Your natural gas cooking stove may leak climate-warming methane even when it is turned off, warns a new Stanford University study.

That's important because methane is a much more potent greenhouse gas than even carbon dioxide, though it doesn't linger in the atmosphere nearly as long.

Stanford scientists measured methane released from gas cooking stoves in 53 California homes. They examined how much methane is leaked each time you turn the knob in that second before the gas lights on fire. They also measured how much unburned methane is released during cooking. And unlike most previous studies, they measured how much methane is released when the stove is off.

In fact, it turned out that's when about 80% of methane emissions from stoves happen, from loose couplings and fittings between the stove and gas pipes.

"Simply owning a natural gas stove and having natural gas pipes and fittings in your home leads to more emissions over 24 hours than the amount emitted while the burners are on," says Stanford professor of earth sciences Rob Jackson, one of the study's authors.

There are surprisingly very few measurements of this "incomplete combustion from appliances," says Eric Lebel, lead author of the study, which was published Thursday in the journal Environmental Science & Technology. Lebel conducted the research as a graduate student at Stanford and is now a senior scientist at PSE Healthy Energy.

Lebel's research shows it didn't matter if the stove was old or new or what brand it was — the presence of leaks was consistent. There were 18 brands of stoves and cooktops in the study, and they ranged from three to 30 years old. Stoves using a pilot light instead of an electronic sparker leaked more.

Researchers estimate that up to 1.3% of the gas used in a stove leaks into the atmosphere. Individually, that's a tiny climate impact compared with things like coal-fired power plants. But Jackson says if you add up the more than 40 million gas stoves in the U.S., the amount of leaked methane every year has about the same climate change effect as the carbon dioxide from 500,000 gasoline-powered cars.

There's a battle over gas stoves in the push to rein in climate change

The U.S. has a goal of zeroing out emissions by 2050 to comply with the Paris climate agreement. And as communities around the country already face climate change impacts from more severe storms, droughts and wildfires, every emission source is coming under scrutiny.

The stove is special because Americans love "cooking with gas." But the Environmental Protection Agency says buildings account for more than a tenth of the country's greenhouse gas emissions each year.

Climate activists are trying to convince people to switch to electric stoves as part of a broader campaign to stop using natural gas in buildings. They believe that once Americans switch stoves, they'll be more likely to electrify bigger sources of emissions too, such as the furnace, water heater and clothes dryer.

Another argument for switching to electric is that the entire natural gas production and supply chain leaks climate-warming methane from start to finish.

The gas utility industry sees the campaign against gas stoves as an existential threat. Utilities and their trade group, the American Gas Association (AGA), are trying to find cleaner replacements, such as "renewable natural gas" from agriculture, and using hydrogen produced with renewable energy.

The AGA says it has not had time to fully review the Stanford study. Still, the trade group says its members are working to reduce emissions across their networks by doing things like replacing older pipes that leak. "Natural gas distribution system emissions have declined 69% since 1990," says Karen Harbert, the AGA's president and CEO.

Utilities also are getting laws passed to preserve their business. According to the Natural Resources Defense Council, 20 states now have laws on the books that prevent cities from banning gas hookups in new buildings. Such bans have become a trend in places like Seattle, Berkeley, Calif., and New York City, as local governments try to meet their increasingly aggressive climate targets.

One defender of natural gas utilities, Frank Maisano, with the energy law and lobbying firm Bracewell, said the methane leakage issue had not emerged in previous testing, which generally focused on indoor air quality. "Certainly, it is new to hear that emissions occur on stoves that are off. That requires further investigation," Maisano said.

There's an easy way to limit methane leaks

Replacing a gas stove with an electric one is not an option for people who can't afford it or for renters. But Jackson says there's still something you can do and all it takes is a wrench.

"Pull the stove out from the wall and tighten the connectors to the stove and to the nearby pipes," he says. That should reduce the leaks. The AGA recommends that only licensed professionals do maintenance on gas lines and appliances.

Still, Jackson is among those concluding that the only way to ensure there are no leaks is to switch to an electric stove. He says research has convinced him it's time to do that.