The greenhouse gas produced by the decomposition of organic matter in landfills is methane. Food scraps, paper, yard waste, and other biodegradable materials are buried in compacted layers, where oxygen becomes scarce. Microorganisms then break down the material anaerobically, generating landfill gas that is rich in methane and carbon dioxide.
Methane is especially important because it traps much more heat per molecule than carbon dioxide over a 100-year period, although it remains in the atmosphere for less time. Landfill operators often collect the gas through wells and pipes, then burn it for energy or flare it to convert methane into carbon dioxide.
The answer is sometimes confused with carbon dioxide because both gases occur in landfill gas. Carbon dioxide is also produced during decomposition, but methane is the characteristic greenhouse gas associated with oxygen-poor decay in landfills. How much methane forms depends on the waste composition, moisture, temperature, landfill design, and effectiveness of gas collection systems.