Why did methane stop increasing, then start again?

Rice paddies near Bandung, Indonesia. (credit: Onny Carr)

The rise of atmospheric carbon dioxide has been unrelenting over the past century. But that isn’t the only greenhouse gas humans are adding to the atmosphere, and other have different stories to tell. Methane levels, for example, actually flattened out in the late 1990s, holding pretty steady until continuing an upward climb in 2006. Why, you ask? Well, you aren’t the only one.

(credit: Ed Dlugokencky, NOAA/ESRL)

Methane is a little harder to get a handle on than CO₂, partly because human emissions from things like livestock, rice growing, and landfills are a little harder to track. It's also partly because the natural terms in the global equation are large and erratic. In wetlands, where water-logged, oxygen-poor sediments host methane-exhaling microbes, the amount of methane released varies strongly between wet or dry years. And then there’s the long-term warming of the Arctic, where thawing permafrost can constitute an additional methane source to account for.

Researchers try to determine trends in methane contributions in several ways. You can do your best to monitor individual processes and add up all your estimates. Or, flipping that around, you can interrogate atmospheric measurements to figure out which processes could be responsible for changes. As it happens, methane molecules come with labels that make that easier—different types of methane sources impart different isotopic fingerprints on the carbon in CH₄.

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