Landfill methane

Why does one food scrap warm the world far more buried in a tip than rotting in a garden heap?

Take two identical apple cores. Put one in a garden heap, and put the other in a wheelie bin bound for landfill. Both will be eaten by microbes. Both contain exactly the same carbon, in the same molecules, and neither one gains or loses an atom on the way. Yet in every serious greenhouse accounting the buried core is charged with vastly more warming than the composted one.

That should look impossible. The carbon is conserved, so whatever the difference is, it cannot be about how much carbon comes out. It has to be about the form it comes out in.

Start with the heap, the simpler case. Air moves through it, so the microbes have oxygen, and with oxygen available they do the energetically obvious thing: oxidise the carbon all the way to carbon dioxide. That carbon dioxide was drawn out of the air by the apple tree a year or two ago, so returning it closes a loop just opened. In greenhouse accounting this is called biogenic carbon and treated as near-neutral — not because carbon dioxide from an apple behaves differently from carbon dioxide out of coal, but because the apple's carbon was atmospheric so recently that the round trip adds nothing to the standing stock.

Now bury the second core under several metres of compacted refuse. What runs out first? Oxygen — not the food, not the water — because nothing resupplies it through a compacted, capped mass. Within weeks the pockets are anoxic.

Here is the step that decides everything. Microbes cannot make carbon dioxide without an oxidant, so the community that thrives there is a different community entirely. Fermenters break the material to acids and alcohols, other organisms take those to acetate, hydrogen and carbon dioxide, and finally the methanogens — archaea, not bacteria — combine hydrogen with carbon dioxide, or split acetate, to make methane. The conditions selected for them; they are the only thing that can make a living down there.

The end product is roughly a half-and-half mixture: landfill gas typically runs something like forty-five to sixty per cent methane by volume, most of the rest carbon dioxide. So roughly half the buried core's carbon now leaves as methane. Not more carbon. The same carbon, in a reduced form.

Why does that matter so much? Because methane is a fuel — not a metaphor, but the physical fact behind everything else. Look at the energy. Fully oxidising a mole of glucose to carbon dioxide releases about 2800 kilojoules. Routing the same mole down the methanogenic path yields three molecules of methane and three of carbon dioxide, and those three methanes still hold about 2670 kilojoules of combustible energy between them. The whole microbial community shared out only the small remainder. The carbon left the tip still carrying almost all its chemical energy.

Weight for weight, methane's warming effect over a hundred years is put at roughly twenty-seven to thirty times that of carbon dioxide, and over twenty years at something near eighty. So a carbon atom sent out as methane rather than as carbon dioxide does not do slightly more damage. It does more than an order of magnitude more, and about half the buried carbon takes that route.

Does this predict anything checkable? It predicts the tip should be cooler than the heap, since its microbes extract only a few per cent of the available energy. It is. A compost heap runs hot enough to be uncomfortable to touch; a landfill body never approaches it. The heat a heap gives off is exactly the energy a landfill sends off in a pipe instead.

Several things want stating carefully, because this topic attracts sloppy claims in both directions.

First, the dividing line is oxygen, not the words "tip" and "compost". A garden heap that is too wet, too dense or never turned goes anaerobic in its middle and makes methane too. A landfill's freshly tipped surface layer is aerobic and does not. The mechanism is indifferent to what we call the site.

Second, modern landfills are not simply left to vent. Collection wells extract the gas for flaring or electricity generation, and methane escaping collection is partly eaten on the way out by methane-oxidising bacteria in the cover soil, conventionally reckoned at around a tenth of what passes through. How much is actually captured over a cell's whole life is genuinely disputed: regulatory defaults have long assumed around three-quarters, while aircraft and satellite campaigns have repeatedly found real emissions above reported figures. Treat any capture number, including the one below, as illustration rather than measurement.

Third, the hundred-year comparison is a convention with known weaknesses. Methane's short atmospheric lifetime — around twelve years — means a single pulse behaves quite differently from a sustained flow, and whether the standard metric fairly represents that is an active argument among climate scientists. The direction of the conclusion is not in doubt; the exact multiplier is a modelling choice.

• Why anaerobic digesters deliberately do what landfills do accidentally, and capture the gas instead.

Every term the collection defines is gathered in the glossary.