A belching coal plant is easy to identify as a probable greenhouse gas polluter. Coal emissions are point source pollution—like a chemical spill in a stream, the pollution can be traced back to a specific activity at a precise place.
But is measuring the carbon produced at a power plant the best way to monitor emissions? A team of scientists recently took a different approach to estimating carbon dioxide: the bottleneck method. Instead of considering the pollution emitted only at the end use, burning phase of fossil fuel use, the researchers considered all phases: mining, transport, refining, and burning.
Their study identified the worst emissions offenders, and the results were surprising: oil and gas pipelines. The researchers noted that the companies enabling greenhouse gases emissions are most at risk of climate mitigation lawsuits.
The new study, published in Energies, introduces the bottleneck method. “Most of the work that’s been done in this area in the past is looking at kind of end use because that’s where most of the emissions occur,” said Joshua Pearce, a materials and electrical engineering professor at Michigan Technological University and coauthor of the new study.
Using the bottleneck method, all emissions a facility enables are considered in carbon tallies, including extraction, transport, and end use. “Bottlenecks are the limiting factor for a total amount of emissions,” said Pearce.
“As more science provides unquestionable evidence that certain facilities are creating economic harm to others,” Pearce said, the bottleneck approach could identify the biggest offenders. It’s this carbon parsing that will likely become more important in climate mitigation efforts.
The researchers collected publicly available data for the amount of fuel and the emissions caused by those fuels from coal, oil, and natural gas. They also gathered emissions data from the entire life cycle of the fuel, including extraction, transport, and end use.
Natural Gas Pipelines
The bottleneck analysis showed that 9 of the top 10 carbon polluters were oil and gas pipelines (47% and 44%, respectively), while a coal mine took the remaining spot in the top rankings. In comparison, point source methods revealed that the top 10 polluters were oil pipelines (eight spots) and coal mines.
The top nine emitters were unexpected, said Pearce, adding he was especially surprised “that natural gas showed up at all.”
Pearce noted that natural gas has lower emissions per unit energy than coal, so it can seem like a good solution—a bridge fuel—for reducing carbon. “But our study showed that when you step away from point source emissions and go to the bottleneck, it turns out that natural gas pipelines are some of the worst offenders,” said Pearce. “They’re allowing the most carbon emissions.”
“That really shocked me, especially the gas piece,” said Michael Craig, an energy systems professor at the University of Michigan School for Environment and Sustainability who was not involved in the study. “Oil I can see, but I would have expected a coal mine to be the worst.”
For natural gas, the biggest emissions came from pipeline transport. The sheer length of pipelines—the Transcontinental Gas Pipeline (Transco) alone branches into more than 16,900 kilometers (10,500 miles) of pipeline from Texas to New York—means there are lots of places to emit gas.
“Especially for older pipelines and pipeline networks, you get leaking. And methane is a pretty severe greenhouse gas,” said Pearce. He noted that fixing or not fixing leaks can be an economic decision for pipeline owners—minor leaks might cost more to repair than the loss of gas into the air.
“That’s why there’s this intense debate about fugitive emissions,” said Craig. “Especially what is the rate of fugitive emissions along the natural gas supply chain.” He added that when determining the best way to decarbonize fuel systems, the way emissions are measured makes a difference. “When we account for those upstream emissions, do we come up better or worse by burning gas in the natural gas versus coal?”
The majority of these pipelines, almost three quarters, are owned by multinational corporations. For example, of the “companies that are responsible for the most emissions in the U.S., the big one, unquestionably, is Enbridge. And that’s a Canadian firm,” said Pearce. “So you’ve got a Canadian company responsible for roughly 74% of the oil industry’s carbon emissions in the U.S.”
Mitigation and Litigation
Although the researchers said Transco is the worst emissions offender, Craig pointed out that the pipeline also allowed regions to reduce their dependence on coal plants. “The Transcontinental Gas Pipeline is an essential piece of supporting infrastructure that has allowed New England to decarbonize,” he said.
“I think it’s interesting to think about,” said Craig. “If you were designing policies from scratch, where would it be most cost-effective to direct those policies?” He noted that pipelines are still the most efficient way to move oil and natural gas, and at the moment, the need for those fuels isn’t going away.
The bottleneck method may also contribute to the way litigation of polluters proceeds. As droughts, floods, heat waves, and storms intensify with climate change, new climate litigation will go after the worst carbon offenders. Many of these offenders are the multinational corporations of the Global North, which are responsible for 92% of carbon emissions worldwide.
The researchers noted that their bottleneck approach is more effective than point source calculations at capturing a company’s “true liability” for pollution. Legal actions could be focused on companies that are the major polluters through the whole cycle of fossil fuel use.
The bottleneck method could also point out which companies could benefit from mitigation measures. Pearce said their goal was to dig into the life cycle of fossil fuels with a new perspective that might give companies pause when thinking about new infrastructure.
“If [a company is] trying to make the decision ‘Should I build a new pipeline right now?’ the answer is absolutely not,” said Pearce. “Your money is much better invested elsewhere.”
—Sarah Derouin (@Sarah_Derouin), Science Writer