How much methane if you don't compost it? The greenhouse-gas arithmetic of food waste

That landfilled food waste generates methane has been understood for nearly half a century. But the question of how much methane, nationally, per year, from food waste specifically, did not get an official numerical answer in the United States until October 2023. The EPA report Quantifying Methane Emissions from Landfilled Food Waste covers 1990 to 2020 and estimates that roughly 0.86 tonnes of carbon dioxide equivalent reach the atmosphere for every tonne of food waste buried — about 55 million tonnes CO2e in 2020 alone. Its central observation was uncomfortable: total emissions from municipal solid waste landfills are trending down, while the share attributable to food waste is going up.

What usually falls away when that sentence is quoted in policy documents is how it was counted. Landfill methane does not emerge from a stack at a single point; it seeps across a surface, slowly, for decades. Direct measurement is therefore hard, and in practice the number comes from a model. The 0.86 figure is the output of a calculation about how food waste rots underground, when that rot becomes methane, and what fraction of that methane a gas collection system intercepts. Change one assumption and the number changes. The first thing this article wants to locate is where those assumptions sit.

As our earlier column set out, decomposition splits along a single line: with oxygen it yields mostly carbon dioxide, without it, methane. As physics, that is the whole of it. Entered into a climate ledger, however, the fork becomes heavy. On the IPCC's Sixth Assessment values, non-fossil methane has a hundred-year global warming potential of 27, fossil methane 29.8, and on a twenty-year horizon both sit near 80. The same carbon atom, depending on nothing more than whether oxygen was present, becomes a gas some eighty times weightier over two decades. Everything said about composting as climate policy reduces, in the end, to managing that fork.

Managing the fork, though, does not mean composting is zero. Anaerobic pockets form inside aerobic piles and vent methane. Digesters make methane deliberately and then leak some of it. Landfills collect gas, but never all of it. All three destinations, in other words, belong on a scale of how much rather than a binary of whether. What follows works through the structure of the landfill calculation, the gap between models and aerial measurement, the measured emissions of composting and of digestion, and finally how far plastics labelled 'compostable' actually break down.