Another 12,000 landfills have been closed in the past 25 years. Methane gas emitted from landfills is suspected to be a significant factor in global warming. In the U.S., about 36 percent of the methane released into the atmosphere comes from landfills. Methane has approximately 15 times the global warming potential of carbon dioxide. Landfill waste is now about 85 to 90 percent biodegradable, up from 70 percent before the advent of widespread recycling programs.
Landfill gases are a by-product of decomposing organic materials.
MSW is composed primarily of polymers from paper or cardboard, such as cellulose, hemicellulose and lignin. In the first phase, the polymers are broken down or hydrolized by bacterial enzymes, producing large amounts of organic acids and alcohols that are dissolved in the rain water that seeps through the landfill. These, along with other water soluble wastes, are called leachates. Methane-producing bacteria called methanogens use the organic materials in leachate for energy, producing equal amounts of methane and carbon dioxide. Depending on its size, a landfill could produce methane over a period of 20 to 50 years.