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Study suggests current levels of methane leakage would result in numerous decades of more rapid climate change from a shift to natural gas vehicles

Technology warming potential (TWP) for three sets of natural gas fuel-switching scenarios. (A) CNG light-duty cars vs. gasoline cars; (B) CNG heavy-duty vehicles vs. diesel vehicles; and (C) combined-cycle natural gas plants vs. supercritical coal plants using low-CH4 coal. A TWP >1 means that the cumulative radiative forcing from choosing natural gas today is higher than a current fuel option after t yr. Source: Alvarez et al. Click to enlarge.

Because of methane leakage from the natural gas infrastructure, a shift to natural gas vehicles from gasoline or diesel vehicles could lead to greater radiative forcing of the climate—i.e., increased warming—for 80 or 280 years, respectively, before beginning to produce benefits, according to a new open access paper published in the Proceedings of the National Academy of Sciences (PNAS). However, natural gas vehicles could produce climate benefits on all time frames if the well-to-wheels CH4 leakage were capped at a level 45–70% below current estimates, the authors suggest.

By contrast, using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, and reducing CH4 losses from the production and transportation of natural gas would produce even greater benefits, they found.

Recent reports in the scientific literature and popular press have produced confusion about the climate implications of natural gas. On the one hand, a shift to natural gas is promoted as climate mitigation because it has lower carbon per unit energy than coal or oil. On the other hand, methane (CH4), the prime constituent of natural gas, is itself a more potent GHG than carbon dioxide (CO2); CH4 leakage from the production, transportation and use of natural gas can offset benefits from fuel-switching.

The climatic effect of replacing other fossil fuels with natural gas varies widely by sector (e.g., electricity generation or transportation) and by the fuel being replaced (e.g., coal, gasoline, or diesel fuel), distinctions that have been largely lacking in the policy debate. Estimates of the net climate implications of fuel-switching strategies should be based on complete fuel cycles (e.g., “well-to-wheels”) and account for changes in emissions of relevant radiative forcing agents. Unfortunately, such analyses are weakened by the paucity of empirical data addressing CH4 emissions through the natural gas supply network, hereafter referred to as CH4 leakage. The U.S. Environmental Protection Agency (EPA) recently doubled its previous estimate of CH4 leakage from natural gas systems.

—Alvarez et al.

Using EPA’s estimate of CH4 emissions from the natural gas supply, the authors—Ramón A. Alvarez, Environmental Defense Fund (EDF); Stephen W. Pacala, Princeton University; James J. Winebrake, Rochester Institute of Technology; William L. Chameides, Duke University; and Steven P. Hamburg, EDF—assessed the climate-forcing implications of three US-specific fuel-switching scenarios: from gasoline, diesel fuel, and coal to natural gas.

The authors used technology warming potentials (TWPs) rather than global warming potentials (GWPs) to compare the cumulative radiative forcing created by alternative technologies fueled by natural gas and oil or coal.

GWPs were established to allow for comparisons among GHGs [greenhouse gases] at one point in time after emission but only add confusion when evaluating environmental benefits or policy tradeoffs over time. Policy tradeoffs like the ones examined here often involve two or more GHGs with distinct atmospheric lifetimes. A second limitation of GWP-based comparisons is that they only consider the radiative forcing of single emission pulses, which do not capture the climatic consequences of real-world investment and policy decisions that are better simulated as emission streams.

To avoid confusion and enable straightforward comparisons of fuel-technology options, we suggest that plotting as a function of time the relative radiative forcing of the options being considered would be more useful for policy deliberations than GWPs. These technology warming potentials (TWP) require exactly the same inputs and radiative forcing formulas used for GWP but reveal time-dependent tradeoffs inherent in a choice between alternative technologies.

—Alvarez et al.

Their TWP approach extends the standard GWP calculation in two ways: by combining the effects of CH4 and CO2 emissions from technology-fuel combinations and by considering streams of emissions in addition to single pulses. Considering streams of emissions is more reflective of real-world scenarios that involve activities that occur over multiyear time frames, they suggest.

Using this approach, they found that converting a fleet of cars from gasoline to CNG would result in decades of more rapid climate change because of greater radiative forcing in the early years after the conversion. This would eventually be offset by a modest benefit. After 150 years, a CNG fleet would have produced about 10% less cumulative radiative forcing than a gasoline fleet: a benefit equivalent to a fuel economy improvement of 3 mpg in a 30 mpg fleet. CNG vehicles fare even worse in comparison to heavy-duty diesel vehicles.

However, in contrast to the transportation cases, a fleet of new, combined-cycle natural gas power plants reduces radiative forcing on all time frames, relative to new coal plants burning low-CH4 coal—assuming current estimates of leakage rates (Fig. 1C). The findings differ primarily because of coal’s higher carbon content relative to petroleum fuels; however, fuel-cycle CH4 leakage can also affect results. This study does not consider potential changes in sulfate aerosols and black carbon from the fuel-switching scenarios.

The authors drew three main conclusions from their study:

  • The TWP approach offers policymakers greater insights than conventional GWP analyses. GWPs are not sufficient when thinking about fuel-switching scenarios, the suggested. TWPs provide a transparent, policy-relevant analytical approach to examine the time-dependent climate influence of different fuel- technology choices.

  • Improved science and data are needed. Despite recent changes to EPA’s methodology for estimating CH4 leakage from natural gas systems, the actual magnitude remains uncertain and estimates could change as methods are refined. Ensuring a high degree of confidence in the climate benefits of natural gas fuel-switching pathways will require better data than are available today.

    Reductions in CH4 leakage are needed to maximize the climate benefits of natural gas. While CH4 leakage from natural gas infra- structure and use remains uncertain, it appears that current leakage rates are higher than previously thought. Because CH4 initially has a much higher effect on radiative forcing than CO2, maintaining low rates of CH4 leakage is critical to maximizing the climate benefits of natural gas fuel-technology pathways. Significant progress appears possible given the economic benefits of capturing and selling lost natural gas and the availability of proven technologies.


  • Ramón A. Alvarez, Stephen W. Pacala, James J. Winebrake, William L. Chameides, and Steven P. Hamburg (2012) Greater focus needed on methane leakage from natural gas infrastructure PNAS doi: 10.1073/pnas.1202407109



In UK there is leakage out of low pressure gas pipelines. These were built pre 1960 and around 0.5% of the methane leaks out. The grid is progressivly being replaced with new plastic pipes. However, the leakage is not related to throughput. So adding CNG vehicles does not increase the leakage.

In practice, CNG stations are usually fed from higher pressure grids that are newer, steel, do not leak at what this study is saying is that there should be investment in replacing leaking pipelines in the domestic heating market. No argument with that, but this has nothing to do with CNG for vehicles which reduces CO2 with NO increase in leaks of methane.


So the argument goes that CNG should not be used for vehicles because humans don't know how to handle CNG without having a whole bunch of it leak into the atmosphere. It suggests immediately that we could have a significant impact on it's release and thus climate change by fixing our infrastructure, and that humans are rather inept at very simple tasks. We've apparently become to stupid to survive, because whatever we do, we do it in a very cheap and sloppy way.

Account Deleted

Frankly, the argument that we should not substitute expensive oil and dirty coal with cheap and abundant natural gas that burns far more clean because we can’t prevent massive leakage of natural gas to the atmosphere is one of the most idiotic arguments I have ever head on this topic. Such leakage is fully controllable and it can be prevented at fairly low costs.

Leakage of methane from farting cows and other animals including humans is a much more serious problem because there is no easy or inexpensive ways to contain it.


I agree with all 3 of you. And with the statement: Significant progress appears possible given the economic benefits of capturing and selling lost natural gas and the availability of proven technologies.

As more people start to use this "cheap fuel" its true value will be seen and we'll make the necessary investments in infrastructure rather quicky and leakage rates will likewise drop.


BTW, I note that this study dealt only with fuel switching using CNG. What about LNG? LNG is more likely to be used in those larger vehicles that use diesels because they'd get greater range.


Another thought: What about Hythane? Switching to NG would allow you to mix in some hydrogen. What's the relevant radiative forcing for H2?


Is it too much to ask that the National Academy of Sciences exercise some minimal level of screening?


This is a complex issue and the proper environmentally acceptable solutions may not be compatible with higher profits. We all know what the most probable choice will be. Of course, the majority will easily be convinced that profits must be favored regardless of higher leakages.

Why not reduce leakage with large NG power plants instead of 100,000,000 CNG vehicles and 400+ dirtier coal fired power plants?

Chad Snyder

How convenient Harvey?

Any question of plug-in viability and you suggest big oil conspiracy.

Here, the culprit in question is, essentially, a usable fuel. Why not just harness that fuel? Would methanol fuel cell hybrids really be so bad?

Yes, instead, nat gas powered power plants and plug-ins would be more efficient, but if the auto industry isn't building them and consumers don't want to buy them, how much is your greater efficiency-potential worth in the real world?

This isn't communist America. Maybe it should be, but unless you can pass that via the ballot, consumers are much more willing to embrace CNG vehicles than plug-in ones until there is a major battery breakthrough that makes plug-ins equivalent to guzzlers -- in EVERY way.

I'm sorry, but it really bothers me how utopian dreams, despite a plethora of evidence, are consistently an argument against rational energy policy. Plug-ins have become the modern day fuel cell.

Chad Snyder

Also ironic that the Environmental Defense Fund was a key contributor. But they're completely unbiased, of course. They certainly have no agenda.

Nick Lyons

Solution: fix the leaks. This means better regulation of the owners of gas production and transmission lines. You may recall that PG&E recently blew up a whole neighborhood south of San Francisco because of lax inspection of its gas line network. CH4 is becoming the major US energy source for the early 21st century. So be it--let the producers produce and the transporters transport, but lets keep them honest. Everyone needs to play by the rules, and the referees need to have sufficient clout to keep the players honest.


With current NG prices in Europe there is no any rational switching from diesel to CNG.

CNG since it it is under high pressue leaking more than NG.


GTL would turn natural gas into liquid fuels as close to the well head as possible. The liquid fuels can be transported and stored just like refined product and blended to reduce oil imports.


You forget that GTL gasoline can also leak.. we are doomed!


Im interrested to buy a natural gas conversion kit for my 2005 4 cylinders ice car. The kit is added to the car and the gasoline tank is kept intact so the car become a hybrid fuel nat gas plus gasoline so it is just a way to choose the cheapest fuel and gasoline price will lower if there is direct competition between them. Actually big oil with the help of goverments and actual car manufacturers have avoided clean abondant cheap nat gas to promote high gasoline price and the money that is given to saudi-arabia for petrol imports is deposited secretly in swiss banks accounts in the name of exxon and friends. That's why they import petrol it's because the money is transferred directly in secret internationnal bank accounts tax free. They are not interrested in keeping the money in official taxed contries like usa or canada, they prefer swiss banks via saudi-arabia.



Yeah, I forgot about that :) It seems like there is always a downside.

Shell is sending GTL platform ships to the arctic where they are turning natural gas into liquid fuels. Shell has the huge Pearl facility turning gas into fuels in the middle east.

There is talk about GTL in the U.S. as well, it is a matter of economics, which is the case with most if not all business decisions. As I have said, I would start with natural gas and use more biomass as time goes on for a more sustainable energy source.


this is kind of a no-brainer. We know that the greenhouse gas benefits of taking coal offline are greater. So it depends on your priority: ending US dependence on foreign petroleum (then you want natural gas vehicles), or lowering GHG (then you want natural gas to replace coal).

The latter is much easier and could be deployed much more rapidly. The former goal might be achieved more readily through upgrading to more efficient drive technologies (downsizing and electrification).

Roger Pham

This is an additional support for the use of H2 in transportation instead of NG. Atmospheric H2 has much shorter lifespan than methane, and H2 has much lower GHG index than methane. H2 is easily oxidized in the upper atmosphere by UV light, whereas methane stays around much longer.


Thanks Roger, that answers my question about Hythane, but as I recall there's still a problem with the energy content of a volume of H2 when compared to methane as it flows through a pipeline. And whereas NG is stored energy that can be pumped out of the ground H2 requires energy more to release it from some chemical feedstock than we'd get back from using it. What are the trade-offs here?


Here is a company that can turn natural gas into ethylene and then gasoline, ethylene can be made into ethanol as well. So with one small plant, you could make ethanol as an oxygenate and gasoline from natural gas.

Roger Pham

@ai vin,
H2 should be made from excess solar, wind and nuclear energy and not from chemical feedstocks in order to be environmental sustainable. Gradually upgrading the NG pipeline to where it can also support H2, or building dedicated H2 pipeline. H2 can be stored in used-up oil or gas well as well as used up salt mine. There is no problem with the lower energy content of a volume of H2, since with higher efficiency of fuel cells and with better home insulation and multi-zone HVAC, we will use less of it. H2 forces us to be more energ efficient. Necessity is mother of invention!


H2 should be made from excess solar, wind and nuclear energy and not from chemical feedstocks in order to be environmental sustainable.

Just as a point of clarity, when I said "chemical feedstocks" I was including H2O: A substance made of multiple atoms joined by shared electrons.

Using water as a feedstock, while more environmentally substainable, still takes more energy (excess solar, wind and nuclear or not) than we get back when the H2 is recombined so the question: What are the trade-offs here? still stands.

Roger Pham

<<"What are the trade-offs here?"

H2 is the most efficient synthetic chemical fuel derived from the energy of renewable energy and nuclear energy. As a chemical fuel, it can be used to store excess renewable energy produced in the springs and falls for use in winters. To combat global warming, we must be serious about going toward renewable energy and nuclear energy and wean off from fossil fuels completely. H2 will enable us to do that. Cost is more relevant than efficiency. H2 as energy storage is not as efficient as battery electricity, but for seasonal energy storage, the cost of battery electricity is prohibitive.


Now that's the kind of answer I was looking for.

And I now have another question regarding your idea of using H2 for seasonal energy storage. As I understand it hydrogen, in its elemental H2 form is rather hard to store long term. The molecule being so small leaks from the smallest of openings, it can even slip between the atoms of a steel pressure vessel. If the goal is seasonal energy storage would it not be easier to store it as part of a larger molecule? We could combine it with carbon and make CH4, or SNG and use the same infrastructure we have now.


Fundamental laws of physics expose the weakness of a H2 economy. Energy cannot be created, it can only be converted from one state to another, and some energy is always lost during the conversion, H2 is an artificial energy carrier, and can never compete with its own energy source, either natural gas or electricity, in a sustainable future.

By creating a link between the power generation and transport sectors, natural gas-fired electricity generation in combination with PHEVs, will be one of the pathways that are both highly efficient and sustainable.

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