Methane - carbon dioxide on steroids
Substantial volumes of methane are generated from the decomposable organic fraction of our municipal and industrial waste.
Between half and two-thirds of our household and commercial discards are organic.
When garbage and its organic fraction are buried in landfill they decompose anaerobically (i.e. in oxygen-starved conditions), and methane (CH4) is produced among the decomposition byproducts.
A ton of wet organic material buried in a landfill is reflective of what one family might throw out in a year and will generate approximately 1400 kg of methane spread out over decades. Some fraction of that methane will escape from the landfill into the atmosphere, whether or not some of the methane is collected and burned. Those escaped landfill gases are commonly known as fugitive or uncontrolled emissions. Similarly methane is produced at waste water treatment facilities.
Methane is carbon dioxide on steroids. The difference between releases of CH4 (from landfills alone) and CO2 (from almost any other alternative) holds enormous consequences for climate change. Methane emissions have at least 25 times the warming potential of CO2 emissions when climate impacts are counted over the longer term.
Methane produced at landfills can be captured and utilised for generation of electricity or the production of heat. A good summary is available here.
Estimates of how much methane is produced by waste in landfills, the timing of that methane generation, and by extension, how much methane is actually emitted to the atmosphere, after gas collection and oxidation is outlined here and here.