Methane is among the six GHGs restricted under the Kyoto Protocol. Its global warming potential is more than twenty times that of CO2.

Methane is a colorless, odorless gas with the molecular formula CH4. It is the main chemical component of natural gas (accounting for 70-90 percent of such gas). Natural gas makes up to 20 percent of the U.S. energy supply. Methane was discovered by the Italian scientist Alessandro Volta, who collected it from marsh sediments and demonstrated that it was flammable. He called it "combustible air." As with all greenhouse gases (GHGs), methane in the atmosphere acts similarly to glass in a greenhouse.

It allows light energy from the Sun to reach Earth's surface, but it traps heat energy radiated back from the surface in the form of infrared radiation.

Since the beginning of the Industrial Revolution in the mid-eighteenth century, methane concentrations have more than doubled in the atmosphere, causing nearly one-quarter of the planet's anthropogenic global warming. Continuous release of methane into the atmosphere causes rapid warming, because methane's contribution to the greenhouse effect is much more powerful than that of carbon dioxide (CO2).

Global warming itself may trigger the release of methane trapped in tundra permafrost or ocean deposits, thereby accelerating climate change in a positive feedback loop. The release of large volumes of methane from such geological formations into the atmosphere has been suggested as a possible cause for global warming events in the past. Methane oxidizes to CO2 and therefore remains in the atmosphere for a shorter time-period of nine to fifteen years, compared to CO2, which may remain in the atmosphere for one hundred years.

According to the U.S. Environmental Protection Agency (EPA), about 60 percent of global methane emissions are a direct result of human-related activities. These activities include creating landfills, treating wastewater, animal husbandry (through enteric fermentation and manure production), cultivating rice fields, mining coal, and producing and processing natural gas. For instance, the livestock sector (including cattle, chickens, and pigs) generates 37 percent of all anthropogenic methane. Landfills are the second largest anthropogenic source of methane in the United States.

Natural sources of methane include wetlands, lake sediments, natural gas fields, termites, oceans, permafrost, and methane hydrates. Wetlands are responsible for up to 76 percent of global natural methane emissions. Surprisingly, according to EPA data, termites contribute about 11 percent of global natural methane emissions. In most of these processes, methane is produced by microorganisms called archaea as the integral part of their metabolism.

Such microbes are called methanogens, and the route of methane generation is called methanogenesis (Archer, 2007). Archaea live in oxygen-depleted habitats, because the presence of oxygen would kill them instantly. For their food source, methanoges use products of bacterial fermentation such as CO2 and molecular hydrogen (H2); different acids such as acetate, pyruvate, or formate; or even carbon monoxide. That is why methanogenic archaea usually exist in consortium with other microorganisms (bacteria). They also live in symbiotic relationships with other life forms, such as termites, cattle, sheep, deer, camels, and rice crops.

In the 1985 science-fiction film Mad Max Beyond Thunderdome starring Mel Gibson, a futuristic city was run on methane generated by pig manure. In reality, methane can be a very good alternative fuel. It has a number of advantages over other fuels produced by microorganisms. It is easy to make and can be generated locally, obviating the need for long-distance distribution. Extensive natural gas infrastructure is already in place to be utilized. Utilization of methane as a fuel is a very attractive way to reduce wastes such as manure, wastewater, or municipal and industrial wastes. In local farms, manure is fed into digesters (bioreactors), where microorganisms metabolize it into methane. Methane can be used to fuel electrical generators to produce electricity.

In China, millions of small farmers maintain simple, small, underground digesters near their houses. There are several landfill gas facilities in the United States that generate electricity using methane. San Francisco has extended its recycling program to include conversion of dog waste into methane to produce electricity and to heat homes (Madigan, 2009).

With a city dog population of 120,000, this initiative promises to generate significant amounts of fuel with a huge reduction of waste at the same time. Methane was used as a fuel for vehicles for a number of years. Several Volvo car models with bi-fuel engines were made to run on methane, with gasoline as a backup. Methane is more environmentally friendly than are fossil fuels. Burning methane results in production of CO2 and contributes to global warming, but with less impact on Earth's climate than methane itself would have in the atmosphere, for a net benefit. Even though the use of methane as an energy source releases CO2, the process as a whole can be considered CO2 neutral, in that the released CO2 can be assimilated by archaea.

The natural mechanism of methane removal from the atmosphere involves its destruction by the hydroxyl radical (OH). Significant amounts of methane are also consumed by microorganisms called methanotrophs, which use the methane for energy and biosynthesis. These bacteria are prevalent in nature and potentially could be used for methane mitigation.

Since methane is a powerful contributor to global warming, any efforts to reduce methane emissions will have a rapid impact on Earth's climate. One way to avoid methane release into the atmosphere is to turn it into a fuel. Supply of fossil fuels, particularly oil, is limited and does not satisfy world energy demands, which consistently increase. The extensive use of fossil fuels causes global warming. Methane utilization in place of fossil fuels as an energy source can provide significant environmental and economic benefits. In the future, landfills and wastewater treatment facilities can possibly be redesigned to optimize methane production. However, further research is needed to better understand archaean-bacterial methanogenic communities in landfills and wastewater treatment facilities in order to improve methane generation.

Some technical obstacles exist to efficiently converting landfill wastes that primarily contain plant lignocellulosic material. Lignocellulose is a combination of lignin, cellulose, and hemicellulose that strengthens plant cell walls. Microbial communities in landfills cannot utilize lignin. Creating efficient methane-producing facilities that are also capable of reducing waste is a feasible option for sustainable development to provide fuel to heat homes, run cars, generate electricity, and eliminate powerful GHG and health hazards.

References

1. Archer, David. Global Warming: Understanding Forecast. Malden, Mass.: Blackwell, 2007.

2. Madigan, Michael T., et al. Brock Biology of Microorganisms. 12th ed. San Francisco, Calif.: Pearson/ Benjamin Cummings, 2009.

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