The role of natural gas as a “bridge fuel” has been called into question by critics who argue that methane leakage rates from gas production and transport could outweigh the lower CO2 emissions associated with gas. However, according to Alex Trembath, policy associate at the Breakthrough Institute, this is not what most of the scientific literature says. “Methane leakage is a minor factor in determining the benefit of gas versus coal and methane leakage levels are well within acceptable ranges.”
Public positions on natural gas are strongly influenced by interpretations of the science on fugitive methane emissions. These vary significantly. In the U.S., the self-identified anti-natural gas wing includes professors like Robert Howarth and popular media figures like filmmaker Josh Fox. Other scholars, such as Cornell’s Lawrence Cathles and Council on Foreign Relations’s Michael Levi, have essentially concluded that fugitive methane is mostly a red herring in the coal-versus-gas conversation, and that natural gas can be a suitable “bridge fuel” in power-sector decarbonization. Institutions like the Environmental Defense Fund concede that natural gas can be an “exit ramp” toward a clean energy future, but insist that fugitive methane must be tightly regulated to ensure that a coal-to-gas transition provides a warming benefit.
This article summarizes the existing literature on natural gas’s climate impact and the available data on methane leakage. While fugitive methane gets most of the press, it appears that other factors – like power plant efficiency and longevity of power plant operations – play a more significant role in determining natural gas’s potential as a bridge fuel.
A look at the literature
How did methane leakage become such a divisive subject? A review of the scientific literature shows that the discourse around fugitive methane has centered around two main controversies.
First is methodology. Comparisons of coal and gas for power generation depend crucially on factors like atmospheric residence of the greenhouse gases, the efficiency of power generation, and the availability of zero-carbon technologies to complement or displace gas in the future. These factors are often overlooked in favor of simple molecule-versus-molecule comparisons of greenhouse gases.
Second is the data. Some papers have used assumptions about leakage rates that significantly exceed leakage measurements in the real world. Various accounts estimate methane emissions from natural gas systems as high as 7 to 9 percent, while a growing literature shows they are more likely in the range of 1 to 4 percent.
For a long time, natural gas was considered an obvious improvement over coal power. Hayhoe et al. 2002 produced a respected analysis that analyzed carbon dioxide (CO2), sulfur (SO2), methane (CH4), and black carbon (BC) emissions and confirmed that the net atmospheric impact of coal-to-gas switching is a cooling effect.
The consensus was challenged by Robert Howarth and colleagues in a more recent paper (Howarth et al. 2011). That paper concluded that natural gas from shales had a higher warming impact than conventional natural gas, and that, “Compared to coal, the footprint of shale gas is at least 20 percent greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.”
Global warming potential
Since 2011, the Howarth paper has attracted lots of attention and lots of criticism. In a 2012 response, Cornell’s Lawrence Cathles charges the Howarth analysis with using estimates for methane leakage that are “unreasonably large and misleading.” For one thing, as Cathles noted, the Howarth paper combines data from a few US Haynesville Shale sites with leak rates from Russian pipelines and extrapolations. A systematic estimate of average methane leakage is implied, but not performed. (See below for more comprehensive estimates of methane leakage from natural gas systems.)
The challenges to the Howarth team’s data collection – essentially that the authors cherry-pick and misrepresent industry totals – is demonstrated in other analyses as well. A 2012 paper published by the National Oceanic and Atmospheric Administration (NOAA) performed a top-down estimate of leakage from natural gas systems, which was likewise found to use flawed data interpretation.
Michael Levi identified serious flaws in the measurement of atmospheric methane, concluding that the data in the NOAA paper actually “results in a new set of estimates that are consistent with current inventories.” Essentially, the NOAA paper authors assume that the gas vented in the fields they studied had high compositions of methane, assumptions that were contradicted by their own data. The NOAA paper is also a case study in extrapolating single-field results to draw system-wide conclusions.
Over the long term, CO2 accumulates in the atmosphere, like mercury in the body of a fish, whereas methane does not
In addition to controversial data, many analyses – including Howarth 2011 – have relied on the “Global Warming Potential” (GWP) methodology for comparing coal versus gas. GWP is the metric used when it is said that methane is 28 times as powerful a greenhouse gas on a 100-year basis and 86 times as powerful on a 20-year basis. It’s a rule of thumb endorsed by the IPCC, but not actually used in any physical models of the global climate. For this reason, Wigley 2011 writes that GWPs “are a poor substitute for a full calculation.”
As Wigley and others have explained, GWP is a molecule-versus-molecule comparison of greenhouse gases. This type of comparison omits important atmospheric dynamics like cumulative radiative forcing, concentration, mixing, and the like. GWPs are further abused by focusing on the 20-year comparison, as Howarth does, and underemphasizing the 100-year comparison, which is much more relevant to discussions about total temperature increases.
The Howarth analysis has been widely criticized, both by authors who agree with its broad policy conclusions (eg, Wigley 2011) and authors who do not (eg, Levi 2013). But that still leaves the broader question on the table: What is the climate benefit of coal-to-gas switching?
Many papers have been published that imply that the rate of methane leakage (in percentage terms) is the determining factor in this question. But looking at the various analyses, it seems that a) power plant efficiency, b) lifetime of the power plants, and c) emissions of other gases like SO2 and BC are just as important, if not more so.
Let’s explore these in the literature. There are a couple of techniques that scholars have used to compare the respective climate impacts of coal and gas. One is by comparing global power plant fleet impacts in major climate-energy models (eg, MERGE, MiniCAM, MINREF, etc.) and another is by comparing prototypical power plants. Both of these compare coal and gas using assumptions about combustion efficient, power plant/fleet lifetime, and system-wide emissions rates in combination with climate modeling. Both of these are more sophisticated and widely employed than the simplistic molecular comparison achieved by the GWP method.
Let’s look at the fleet method first. Myhrvold and Caldeira 2012 build a simple model to compare coal-mitigating technologies on a lifecycle basis, including emissions from construction, operation, waste heat, and transmission in their analysis. They do not perform a sensitivity analysis for methane leakage, assuming across the board in their scenarios that methane and NO2 emissions account for <10 percent of the radiative forcing caused by natural gas power.
Where natural gas is replacing coal – as in the massive coal-to-gas transition in the United States – it is buying time to develop and deploy zero-carbon energy technologies
Their conclusion is that transitioning to natural gas from coal wouldn’t result in “significant” warming benefits for centuries, though gas plants do immediately reduce warming when replacing coal plants in their model. Wigley 2011 similarly models a fleet-wide transition from coal to gas using the MINIREF and MAGICC models, concluding that a coal-to-gas transition in and of itself is not suitable for climate stabilization.
Neither of these papers’ results is surprising: they essentially conclude that zero-carbon replacements for coal fleets mitigate more emissions than natural gas does. What is interesting is that their results are largely independent of methane leakage. Myhrvold and Caldeira do not consider variable leakage rates. Wigley does, but notes, “Even with zero leakage from gas production…the cooling that eventually arises from the coal-to-gas transition is only a few tenths of a degC.” Again, this shouldn’t be surprising, since Wigley’s model assumes that coal generation is cut by 50 percent by 2050, replaced by natural gas that then stays in place for centuries.
What about analyses that model a “gas bridge?” Cathles 2012 builds his own model that he compares to MAGICC results, creating climate stabilization scenarios for 0.9, 2.0, and 2.7 degrees Celsius. He found that “gas is a natural transition fuel because its substitution reduces the rate at which low-carbon energy sources must be later introduced and because it can facilitate the introduction of low-carbon energy sources.” Likewise, Levi 2013 modifies stabilization scenarios in the MiniCAM, MERGE, and IGSM models to create 450-ppm and 550-ppm “gas bridge” scenarios, concluding that “it may be useful to think of a natural gas bridge as a potential hedging tool against the possibility that it will be more difficult to move away from coal than policy makers desire or can achieve.”
Methane leakage does not turn out to be a substantial factor in either of the “bridge” analyses. Levi concludes, “Even high rates of methane leakage do not fundamentally alter the conclusion that replacing coal with gas can substantially lower peak temperatures.” Cathles, similarly, writes: “Under the fastest transition that is probably feasible (our 50-year transition scenario), substitution of natural gas will be beneficial if the leakage rate is less than about 7 percent of production. For a more reasonable transition of 100 years, substituting gas will be beneficial if the leakage rate is less than ~19 percent of production. The natural gas leakage rate appears to be presently less than 2 percent of production and probably ~1.5 percent of production.”
So in the major papers that model a fleet-wide coal-to-gas bridge, methane leakage appears trivial to the conclusions in comparison to other factors.
A similar but slightly different type of analysis compares a single natural gas power plant to a coal power plant and is exemplified by a recent paper by Xiaochun Zhang and coauthors (Zhang et al 2014). That paper compares a) a “typical” natural gas power plant and the “most efficient” natural gas power plant to b) a “typical” coal plant and the “most efficient” coal plant. The authors model methane leakage rates between 0 and 9 percent, and in almost no circumstances do the warming impacts of the natural gas plant exceed the warming impacts of the coal plant after 100 years.
Even assuming high rates of methane leakage, the assumed efficiency of the various modeled power plants outweighs the impact of methane leakage. As the authors write: “By the end of the century, the most efficient natural gas plant is producing 6.3 percent-35.0 percent less warming than the most efficient coal plant, and 40.0 percent-58.4 percent less warming than the typical coal plant, depending on the methane leakage rate in the 0 percent-9 percent range.”
So assumptions about power plant efficiency yield a 33.7 percent difference (40.0 percent minus 6.3 percent) in the modeled warming impact compared to a 28.7 percent difference (35.0 percent minus 6.3 percent) as a result of different levels of methane leakage. As these results show, in all cases, a coal-to-gas transition reduces long-term atmospheric warming.
Hausfather 2015 reaches a similar conclusion, noting that GWPs are an imperfect methodological tool for comparing coal and natural gas. Since a primary unit of natural gas generates more final energy than a primary unit of coal, the “molecule-for-molecule” GWP methodology is flawed – an insight also found in the work of Cathles, Levi, among others. As Hausfather writes, “With no [zero-carbon energy] delay, leakage rates could be 5.2%–9.9% for same 100-yr mean forcing. At 2% leakage over a 100-yr forcing period, gas could be used 1.5–2.4 years per year of coal displaced.”
Likewise, climate scientist Raymond Pierrehumbert drew this conclusion: “Over the long term, CO2 accumulates in the atmosphere, like mercury in the body of a fish, whereas methane does not. For this reason, it is the CO2 emissions, and the CO2 emissions alone, that determine the climate that humanity will need to live with.”
Real-world leakage rates
The sensitivity to methane emissions in Zhang et al 2014 is exaggerated by assuming much higher rates of fugitive methane (as high as 9 percent) than have been observed at a wide scale in the United States. (NB: Their “exaggeration” can be construed as perfectly reasonable within their analysis to demonstrate the sensitivity of their results.)
In its 2014 Greenhouse Gas Inventory, the EPA (Environmental Protection Agency) estimated that total methane emissions from the natural gas sector have fallen 17 percent since 1990 even as production rose by over 40 percent. Current leakage stands at 1.8 percent nationally according to the Inventory.
As Levi writes in his 2013 paper, “Most recent publications have indicated that leakage in the United States is likely to be 1 to 2 percent and have all but rejected the possibility of leakage on the order of 5 percent.” Several major reviews (Miller et al. 2013, Brandt et al. 2014, and Schwietzke et al. 2014) find that system-wide leakage is likely under 4 percent and, as Schwietzke et al 2014 write, “trending downward.”
The two broad conclusions from the literature – that methane leakage is a minor factor determining the benefit of a coal-to-gas transition and that methane leakage levels are well within acceptable ranges – appear to be getting more robust with each added analysis.
The policy conclusion from the literature then is that natural gas can and is acting as a bridge fuel to a clean energy future in the power sector. Government and industry can and are making efforts to reduce methane leakage, and this is encouraging. In the mean time, where natural gas is replacing coal – as in the massive coal-to-gas transition in the United States – it is buying time to develop and deploy zero-carbon energy technologies. How quickly those technologies become cheap and scalable is the most important factor determining how strong our climate action will be. Fugitive methane doesn’t come close.
Alex Trembath is a policy associate in the Energy and Climate Program at Breakthrough. He is the lead or co-author of several Breakthrough publications, including the 2012 report “Beyond Boom and Bust: Putting Clean Tech on a Path to Subsidy Independence” and “Where the Shale Gas Revolution Came From. This article was first published by The Breakthrough Institute and is republished here with permission.