Scientists highlight a worrying—and not well understood—spike in atmospheric methane.
By Nathan Collins
(Photo: Scott Olson/Getty Images)
Though carbon dioxide is arguably the most notorious of the greenhouse gases, it’s certainly not the only one. In fact, an unexpected surge in one of those other gases — methane — has researchers both worried and confused.
“Unlike CO2, atmospheric methane concentrations are rising faster than at any time in the past two decades,” and in fact faster than in the worst-case scenarios used to predict future climate change, Marielle Saunois and her colleagues write in an editorial published in Environmental Research Letters. Yet the “reasons for this renewed growth are still unclear, primarily because of uncertainties in the global methane budget.”
Carbon dioxide is by far the most abundant greenhouse gas, accounting for around four-fifths of emissions in the United States in 2014. But it is hardly the most potent. That title goes to fluorinated gases, such as nitrogen trifluoride, that have tens of thousands of times the 100-year global warming potential of carbon dioxide (those gases are quite rare though). Methane, with a global warming potential roughly 30 times that of carbon dioxide, lies in between: Although it stays in the atmosphere for a much shorter amount of time compared with carbon dioxide, it retains much more heat.
The fact that methane emissions appear to be going up even as carbon emissions are leveling off is cause for concern. In the 1980s, atmospheric methane concentrations went up by around 11 or 12 parts per billion per year, then dropped to around two or three parts per billion per year during the 1990s, according to National Oceanic and Atmospheric Administration data. For several years in the early 2000s, methane concentrations actually dropped, but since 2006 methane emissions have grown more than 10-fold, returning in some years to 1980s rates.
Reviewing a number of recent studies on methane sources, Saunois and her team conclude that the most likely source of the increase is not, as one might suspect, leaking natural gas wells, something the Environmental Protection Agency said earlier this year it would begin regulating. Instead, the methane surge appears rooted in expanding agriculture—in other words, there seem to be many more methane-belching cows than there used to be, and much more decaying agricultural waste, another significant source of methane.
There are a number of possible approaches for dealing with the high methane levels, the researchers write, some of which are already in use—for example, farm digesters, which extract methane from manure to help heat and power the farm itself. Other possibilities include changing livestock diets to emit less methane, or changing rice growing practices.