In recent years, as the natural gas boom has led to the fuel playing an increasing role in the U.S. energy mix, a debate has been raging over its climate benefits.
A number of studies measuring emissions of methane, a key component of natural gas with 30 times the warming potential of CO2, have measured significant leakages of the gas.
If such leaks are common, the climate benefits of burning natural gas rather than coal diminish.
"Methane is a potent greenhouse gas," said Adam Brandt, a Stanford University professor who studies methane leaks. "So relatively small leaks can have a significant impact on the overall greenhouse gas intensity of using natural gas."
Recent studies have looked at methane emissions on a variety of scales, from measurements across the entire United States to on-site leak rates from various components in the drilling process. Some studies, particularly in Los Angeles and Utah, have found very high leak rates, as much as 17 percent (ClimateWire, May 15, 2013), while others have not.
Because of this, Brandt said, it has been difficult to get an overall picture of methane leaks in the United States (ClimateWire, Aug. 1, 2013).
In a policy paper published yesterday in Science, Brandt and his co-authors, all experts in various aspects of methane leak measurement, synthesized the results of those varying studies.
Could be 50% higher than EPA's measure
They found that methane emissions overall are likely 50 percent higher than U.S. EPA inventories, with an uncertainty range of 25 to 75 percent. This includes methane from all sources, natural and human-caused.
"This amounts to 7 [million] to 21 million tons of [excess] methane per year with a central estimate of 14 million tons," Brandt said.
The researchers said that although they also believe emissions from the natural gas industry are higher than EPA emissions inventories state, there are not enough data at this point to know exactly how much leaks from natural gas contribute to excess methane in the atmosphere.
"There is not enough scientific evidence to give a firm answer about how much of the problem is being contributed by the natural gas industry," Brandt said.
EPA estimates a total leak rate of 1.5 percent from the natural gas industry; in a figure accompanying the article, the researchers noted that six out of 10 other studies have found natural gas system leak rates higher than the EPA rates; the rest found similar leak rates. No study in their review found a lower rate of leakage.
EPA did not respond to requests for comment sent early this week, but the scientists said they were "in discussion" with EPA scientists who expressed interest in the research.
The disparities between emissions inventories and the amount of methane measured in the atmosphere likely come from a number of factors. First, much of the sampling used to calculate leak rates took place in the early 1990s, and many practices in the natural gas industry have changed since then.
Second, since it is expensive to make the leak measurements used to calculate emissions inventories, there are not very many of them, so the sample size is small.
Where are the 'super-emitters'?
Third, if a small number of sources, known as "super-emitters," are likely responsible for a large fraction of the methane leaking from the natural gas industry, those would be easy to miss when sampling.
The scientists said the research to date leads them to believe super-emitters are likely responsible for a large fraction of the leaking methane, which may account for the findings of very high leak rates in certain areas.
This provides both a challenge and an opportunity, they said.
"If the super-emitter argument is correct, which maybe it is -- we don't know for sure -- those are situations where almost surely somebody can invest something and get their money back," said Robert Socolow, a professor emeritus at Princeton University who has conducted research across much of the energy sector.
If it is the case that a few leaky pieces of equipment or certain practices are leading to a large number of the leaks, that provides an opportunity to quickly reduce the amount of leaking methane and probably save the gas industry money, scientists said.
"There are fewer places you have to repair or correct to get a large positive benefit to reduce emissions," said Steven Hamburg, chief scientist at the Environmental Defense Fund, who is coordinating a large number of studies aimed at understanding leakage from the natural gas system.
On the other hand, since super-emitters tend to be distributed randomly throughout the natural gas system, there is a challenge in identifying them, Brandt said.
Brandt added he is working on a project that aims to reduce the cost of identifying natural gas leaks by a factor of 100.
"We need to give [industry] a cheap way to find these leaks," he said.
Even with a higher leak rate than estimated by EPA, the researchers calculated that on a 100-year time scale, compared with burning coal, natural gas is better for the climate.
They also pointed out, however, that substituting natural gas for diesel fuel in bus fleets, as many cities have already done, is likely not better for the climate, although natural gas has other benefits, such as burning cleaner.
"There's all sorts of reasons we might want to substitute away from diesel buses, but from a climate perspective, it appears that it is not going to be a big win," Brandt said.
Shorter-lived, but still troublesome
While the paper makes a case for the substitution of natural gas for coal as beneficial, the use of a 100-year time scale raised questions in the minds of other researchers, including those who have investigated the impacts of short-lived climate pollutants.
These pollutants, which include methane, do not stay in the atmosphere very long but can have significant effects on the rate of warming.
Late last year, a different policy forum paper in Science, led by researchers Julie Shoemaker and Daniel Schrag at Harvard University, argued that the decision to focus on short-term versus long-term climate pollutants missed the point and that both time scales must be considered when making policy on greenhouse gases.
Drew Shindell, a NASA researcher who has focused on the impacts of short-term climate pollutants and who published a 2012 paper in Science on that topic, said by looking only at the 100-year time scale, the paper was "lacking in perspective."
"[It] doesn't really weigh in on the rationale for emphasizing methane as a way to reduce near-term climate change and improve human and ecosystem health, as discussed in my 2012 Science paper, which I think still holds," Shindell wrote in an email.
Robert Howarth, a professor at Cornell University who was one of the first to raise questions about the climate change impact of leaks from natural gas, also faulted the paper on this front, particularly given that the Intergovernmental Panel on Climate Change recently stated that methane's impacts ought to be considered on shorter time scales.
"This new paper ignores that guidance from the international science community, and in doing so, highly biases their comparison of natural gas and coal. Had they used the shorter time frames, they would have reached a fundamentally different conclusion: Natural gas is worse than coal for climate warming," Howarth wrote in an email.
Need for national standards
But from the perspective of rounding up what is known to date on methane leakages in the United States, David McCabe, an atmospheric scientist with the Clean Air Task Force, a group that works on air pollution and climate change, praised the study as "quite comprehensive."
The value in a synthesis paper like this one is that it places all the past research done at a wide range of scales with seemingly different findings on a common base line, in order to compare them with one another, McCabe said.
McCabe agreed with the paper's finding that some of the very high leak rates measured over natural gas fields were unlikely to represent the industry as a whole, simply because multiplying that leak factor industrywide would lead to more methane in the atmosphere than has actually been measured.
He also pointed out that since some parts of the natural gas system were found to be far less leaky than others, it is clearly possible for the industry to produce and transport gas without a lot of leaks -- but that standards that result in low leakage need be applied nationwide, similar to how catalytic converters in vehicles were required nationally to reduce pollution.
McCabe also said the question of whether natural gas is better for the climate than coal misses the point to some degree. "In the big picture, gas is not good enough for climate without carbon capture and storage," he said.
"Cleaner than coal isn't nearly clean enough, and that's not the only question we should be asking about emissions from natural gas."
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