Carbon dioxide (CO2) is not the only greenhouse gas on the rise. Since 2007, methane—which molecule-for-molecule has 30 times the warming effect of CO2—has risen by more than 3%. Befuddled scientists have tried to pin the growth on increased natural gas drilling, rising rice cultivation, and a surge in bovine belches. But none of these explanations has stuck.
Now, two more processes have gained ground as possible culprits, according to new work presented here last week at a meeting of the American Geophysical Union (AGU). In one scenario, methane’s rise may come in part from a drop in hydroxyl, a chemical that acts as an atmospheric detergent; in the other, the gas is emanating from tropical wetlands flooded by heavy rains in recent years. Because climate change is expected to increase tropical rainfall, this methane could be a new signal that “the tropics are changing fast,” says Euan Nisbet, a climate researcher at Royal Holloway, University of London—and a warning that methane may continue to rise as the world warms in a positive climate feedback. “Methane may be a tropical parallel to Arctic sea ice,” Nisbet says.
Methane is normally held in check by the hydroxyl radical (OH), which scrubs nearly all of it out of the atmosphere within a decade. Formed in the presence of sunlight by water vapor and pollutants like ozone and nitrogen oxides, hydroxyl is hard to measure, because it persists for just a second in the air before it reacts away. Instead, scientists gauge OH abundance by looking to proxies— chemicals that react with hydroxyl. One proxy is methyl chloroform, banned years ago for contributing to the ozone hole. A steady decline of that compound in the wake of regulations would suggest that hydroxyl is relatively constant, an assumption baked into most models of the methane rise. “OH tends to get ignored a bit in discussions even in the science community,” says Michael Newland, an atmospheric chemist who recently completed a postdoc at the University of East Anglia in Norwich, U.K.
But a close look at the methyl chloroform trend shows that the decline isn’t as steady or certain as many have assumed. Instead it reveals bumps that could indicate a loss of OH, according to research presented at the AGU conference by Alexander Turner, a graduate student in atmospheric chemistry at Harvard University. The overall oxidative capacity of the atmosphere could be declining, allowing methane to linger longer, he says. Newland says that a fall in nitric oxide and nitrogen dioxide from clean air regulations could have slowed the production of OH. But Turner cautions that the case for declining OH is far from closed: With different assumptions, his sparse data could just as easily chart a rise in methane emissions rather than a hydroxyl decline.
*Correction, 23 December, 11:45 a.m.: A previous version of this story incorrectly described the hydroxyl radical as OH-, which refers to the hydroxide ion. Moreover, the article misstated a pollutant. In fact, nitrogen oxides, not nitrous oxide, are the pollutants that play an important role in hydroxyl chemistry.
And so scientists continue to look for unrecognized new sources. Cows are unlikely, as their numbers saw their steepest increase between 2000 and 2006, when methane levels were flat. There’s little evidence of thawing Arctic permafrost pumping out more methane. Expanded rice growing could play a role. But Ed Dlugokencky, an atmospheric chemist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colorado, sees another potential source: the heavy rains that washed over the tropics from 2008 to 2014, creating a surge in wetlands and methane-spewing microbes. “This is all very anecdotal,” says Dlugokencky, who gave a talk at the AGU meeting. “But I think it paints a consistent picture of what’s going on.”
In recent years, researchers have noticed another clue to the puzzle: The carbon atoms in atmospheric methane molecules have shifted toward lighter isotopes. Because life prefers lighter carbon, the isotopes suggest to some scientists that the atmospheric rise must be due to extra microbial production, and not a boost due to leaked gas from fracking operations, which has a heavier isotopic signature.
But the lighter carbon could also signal that hydroxyl levels are falling, says Joe McNorton, a climate scientist at the University of Leeds in the United Kingdom. Because OH prefers to react with lighter carbon, having less of it around would allow more of the light, microbial methane to linger in the atmosphere.
It’s clear that methane scientists will need to come together to resolve this debate, as several factors are likely playing a role, says Eric Kort, an atmospheric chemist at the University of Michigan in Ann Arbor. “Trying to use any one single source or sink, or single measurement technique, to define exactly what’s happening is perhaps too simple.” The payoff will be a clearer sense of the future: whether the methane rise is a long-term trend, driven by climate change, or a blip that could reverse next year.