increase and atmospheric levels may begin to rise again. Methane may also rise if methane hydrate ice in permafrost melts in large quantities. This is more likely in the near future than increased melting of marine methane ices, which are not as quickly reached by atmospheric warming as Arctic tundra.
Khalil, Mohammad, and Aslam Khan. Atmospheric Methane: Its Role in the Global Environment. New York: Springer, 2000.
Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Dickens, Gerald R. “A Methane Trigger for Rapid Warming?” Science 299 (2003): 1017.
Giles, Jim. “Methane Quashes Green Credentials of Hydropower.” Nature 444 (2006): 524-525.
Kasting, James F. “When Methane Made Climate.” Scientific American (July 2004): 78–85.
Keppler, Frank, and Thomas Römann. “Methane, Plants, and Climate Change.” Scientific American (February 2007): 52–57, 78–85.
Kerr, Richard A. “A Smoking Gun for an Ancient Methane Discharge.” Science 286 (1999): 1465.
Suess, Erwin, et al. “Flammable Ice.” Scientific American (November 1999): 76–83.
Methane (METH-ane) is a colorless, odorless, tasteless, flammable gas that is less dense then air. It is the primary component of natural gas. Methane is the simplest of all hydrocarbons, organic compounds that contain carbon and hydrogen and no other elements.
Marsh gas; methyl hydride
Hydrocarbon; alkane (organic)
Very slightly soluble in water and acetone; soluble in ethyl alcohol, methyl alcohol, and ether
HOW IT IS MADE
Methane formed millions of years ago from microscopic underwater plants and bacteria that dropped to the bottom of the ocean when they died. Over millions of years, they were crushed and heated by the pressure of layers of sand, dirt, and other materials that accumulated on top of them. The mineral components of the undersea mud gradually turned into a type of rock known as shale. Some of the organic components turned into natural gas, which is mostly methane. The natural gas became trapped in porous rocks called reservoir rocks and in larger pockets of the rock called reservoirs or geologic traps. Natural gas is now found in pockets by itself, but is more commonly found floating on top of petroleum lakes in underground reservoirs.
Methane is also found in conjunction with pockets of coal. The largest reserves of natural gas in the United States are in Texas, Alaska, Oklahoma, Ohio, and Pennsylvania. Oil and gas companies remove natural gas from the ground by drilling. They then purify the natural gas by separating the components of which it is made, such as methane, ethane, propane, and butane. After isolation from natural gas, methane is often liquefied, which makes its easier to store and transport.
Although abundant supplies of methane exist, it can also be produced synthetically. For example, the reaction between steam and hot coal results in the formation of synthesis gas, a mixture of hydrogen and carbon monoxide. When this mixture is passed over a catalyst containing nickel metal, methane is formed. A very similar process, called the Sabatier process, uses a mixture of hydrogen and carbon dioxide, rather than carbon monoxide, also resulting in the formation of methane. Finally, methane produced during the anaerobic decomposition of manure can be captured and purified.
COMMON USES AND POTENTIAL HAZARDS
When methane burns, it releases a large amount of energy, making it useful as a fuel. Humans have known about methane as a source of energy for thousands of years. Temples in the ancient world often burned "eternal flames" that may have been fueled by natural gas. In the early nineteenth century, people began using natural gas as a light source. Once oil was discovered in the 1860s, however, its use, and the electricity produced by burning oil, became much more popular, and people abandoned natural gas as a fuel except for limited use in cooking.
Natural gas has become more popular in recent years because its use results in less pollution than petroleum and other fossil fuels. Some uses include heating homes, offices, and factories; powering room heaters and air conditioners; and operating home appliances such as water heaters and stoves.
- Methane is sometimes called marsh gas because it forms in swamps as plants and animals decay under water.
- Methane is odorless, but gas companies add traces of sulfur-containing compounds with strong odors so that people will be able to smell gas leaks and avoid suffocation or explosions.
- Some experts estimate that enough methane is present in the Earth's surface to last as much as two hundred years, although extraction of some methane resources may prove to be difficult.
Scientists are now exploring other uses for methane and natural gas with the hope that they might eventually become the most important fuels used by humans. Methane has some advantages over petroleum and coal as a fuel. It burns more cleanly than either of these other fossil fuels, producing only carbon dioxide and water as combustion products. Some experts believe that methane could be used as a power source of fuel cells, cells that burn hydrogen to produce electricity. Adding natural gas to oil- or coal-fired burners would also help reduce the greenhouse gas emissions of these appliances.
In addition to its applications as a fuel, methane is used in the manufacture of a number of organic and inorganic compounds. For example, ammonia, which is the tenth most important chemical compounds in the United States, based on quantity produced, is made from hydrogen and nitrogen gases. Over 90 percent of the hydrogen used to make ammonia is now obtained by reacting methane with water at high temperatures over a catalyst of iron oxide (Fe3O4). Other compounds produced from methane include methanol (methyl alcohol), acetylene (ethyne), formaldehyde (methanal), hydrogen cyanide, carbon tetrachloride, chloroform, methylene chloride, and methyl chloride.
Methane is not toxic, but it can cause suffocation by reducing or eliminating the oxygen a person needs to breathe normally. The primary hazard posed by the gas is its flammability and explosive tendency.
Words to Know
- Describing a process that takes place in the absence of oxygen.
- GREENHOUSE GAS
- One of several gases, including carbon dioxide and ozone, that causes the greenhouse effect on Earth.
FOR FURTHER INFORMATION
"Chemical of the Week: Methane." Science Is Fun. http://scifun.chem.wisc.edu/chemweek/methane/methane.html (accessed on October 17, 2005).
"Methane." U.S. Environmental Protection Agency. http://www.epa.gov/methane/ (accessed on October 17, 2005).
"Methane Madness: A Natural Gas Primer." The Coming Global Oil Crisis. http://www.oilcrisis.com/gas/primer/ (accessed on October 17, 2005).
Sherman, Josepha, and Steve Brick. Fossil Fuel Power. Mankato, MN: Capstone Press, 2003.
The hydrocarbon methane (CH4) is the major component of natural gas (around 90 percent) that is found in oil and gas wells throughout the world. Since the beginning of time, methane has also been produced by a number of biological sources—both natural and human—by the decomposition of organic material. From 1800 to 2000, atmospheric concentrations of methane, which are approximately 0.00017 percent, have grown around 150 percent. However, the patterns of methane emission is highly irregular and, for reasons yet unclear, the rate of increase slowed considerably from 1980 to 2000. The major natural releases of methane are from wetlands (marsh gas) and termites; the major human releases are from energy use, rice paddies, gaseous emissions from animals, human/animal wastes, landfills and biomass burning. Methane research is proceeding in two major directions: the energy course, looking for ways to make bioconversion of wastes to methane more economically attractive as an alternative fuel; and the environmental course, looking for ways to limit its release into the atmosphere since it is a much more potent greenhouse gas than carbon dioxide—a thermogenic effect four to six times that of carbon dioxide. The shared goal is finding ways to "harvest" for energy production much of the methane now being released into the atmosphere.
In the energy sector, many coal mines are looking at ways to put the methane produced as a result of the mining process to work instead of venting it into the atmosphere. The U.S. Environmental Protection Agency estimates that up to 40 percent of the methane that migrates to the atmosphere can be used for power generation (electricity and heat), injection into pipeline systems, methanol production, or onsite applications like coal drying. Besides methane sales revenue and greenhouse gas reductions, the removal of methane from coal seams could serve the vital function of decreasing the risk to workers of firedamp—methane-air mixtures igniting inadvertently.
Other energy sector concerns are methane emissions from unburned fuel, and from natural gas leaks at various stages of natural gas production, transmission and distribution. The curtailment of venting and flaring stranded gas (remotely located natural gas sources that are not economical to produce liquefied natural gas or methanol), and more efficient use of natural gas have significantly reduced atmospheric release. But growth in natural gas production and consumption may reverse this trend. Methane has the highest ratio of hydrogen to carbon of any fossil fuel (4:1), so switching to natural gas is increasingly seen as an attractive option for cleaner air and carbon dioxide reduction.
Unlike natural gas production, the biological production of methane is attractive from a sustainability perspective. Whereas the methane that sits in underground natural gas reservoirs is finite, the methane production from biological sources is potentially very large as well as renewable. Anaerobic bacteria digestion of organic materials, in the absence of air, can produce a biogas that is 60 to 70 percent methane (state-of-theart systems have reported producing 95 percent pure methane), the rest being carbon dioxide and other trace gases. Burning this gas can provide energy for cooking and space heating, or electricity generation.
Bioconversion of manure-to-methane is accomplished with biogas devices called digesters: organic material fed into the digester tank is heated to increase the natural decomposition rate by microorganisms, and then a pipe carries the biogas to where it will be used. There is an outlet for digested residues that usually are used as fertilizer. Several demonstration projects are taking place that are showcasing the technology. The Mason Dixon Dairy located in Gettysburg, Pennsylvania, produces enough methane from cow manure to meet its power needs, with excess power sold to the local utility. However, wide-scale development is unlikely because the low margins characteristic of farming do not support the additional capital investment to build such an operation.
Landfill gas-to-energy is another promising way to reduce atmospheric release and provide inexpensive energy to local industry and communities. Boilers for steam heat, hot water and the generation of electricity can be designed to burn a blended fuel or fueled exclusively on landfill gas. Because large-scale landfill methane recovery projects are most economical when an industrial facility is located nearby, landfill gas-to-energy projects are few and can only provide a limited amount of energy.
Controlling methane release from wetland, rice paddies and gaseous emissions from animals is more problematic. The release from rice paddies and wet lands is slow, intermittent and takes place over a wide geographic area, and thus very difficult to control. Gaseous emissions from agricultural animals contribute to atmospheric accumulation of methane due to fermentative digestion that produces methane in the rumen (stomach). Although the rate of release is highly variable, affected by factors such as quantity and quality of feed, body weight, age and exercise, beef cattle and draught animals are suspected of contributing around 50 percent, dairy cows around 20 percent, and sheep around 10 percent. Higher quality feed standards, which increase the efficiency of nutrient use, are being recommended as a way to curtail these emissions.
Methane from renewable biological sources will never be a major energy resource, yet it can be a valuable addition to the energy supply mix. Nevertheless, whether methane comes from fossil fuel reservoirs or from bioconversions, it is certain to provide useful energy for many years to come.
Buell, P., and Girard, J. (1994). Chemistry: An Environmental Perspective. Englewood Cliffs, NJ: Prentice-Hall.
Leng, R. A. (1993). "Quantitative Ruminant Nutrition—A Green Science." Australian Journal of Agricultural Research 44:363-80.
Olah, G. A., and Molnar, A. (1995) Hydrocarbon Chemistry. New York: John Wiley.
Methane is an invisible, odorless, and combustible gas present in trace concentrations in the atmosphere. It is the major component of natural gas, a fossil fuel commonly used for heating and cooking. The molecule consists of one carbon atom bonded to four hydrogen atoms (CH4), making it the simplest member of a chemical family known as hydrocarbons. Other hydrocarbons include ethane (C2H6), propane (C3H8), and butane (C4H10).
As a greenhouse gas , methane ranks second to carbon dioxide. Methane levels, based on ice core samples, have more than doubled since 1750 (from 0.7 to 1.7 parts per million), largely due to human activity. On a molecule-for-molecule basis, methane is twenty-three times more potent as a greenhouse gas than carbon dioxide. Both gases are targeted for emissions reduction in the Kyoto Protocol.
Methane enters the atmosphere from both natural (30 percent) and anthropogenic (70 percent) sources. Methanogens (methane-producing bacteria in swamps and wetlands) are the largest natural source.
Anthropogenic sources of methane include leaks during fossil fuel mining, rice agriculture, raising livestock (cattle and sheep), and municipal landfills. Methanogens thrive in the oxygen-free (anaerobic) environment of landfills, releasing the gas in significant quantities. The gas is purposefully ignited to prevent explosion or captured for its commercial value as a fuel.
Livestock such as sheep, goats, camel, cattle, and buffalo currently account for 15 percent of the annual anthropogenic methane emissions. These grass-eating animals have a unique, four-chambered stomach. In the chamber called the rumen, bacteria break down food and generate methane as a by-product. Better grazing management and dietary supplementation have been identified as the most effective ways to reduce livestock methane emissions because they improve animal nutrition and reproductive efficiency. This general approach has been demonstrated by the U.S. dairy industry over the past several decades as milk production increased and methane emissions decreased.
see also Fossil Fuels; Global Warming; Greenhouse Gases; Landfill; Petroleum.
DeLong, Eward F. (2000). "Resolving a Methane Mystery." Nature 407:577–579.
Simpson, Sarah. (2000). "Methane Fever." Scientific American 282(2):24–27.
Marin Sands Robinson
An organic compound with the chemical formula CH4, methane occurs naturally in air at a concentration of about 0.0002 percent. It is produced in processes such as the anaerobic decay of organic matter, the growth of certain types of plants, and the belching of cattle. Methane is the major component of natural gas , making up about 85 percent of that fuel. Environmental scientists are increasingly concerned about methane as a possible greenhouse gas. Like carbon dioxide , methane traps heat reflected from the earth and, therefore, may contribute to global warming. Increases in agricultural and dairying activities have resulted in an increase in methane production, possibly contributing to climate change.
meth·ane / ˈme[unvoicedth]ˌān/ • n. Chem. a colorless, odorless flammable gas, CH4, that is the main constituent of natural gas. It is the simplest member of the alkane series of hydrocarbons.