A new analysis published in Science today ($ub. required) concludes that more methane is leaking from natural gas wells and pipelines than the federal government has estimated, eroding some of the climate benefits of the cleaner-burning fuel.

The sixteen researchers — from Stanford, the National Renewable Energy Laboratory, University of Michigan, MIT and elsewhere — reviewed more than 200 studies estimating how much methane, a potent greenhouse gas, escapes into the atmosphere. The panel concludes that actual methane emissions are 25 to 75 percent higher than the estimates published by the Environmental Protection Agency's national Inventory of Greenhouse Gas Emissions and Sinks

That said, the ability of natural gas to help reduce emissions of the greenhouse gases (GHGs) responsible for climate change hinges on the sector in which it used: the ongoing shift from coal to natural gas in the electric power sector continues to have "robust climate benefits" the authors conclude, while using natural gas as a transportation fuel in place of diesel or gasoline is more suspect.

With those mixed findings, the report is sure to add fuel to both sides of the debate over the net benefits of America's growing reliance on natural gas, which has been touted as either a "bridge fuel" towards a lower-carbon energy system or the next big environmental scourge.

Let's dig into four big takeaways from the new Science study.

1. Methane leaks are worse than EPA estimates, but fracking isn't the main culprit

The study surveys 20 years of research on methane emissions from the nation's energy infrastructure and concludes that bottoms-up inventories of the kind employed by the EPA routinely undercount emissions rates.

That's a big deal, because "methane as a molecule is a very potent greenhouse gas — about 30 times more potent than carbon dioxide on a 100-year basis, and much more so over a shorter-term basis," explains Francis O'Sullivan of the MIT Energy Initiative, one of the study's authors.

As a result, "even small leaks can have a very significant impact on climate change," O'Sullivan says. 

The EPA's inventories are constructed by sampling emissions rates at a small subset of equipment used to produce or transport natural gas, oil, or coal, and then scaling up those samples to derive an estimate for the nation's full energy infrastructure. As a result, these inventories produce estimates with a wide uncertainty range, which has been well known.

The new study concludes that there are at least four reasons to believe EPA's estimate consistently undercount methane emissions as well:

  • First, the sampled devices are often older and not representative of modern techniques used in the energy sector, including hydraulic fracturing and horizontal drilling, which were not widely employed during much of the sampling period used to estimate the EPA emissions factors.
  • Second, collecting samples is expensive and tends to be provided by "self-selecting cooperating facilities," which means the sample size is small and populated by estimates volunteered by precisely those firms most likely to have strong environmental practices in place.
  • Third, there is considerable evidence that overall methane emissions are driven by a few "super-emitters" (more on that later), in which case sample methods assuming a normal bell-curved distribution of emissions rates won't capture the "heavy tails" really at play.
  • Fourth, the quality of the data and device counts used to construct the nation-wide estimates are incomplete and "of unknown represenativeness" as well.

Based on a survey of national-scale atmospheric measurements of methane levels, the study concludes that total emissions are actually 1.25 to 1.75 times higher than the EPA's bottoms-up inventory reports.

The author's best guess: U.S. methane emissions are 50 percent higher than the EPA's estimates.

So is fracking to blame for the excess emissions? Actually, no.

"[H]ydraulic fracturing for [natural gas] is unlikely to be a dominant contributor to total emissions," the study states. 

The recent rise of hydraulic fracturing, or "fracking," is likely responsible for only about 7 percent of the excess methane, the authors estimate. 

"This is a lot of methane — it’s not trivial," says Stanford's Adam Brandt, the study's lead author. "But this doesn’t appear to be the main contributor. The math just doesn’t work out."

Excess emissions are more likely coming from a broad range of activities across the oil and gas sectors, as the following figure illustrates.

Methane Leaks from U.S. Oil and Gas Infrastructure

Fig. Methane Leaks from U.S. Oil and Gas Infrastructure 
EPA estimate in blue, literature estimates in red with central estimate and uncertainty ranges. Source

2. Switching from coal to natural gas for electricity generation still offers "robust" climate benefits

Despite higher estimates of fugutive methane emissions from across the natural gas system, switching from coal to gas in power plants is still a net boon for the climate, the study finds.

"Although our study found natural gas leaks more methane than previously thought, the shift to natural gas is still a positive move for climate-change-mitigation efforts," says MIT's O'Sullivan. 

Burning natural gas to produce electricity produces 40 to 60 percent less CO2 than a coal-fired power plant, and an historic shift from coal to gas in the electric power sector has helped drive down U.S. energy-related CO2 emissions to their lowest levels since 1994.

But emissions at the smoke stack are only part of the story, and any methane emissions upstream undermine the net benefits of switching from coal to gas.

Still, replacing EPA's estimates with the higher range of methane emissions estimates from the study "still supports robust climate benefits from [natural gas] substitution for coal in the power sector over the typical 100-year assessment period," the authors write in Science.

3. Using natural gas as a transportation fuel may not hold any climate benefits.

While using gas in the power sector can help cut total U.S. greenhouse gas emissions, running our cars, trucks, and buses on natural gas may not help climate matters much, the study finds.

"Substituting natural gas for gasoline appears to yield no appreciable climate benefits," says O'Sullivan. "In the case of diesel fuel, switching to natural gas would actually be a negative change for climate efforts."

That's because using compressed natural gas (CNG) to fuel our vehicles produces only 30 percent less CO2 than burning diesel. With a narrower CO2 benefit to begin with, the study's higher estimates for methane leaks would more than offset the net climate benefits of CNG-fueled buses and trucks.

It's worth noting that all those CNG-fueled buses and dump trucks running around American cities do deliver real improvements in air quality over their diesel-fueled brethren. Those health benefits have been the real motive behind the shift to buses and other urban heavy-duty fleets running on "clean burning natural gas."

Still, the climate implications of this study are an important finding, as the nation considers an even greater reliance on natural gas as a substitute for oil in the transportation sector.

In his 2014 State of the Union address, President Obama declared his support for "building fueling stations that shift more cars and trucks from foreign oil to American natural gas."

Yet whatever the benefits for air quality or energy secutiry, "that’s not a good policy from a climate perspective" said Stanford's Brandt.

4. A few "super-emitters" are responsible for the bulk of methane leaks, pointing the way to reducing climate impacts

While the study paints a darker picture of the net climate impacts of natural gas, "opportunities abound" to reduce methane leaks and improve that picture, the authors write.

Better regulations could improve reporting requirements and clamp down on methane leaks — often just by requiring techniques that are already profitable but not uniformly adopted across the industry. That includes reduced emissions completions of oil and gas wells, also known as "green completions." The EPA was set to require green completions at all hydraulically fractured natural gas wells, but delayed implementation of the rule until 2015.

"This report justifies EPA taking action on regulation of methane pollution and to focus that regulation on existing wells," said Mark Brownstein, chief counsel for the American climate and energy program at the Environmental Defense Fund.

The other major opportunity comes in finding better ways to identify, and halt, a few large emitters, which the study finds are responsible for a disproportionate amount of methane emissions.

For example, one study surveyed by the Science paper measured emissions rates at about 75,000 components and found that more than half (58 percent) of overall emissions came from 0.06 percent of possible sources.

Or in Science-speak: "The heavy-tailed distribution of observed emissions rates presents an opportunity for large mitigation benefits if scientists and engineers can develop reliable (possibly remote) methods to rapidly identify and fix the small fraction of high-emitting sources."

Eliminating methane leaks from these "super-emitters" will be key to unlocking the potential climate benefits of natural gas.

"These super-emitters are often simply a mistake, like someone left a hatch or valve open in the system," explains MIT's O'Sullivan. That means "you can have a very significant impact on reducing fugitive emissions if you can more effectively sample, sense the system, identify those super-emitters, and then deal with the problem."