Snaking beneath the surface of many Eastern cities is a network of aging, cast-iron pipes carrying natural gas. The pipes, buried underground, have been shifted for decades by winter freeze-thaw cycles, and some are simply cracked from age. Because of this, some pipes leak.
Just how much gas from those older pipes and their newer replacements in the pipeline distribution system leaks out and rises into the atmosphere, though, is up for debate. Because methane, which makes up about 95 percent of the natural gas in pipelines, is about 25 times more potent as a greenhouse gas than carbon dioxide, the leakage raises a troubling climate question: How clean is natural gas?
The growing role of natural gas in the United States' energy mix makes it more important to quantify the leakage. If that number is significant, it could negate the climate benefit of natural gas -- measured against coal -- unless the leaks are plugged.
"It's outrageous and it's astounding, how little we know [about leaks]," said Nathan Phillips, a Boston University researcher who is working to figure out how much methane is leaking from cities.
Getting accurate measurements of the exact amount of gas leaking from any given city system is difficult. Phillips should know: Last year, his research team found the city of Boston's pipeline distribution system had more than 3,000 leaks.
But though they know there are a lot of leaks, they have yet to determine how much gas is coming out of them. That's what Phillips is working on now. Scientists have proposed other ways of estimating methane emissions from distribution systems, but they all suffer from significant limitations.
Cornell University researcher Robert Howarth and others have suggested using a number the government collects from every gas distribution company in the country. This metric is called "lost and unaccounted for" gas. The federal Pipeline and Hazardous Materials Safety Administration defines it as the difference between what the gas company sends out through its pipeline system and what gets metered at the receiving end.
Missing in action in the gas business
Say the gas company sends out 100 cubic feet of gas. Some of it might leak out of a pipe and into the soil. Cooling temperatures might make the gas contract, so the meter on the other end reads it as less gas. Some might waft through another leaky pipe out of a manhole or a crack in the asphalt, and into the atmosphere.
At the end, only 97 cubic feet get to customers. The missing 3 cubic feet? That's what industry calls "lost and unaccounted for."
According to PHMSA, there are two main reasons for this "lost" gas. The first is leaks. The second is measurement issues caused by inconsistent meters or those temperature and pressure variations that cause meters to measure more or less gas, depending on environmental conditions.
Logically, say Howarth and other researchers interested in how much methane leaks to the atmosphere, a higher lost and unaccounted for percentage would mean more gas is escaping the system and warming the planet.
"If one company reports 4 percent lost gas consistently across years and another reports 1 percent, wouldn't you expect the first company's pipelines to be responsible for more methane leakage to the atmosphere?" asked Robert Jackson, a Duke University scientist who is conducting research into methane losses from cities.
The numbers do vary, and some utilities are consistently higher than others in their percentages of lost and unaccounted for gas.
Southern California Gas Co., the largest gas distribution company in the nation, reported a 0.87 percent loss rate in 2012; in 2011, that rate was 0.84 percent. In comparison, Washington Gas Light Co., which serves the greater District of Columbia, had a 3.65 percent loss rate in 2012; in 2011 it was 4.04 percent.