A new study says the amount of methane being released from the natural gas system into Boston's atmosphere is six times higher than estimates used by the state Department of Environmental Protection.
Methane is a powerful greenhouse gas that warms the climate 80 times more than carbon dioxide in its first 20 years in the atmosphere. And despite laws mandating utility companies repair leaky natural gas pipelines, the research indicates methane emissions did not decrease between 2012 and 2020.
The study, conducted by scientists at Harvard University and Boston University, was published Monday in the journal Proceedings of the National Academy of Sciences.
The scientists used a different approach to measure methane than the traditional method — and one that they say is more accurate. Methane emissions from natural gas infrastructure are usually measured in what the researchers call a "bottom-up" approach.
"They add up what they think the loss is from each compressor station, each mile of pipe, each appliance, your heater in your house," said research scientist Maryann Sargent of Harvard University.
But, she said, studies have shown that just 7% of serious leaks emit half of the overall gas emissions to the atmosphere.
"So if these accounting methods that the state uses don't find enough of those big emitters, they can be significantly undercounting the emissions," Sargent said.
For their study, the Harvard and B.U. researchers used a "top-down" approach by measuring methane in the atmosphere.
"This is a lot better in terms of methane because you can't miss any sources," Sargent said. "Everything is going to get mixed together in the atmosphere."
The researchers installed sensors on the top of buildings at Boston University and in Copley Square. They then compared those recorded emissions to results from three spots outside the city: at Harvard Forest in Petersham, in Mashpee, and at a site in Canaan, New Hampshire. The sensors ran continuously from September 2012 to May 2020.
"We found that the emissions were about six times higher than the emissions number the state is currently using," Sargent said.
The study also found no change in emissions over the eight years of the study, despite state laws passed in 2014 and 2018 requiring gas companies to repair pipeline leaks in a timely manner.
"The goal of those laws was to reduce emissions from these pipelines, and we haven't seen any impact of that when you look at the atmosphere," Sargent said.
As soon as a leak is repaired, another one seems to emerge, said Lucy Hutyra, a professor of earth and the environment at Boston University, and one of the study's authors.
"It's a bit of a game of whack-a-mole," Hutrya said. "They're certainly getting them, but they just keep coming."
The Northeast Gas Association, which represents natural gas utilities, says natural gas serves 1.7 million customers in the Commonwealth, including over half of households.
"We agree that mitigating methane emissions from natural gas systems is a priority for the Boston area," said Charles Crews, President and CEO of the Northeast Gas Association. "Incorporating new assessments is always helpful."
Crews said 1,800 miles of pipeline were replaced in the state during the eight-year period of the study.
"The most effective and efficient way to reduce methane emissions from gas leaks is continued support of the replacement of leak prone pipe through the Gas System Enhancement Program," Crews said. "The natural gas companies have also been eliminating the largest leaks from their systems in accordance with the same law. The utilities have more work to do and are committed to reducing leak-prone pipe to help the state meet its efforts to reduce carbon."
The researchers say the data suggest that the gas leaks are coming from a variety of sources. The pressure in pipelines is relatively constant, so emissions shouldn't increase as consumption goes up. But the researchers found nearly half of the total emissions varied based on seasonal changes in consumption.
"And so this points to sources such as things in your buildings beyond the meter — so your boilers, your furnaces, water heaters," Sargent said. "It could possibly be from the transmission sector, from meters or compressing stations. But this is an area that really is not very well studied, and we need to look closer as to where are the losses coming from in those sectors and how can we target them."
"These losses are not a surprise to anybody, but there hasn't been an emphasis on reducing them," Hutyra said. "It's an engineering problem and it's a maintenance problem, but it's one that is eminently solvable."
But methane emissions persist, she said.
"We've been trying to address this problem, and I think in Boston, we've been trying more aggressively than in many other locales," Hutyra said.
And yet, she said, this study suggests those policies aren't working.
"It's really highlighting that we have more work to do," she said. "And when we think about our climate goals for the city and for the state, the methane problem cannot be swept under the rug. We need to we need to tackle it, and we need to tackle it more aggressively than we have so far. "
