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.

Methane is a little harder to get a handle on than CO 2 , 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 CH 4 .

Past efforts have identified likely causes of methane’s early-2000s plateau, but the post-2006 rise has remained pretty enigmatic. Was it the product of growing natural gas production because of fracking? Or evidence of changes in the Arctic?

A new study from a group led by Hinrich Schaefer of New Zealand’s National Institute of Water and Atmospheric Research compiled as many methane measurements as possible, even including ice core samples to go back to the year 1700. Using a simple numerical model, they twiddled knobs for various methane sources to find combinations of changes that could explain the changes in methane concentrations and isotopic fingerprints.

They examined the early-2000s plateau first. The simplest possibility, a leveling off of all emissions in the 1990s, would not be able to explain the way carbon-13 became a little more scarce in methane at the same time. But the trends could be explained by large reductions of natural gas leakage, perhaps due to the production drop-off (or pipeline improvements) following the collapse of the Soviet Union. That’s the same conclusion others have reached—supported by the fact that atmospheric ethane, which comes almost entirely from natural gas leakage, also declined.

You could also explain the methane trends with combined reductions of microbial and biomass burning emissions, which have less and more carbon-13, respectively, than natural gas leakage. But those two sources tend to vary in opposite directions—El Niño conditions tend to reduce tropical wetland emissions, for example, while increasing wildfires.

For the more enigmatic rise in methane since 2006, things are fuzzier. While the concentration rose, the proportion of carbon-13 decreased. That means the sources of that increase must be pretty light on carbon-13. That points the finger at methane produced by microbes, which favor the lighter carbon-12 isotope. That would rule out much of a contribution from increased natural gas leakage.

The trouble is that microbial methane can come from very different activities, including natural wetlands, agriculture, and thawing permafrost. The researchers attempt to dig out some clues, noting that the isotopic changes show up in the Arctic later than they do in the tropics, and satellite measurements show the biggest action in the tropics as well. As a result, the researchers figure that La Niña rains in recent years probably provided a boost to natural wetland emissions along with increases in agricultural emissions—particularly Chinese and Indian livestock and Southeast Asian rice paddies.

The study emphasizes this explanation may not be exactly right, but it does fit their data best and makes sense in light of other research. And it’s better news than finding that something big had changed in the Arctic.

The researchers point out that while you might have guessed the methane plateau was just a blip in a long-term trend, it actually represents “a reconfiguration of the CH 4 budget.” The story in 2016 is different than it was in 1996, and we’ll be keeping an eye on it to see how it changes in the future.

Science, 2016. DOI: 10.1126/science.aad2705 (About DOIs).