Methane emissions from five major US cities are twice what we previously thought and contributing to climate change, a new study shows.
According to a paper by researchers from the National Oceanic and Atmospheric Administration (NOAA) and the University of Michigan that was recently published in Geophysical Research Letters, leaky pipes are likely sending gas into the atmosphere. Methane is a potent greenhouse gas that, like carbon dioxide, warms the planet in high concentrations.
The researchers flew kitted-out planes over the east coast of the US, tracing urban emissions upwind and downwind of the biggest cities. They took 20 flights in NOAA’s Twin Otter aircraft, totaling 120 flight hours. Equipped with instruments to measure methane, carbon dioxide, carbon monoxide, and ethane, they surveyed Washington, Baltimore, Philadelphia, New York, Providence, and Boston.
According to Eric Kort, co-author and University of Michigan atmospheric scientist, past studies on individual cities have shown that urban centers often have higher methane emissions than reported. His team took that research a step further, quantifying this trend across the five biggest areas surveyed (excluding Providence).
Based on their findings, the researchers estimated that these five large cities—which include about 12 percent of the US population—emit over 900,000 tons of methane each year. Recent Environmental Protection Agency (EPA) estimates placed methane emissions at a little over 400,000 tons, the study said, or half this new estimate.
The researchers attribute about 820,000 tons of those emissions to natural gas leaking, which isn't properly factored into EPA estimates, according to the study authors. To separate methane emissions from natural gas from other sources such as landfills, the researchers tracked ethane, which is found in natural gas but not other methane sources.
The studied cities have old infrastructure, complete with leaky pipes and inefficient appliances, the study authors say. As natural gas (which contains methane) pumps through cities, some is inevitably lost to the atmosphere. Even without decrepit infrastructure, natural gas can leak methane into the atmosphere—for example, when you turn your stove on and it clicks for a few seconds before a flame appears.
Methane turns into carbon dioxide when burned, which is actually less harmful to the environment. Because methane is a more potent greenhouse gas, the impact of letting that natural gas directly into the atmosphere is a lot harsher than if it was used for fuel.
Natural gas is often painted as a cleaner alternative to coal or oil based on carbon dioxide emissions, but there are serious impacts to letting the methane-rich gas into the atmosphere. Just this week, Berkeley just became the first city to ban natural gas in new homes, hoping to move toward an electric-powered city.
“We oftentimes notice that emissions tend to be higher than what they were thought to be, but they’re often addressable problems,” Kort said. “These findings show opportunities for mitigation.”