When scientists talk about the greenhouse gases that are causing global warming, they typically focus on far and away the most important one, carbon dioxide, which is emitted when humans burn fossil fuels or clear forests.

The world is actually making slow progress on CO2 — there’s been no growth in annual CO2 emissions for the past three years, thanks to a slowdown in coal burning in China. That’s not nearly enough to stop climate change (net emissions would have to decline all the way down to zero), but it’s a promising start:

Yet CO2 isn’t the only greenhouse gas around. There’s also methane (CH4), which persists for a much shorter time in the atmosphere, but has a much more powerful warming effect. Over a 20-year period, it’s 86 times as potent at trapping heat as CO2.

And the methane trends are more dire: Two new studies suggest that concentrations of methane in the atmosphere are now rising at their fastest pace in two decades — a rate of increase that’s approaching some of the worst-case scenarios in UN climate projections.

More mysteriously, no one’s exactly sure where all this new methane is coming from, though the expansion of agriculture throughout the tropics remains a prime suspect, particularly new rice paddies and cattle pastures. Methane leaks from growing oil and gas operations may be a smaller contributor here.

“Methane concentrations in the atmosphere were pretty stable in the 2000s,” says Rob Jackson, an earth scientist at Stanford and co-author on both studies. “But in the last decade they’ve gone up ten times faster than they did in 2000-2006, and they’ve gone up faster still in 2014-15.” It’s a crucial piece of the puzzle for figuring out how to stop climate change.

Methane sources are hard to track — though agriculture and oil/gas are two major culprits

It’s relatively straightforward to measure methane levels in the atmosphere via air sampling and satellite monitoring. It’s much harder to figure out where this newfound methane is coming from. Unlike CO2, you can’t just tally up factories and cars.

Methane comes from a boggling array of different sources. Some of it is natural, emitted by soil microbes living in oxygen-poor environments in marshes and wetlands. But most of the recent increase appears to have come from human activities — particularly the continued growth in farming. Cows belch and fart methane, thanks to their peculiar digestive tracts. The flooded soils in rice paddies are rich environments for methane-producing microbes. And methane is a key component in natural gas, which can waft out of oil and gas wells during drilling, or leak from gas pipes.

By analyzing the carbon isotopes of the methane found in the atmosphere, a variety of recent studies have tried to deduce the different sources of the recent methane spike. Jackson estimates that about 60 percent of the rise in methane since 2006 appears to be coming from “biogenic” sources — likely an expansion of agriculture. About one-third appears to be coming from oil and gas operations. The authors visualized the overall “methane budget” like so:

Still, says Jackson, these estimates are imprecise (note the error bars above), and there remains a troubling mismatch between “top-down” atmospheric measurements of methane and “bottom-up” inventories of sources. Different studies disagree on whether fossil-fuel methane emissions are rising or flatlining globally, for instance. And changes in natural methane sinks may be playing a role in the recent spike.

Based on their estimates, the authors of the two new studies note that two-thirds of the rise in methane since 2006 appears to be coming from the tropics — which would, again, point to agriculture:

(Note that the Arctic does not appear to be a major source of methane at this time, though scientists have worried this could change in the future as the planet keeps warming and permafrost melts, releasing more greenhouse gases into the air. But for now, there’s little evidence this is happening yet.)

“Many things are contributing to the increase in methane, and we think it’s agriculture, but we need more work to confirm that,” says Jackson. In particular, one crucial step is developing a more detailed “bottom up” inventory of methane. Scientists also need to better understand methane sinks, the processes that break down methane in the soil and the atmosphere, to develop a more accurate budget. “This is a living paper, and we hope to update it on a regular basis,” he says.

There are ways to curb methane — though CO2 is still the key challenge

If we want to keep global warming at anywhere close to 2 degrees Celsius, the globally agreed-upon climate goal, we’ll almost certainly have to get methane concentrations down. And because methane has so many sources, it’s not easy to control.

But there are important efforts underway. Oil and gas companies can use infrared cameras to track methane leaks and plug them — or capture excess methane and sell it rather than venting it into the air. The Obama administration has ordered US oil and gas companies to monitor and plug leaks from oil and gas wells, though these rules may well get overturned by a Trump administration.

On the farming front, scientists are experimenting with ways to get cows to burp less (really). In some studies, researchers have fed cattle things like infused flaxseed and reduced methane emissions up to 30 percent. Meanwhile, crop scientists are developing new genetically engineered rice varieties that don’t transfer as much methane from flooded paddies into the atmosphere.

That said, scientists agree that carbon-dioxide is still the primary greenhouse gas we should be focused on for global warming. “I would say that methane is important if CO2 is definitely taken care of,” says David Archer, a geoscientist at the University of Chicago who was not involved in these two studies.

Here’s what he means: Because carbon dioxide persists so long in the atmosphere, the level of atmospheric CO2 will affect the Earth’s climate for centuries, if not longer. So we can’t stop global warming until atmospheric CO2 is stabilized, which means getting net annual emissions down to zero. By contrast, the level of atmospheric methane affects today’s climate, but it doesn’t last nearly as long. So methane is mainly important for controlling the peak temperature that global warming ultimately reaches.

Both gases are important in different ways, but zeroing out CO2 — and finding alternatives to the fossil fuels that dominate our energy system — remains the primary task.

Further reading

The Global Carbon Project’s presentation on methane is worth browsing, there are tons of great graphs and data there. You can also read the two new papers on methane, this more detailed analysis in Earth System Science Data and the accompanying commentary in Environmental Research Letters. Or see this op-ed by some of the authors.

Here’s a look at California’s recent efforts to control methane from cow belches.

At the Washington Post, Chris Mooney takes a look at whether a Trump administration might overturn Obama’s rules to curtail methane from oil and gas operations.

A brief history of inaction on climate change