DURHAM, NC – Natural gas is seen by many as the future of American energy; a fuel that can provide energy independence while reducing greenhouse gas emissions.

A study published this week in the online early edition of the Proceedings of the National Academy of Sciences, however, finds that the emissions-reducing benefits of switching to natural gas from other fossil fuels could vary widely due to potential methane leakage.

“Methane is the prime constituent of natural gas and is itself a more potent greenhouse gas than carbon dioxide. Our analysis shows that in some scenarios, the climate benefits of using natural gas as a fuel for motor vehicles in place of gasoline may be offset and delayed for decades because of methane leaks from infrastructure involved in the production, transportation and use of the natural gas,” says William L. Chameides, dean of 51’s Nicholas School of the Environment. “However, the balance sheet would tip strongly in favor of natural gas if those leaks can be eliminated.”

Chameides conducted the study with Ramon A. Alvarez and Stephen P. Hamburg of Environmental Defense Fund, Stephen W. Pacala of Princeton University and James L. Winebrake of the Rochester Institute of Technology. Alvarez and Pacala were the study’s corresponding authors.

By using EPA-estimated methane leak rates for current natural gas technologies and delivery systems, the team projects that a nationwide shift to compressed natural gas vehicles from gasoline or diesel vehicles would lead to greater radiative forcing of the climate – i.e. more rapid climate change – for 80 to 280 years, respectively, before beginning to yield net benefits. Even then, the benefits would be modest: after 150 years, a fleet of compressed natural gas vehicles would have produced only about 10 percent less cumulative radiative forcing than a fleet of gasoline-powered vehicles.

Compressed natural gas vehicles would yield climate benefits immediately and the benefits would increase over time if their well-to-wheels methane leakage could be capped at roughly 45 percent to 70 percent below current EPA estimates, the study finds. While not mutually exclusive, similar climate benefits could be achieved simply by improving the fuel-efficiency of conventional vehicles, Chameides notes.

Using natural gas instead of coal for electric power plants would significantly reduce radiative forcing immediately, the study finds, and these reductions would continue to increase over time. Reducing methane leaks in the technologies and infrastructure used to produce and transport the natural gas to the plants would yield even greater benefits.

“Our calculations assume an average leakage rate for the entire U.S. natural gas supply as well as for coal mining,” the authors write. “Much work needs to be done to determine actual emissions with certainty and to accurately characterize the site-to-site variability in emissions. However, given limited current evidence, it is likely that leakage at individual natural gas well sites is high enough, when combined with leakage from downstream operations, to make the total leakage exceed the 3.2 percent threshold beyond which gas becomes worse for the climate than coal for at least some period of time.”

The scientists believe their study, which employs the well-to-wheels technology-warming approach (TWP) to compare the climate benefits of fuel-switching scenarios, provides policymakers with greater insights than conventional analyses, which use the global warming potential (GWP) approach. GWP analyses are valuable for comparing one gas to another, they note, but TWP is better and more transparent for assessing the time-dependent climate influence of different fuel-technology choices.


Note: William L. Chameides can be reached for additional comment at (919) 613-8004 orbill.chameides@duke.edu.