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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
10 April 2012
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
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