Today, this is still somewhat unclear—CO2 emissions are probably the bigger factor, but methane leaks are still important, and recent studies suggest they are likely undercounted.

In the last 15 years, the United States has seen a dramatic rise in production and use of natural gas. Between 2005 and 2022, domestic production of “dry” gas (the very methane-rich natural gas used for energy) almost doubled to over 35 trillion cubic feet per year (1). During that same period, natural gas eclipsed coal as the largest source of U.S. electricity; in 2022, gas accounted for nearly 40 percent of our electricity generation (2).

This switch from coal to gas has often been credited for a big drop in America’s climate-warming carbon dioxide (CO2) emissions. Coal and natural gas both produce CO2 when burned, but coal produces over twice as much CO2 for every kilowatt-hour of electricity it produces (3). From 2005 to 2019, according to the U.S. Energy Information Administration, 65 percent of the fall in emissions from the U.S. electricity sector can be attributed to natural gas replacing coal-fired power plants (4).

But recent research suggests those emissions reductions are inflated, thanks to another, more potent greenhouse gas associated with natural gas: methane.

“We’re definitely under-accounting natural gas’s contribution to global warming relative to coal,” says Desirée Plata, the Associate Professor of Civil and Environmental Engineering and Director of the MIT Methane Network. Official government calculations of natural gas’s contribution to warming don’t account for leaks, says Plata. And in fact, she says, if methane emissions from natural gas are on the high end of current estimates, natural gas may be no improvement over coal at all.

Methane is the main component of natural gas; it makes up 70% or more of raw natural gas in the ground and well over 95% of the processed gas we burn for energy. When burned, this methane turns into CO2—but before then, it can escape into the atmosphere from all parts of gas infrastructure, like valves and pipes. And those leaks, too, need to be counted when we calculate how much natural gas is contributing to climate change.

The U.S. Environmental Protection Agency estimates that about 6.5 million metric tons of methane leak from the oil and gas supply chain each year (5)—around 1% of total natural gas production. At this rate, methane leaks would account for around 10% of natural gas’s contribution to climate change, and CO2 emissions for the other 90% (6).

But other scientists have reported much larger figures for methane leaks. In a 2022 study focused on gas production in New Mexico, a group of Stanford researchers estimated that leaks equated to more than 9 percent of all production in the area, based on aerial surveys (7). A 2023 study suggested methane emissions were 70 percent higher than U.S. government figures from 2010 to 2019 (8). Plata says there’s no current consensus on the magnitude of methane leaks.

“Leaks are so poorly quantified,” Plata says. “Nobody knows that number for sure. It's hard to sense methane comprehensively and finding those pipe-based leaks can be trickier than it sounds.”

Leaks can start and stop irregularly, in different places along the natural gas supply chain, making them hard to spot even as more methane-sensing satellites are put into space. For now, we’re largely dependent on scientists, industry, or citizen volunteers trying to find leaks one at a time, with equipment that is not consistently accurate (9).

To make matters more confusing, even if we knew exactly how much methane was leaking from our natural gas system, experts disagree about how best to quantify the warming from those methane emissions. Methane has a much shorter lifespan than CO2, but traps much more heat while it’s still floating around in the atmosphere. We’ve previously covered the challenge of comparing methane to CO2 at Ask MIT Climate, but in short, the EPA and other organizations usually say methane is about 28 times more warming than CO2—if you look 100 years in the future. Over 10 or 20 years, though, methane is 80 to 100 times more warming than CO2.

To Plata and others, that suggests we should “value” methane more highly than we do today, which—in addition to the leak rate—would also impact how we view natural gas as compared to coal power. “We're a little late to the game for putting the brakes on our greenhouse gas emissions,” she says. “And one of the unique ways that we could start to change the rate of global warming in our lifetime is actually to target short-lived climate pollutants like methane.”

These debates aside, there is consensus that reducing methane leaks would have a significant climate impact. Plata says improvements to our natural gas system are technically possible, but they require political will and enforcement. “Methane emissions from subterranean pipelines are really hard to fix,” she says. “They cost on the order of a million dollars per mile in an unpopulated city and $8 million per mile in a city like Boston. That's fundamentally why we don't see people coming out and replacing those pipelines.”

Lately, U.S. policymakers have taken some steps to address methane leaks. The Inflation Reduction Act of 2022 included new charges on certain natural gas facilities for their methane emissions, eventually rising to $1,500 per ton of methane (10), and the Environmental Protection Agency has proposed additional rules that it says would cut methane emissions 87 percent from 2005 levels by 2030 (11).

Thank you to Ron Kleist of Santa Rosa, California, for the question. You can submit your own question to Ask MIT Climate here.

Published July 17, 2023.

FOOTNOTES

1 U.S. Energy Information Administration: Where our natural gas comes from. Updated May 30, 2023.

2 U.S. Energy Information Administration: What is U.S. electricity generation by energy source? Updated March 2, 2023.

3 U.S. Energy Information Administration: How much carbon dioxide is produced per kilowatt-hour of U.S. electricity generation? Updated November 25, 2022.

4 U.S. Energy Information Administration: Electric power sector CO2 emissions drop as generation mix shifts from coal to natural gas. June 9, 2021.

5 U.S. Environmental Protection Agency: Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2021. 2023.

6 U.S. Environmental Protection Agency: Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2021. 2023. This inventory reports 6.478 million metric tons of leaked methane from natural gas operations (page 3-92), which equates to around 181 million metric tons of CO2 using the Intergovernmental Panel on Climate Change’s standard “GWP-100” conversion factor of 27.9 tons of CO2 for every ton of methane. This is compared to 1,621 million metric tons of CO2-equivalent emissions from burning natural gas (page 3-8).

7 Chen, Yuanlei, et al. "Quantifying Regional Methane Emissions in the New Mexico Permian Basin with a Comprehensive Aerial Survey." Environmental Science & Technology, Volume 56, Issue 7, 2023, doi:10.1021/acs.est.1c06458.

8 Lu, Xiao, et al. "Observation-derived 2010-2019 trends in methane emissions and intensities from US oil and gas fields tied to activity metrics." Proceedings of the National Academy of Sciences, Volume 120, Issue 17, 2023, doi:10.1073/pnas.2217900120.

9 Ravikumar, Arvind P., et al. "Single-blind inter-comparison of methane detection technologies – results from the Stanford/EDF Mobile Monitoring Challenge." Elementa: Science of the Anthropocene, Volume 7, Issue 37, 2019, doi:10.1525/elementa.373.

10 Congressional Research Service: "Inflation Reduction Act Methane Emissions Charge: In Brief." August 29, 2022.

11 U.S. Environmental Protection Agency: "Biden-Harris Administration Strengthens Proposal to Cut Methane Pollution to Protect Communities, Combat Climate Change, and Bolster American Innovation." November 11, 2022.