A new scientific paper published in the Proceedings of the National Academy of Sciences(PNAS) offers an enhanced method for assessing climate impacts from natural gas development and use using a new approach called “Technology Warming Potential.” Specifically, this approach reveals the inherent climatic trade-offs of different policy and investment choices involving electricity and transportation. It illustrates the importance of accounting for methane leakage across the value chain of natural gas (i.e. production, processing and delivery) when considering fuel-switching scenarios from gasoline, diesel fuel and coal to natural gas.
A new methane leakage model, created by Environmental Defense Fund (EDF) and based on the science described in the PNAS paper, allows anyone to test a range of scenarios to quantify the climate benefits, or damages, of natural gas production and usage given specific methane leakage rates. Users can vary the key system attributes independently to see how they affect net radiative forcing (the primary index used to quantify the effect of greenhouse gases [GHGs] on global temperatures) from U.S. emissions over time.
Natural gas burns cleaner than other fossil fuels when combusted, but methane leakage from production and transportation of natural gas has the potential to remove some or all of those benefits, depending on the leakage rate. Methane is the main ingredient in natural gas and a greenhouse pollutant many times more potent than carbon dioxide (CO2), the principal contributor to man-made climate change. The paper uses the best available estimates on methane emissions from the Environmental Protection Agency (EPA). At the same time, EDF is working to obtain extensive empirical data on methane released to the atmosphere across the natural gas supply chain, since the climatic bottom line of fuel switching scenarios involving natural gas is very sensitive to this parameter.
“Measuring how much gas is lost to the atmosphere and where the leaks are occurring will help to further target leak reduction opportunities to ensure that natural gas will help mitigate climate change. Such a strategy could yield enormous environmental and health benefits,” says Steve Hamburg, EDF’s chief scientist and coauthor of the paper.
The PNAS paper provides illustrative calculations with EPA’s current estimate of the methane leakage rate. The model allows users to plug in different variables and observe the outcome. Thus the paper does not draw hard and fast conclusions about the future implications of any kind of fuel shifting, nor does it answer the question of whether natural gas generation or natural gas-powered vehicles will be better or worse for the climate. What it does do is provide those answers in terms of the leak rates at which fuel switching produces climate benefits at all points in time. It introduces the science required to accurately identify where the challenges lie.
Key findings of the PNAS paper, based on the best available estimates on methane emissions from the EPA, include:
- Assuming the Environmental Protection Agency’s (EPA) 2009 leakage rate of 2.4% (from well to city), new natural gas combined cycle power plants reduce climate impacts compared to new coal plants; this case is true as long as leakage remains under 3.2%.
- Assuming EPA’s estimates for leak rates, compressed natural gas (CNG)-fueled vehicles are not a viable mitigation strategy for climate change because of methane leakage from natural gas production, delivery infrastructure and from the vehicles themselves. For light-duty CNG cars to become a viable short-term climate strategy, methane leakage would need to be kept below 1.6% of total natural gas produced (approximately half the current amount for well to wheels – note difference from well to city).
- Methane emissions would need to be cut by more than two-thirds to immediately produce climate benefits in heavy duty natural gas-powered trucks.
- At current leakage rate estimates, converting a fleet of heavy duty diesel vehicles to natural gas would result in nearly 300 years of climate damage before any benefits were achieved.
A number of scientific papers on the climatic implications of natural gas production and use have been published in the last year, inadvertently figuring into a growing sense of confusion due to conflicting conclusions. The PNAS paper tries to clear up some of this confusion by addressing the analytical challenge of comparing the time-dependent effects on climate of methane by using the Technology Warming Potential approach. The paper specifically illustrates this approach to compare the climate influence (i.e. changes in radiative forcing) of fuel switching scenarios involving natural gas.
“Failing to reduce methane leaks has the potential to eliminate much, if not all, of the greenhouse gas advantage of natural gas over coal,” said Steven Hamburg, EDF’s chief scientist and coauthor of the paper. “If we want natural gas to be an accepted part of a strategy for achieving energy independence and moving to a clean energy future, it’s critical that industry, regulators and other stakeholders work together to quantify the existing methane leakage rate and commit to reducing it to one percent or below if, as expected, the leakage is currently higher than that. One percent is the magic number.”
EDF is currently collaborating with partners on a major scientific study designed to quantify the methane leakage rate across the natural gas value chain in five discrete modules, the first of which – emissions from the production sector – has already been launched. EDF aims to complete the entire study by December 2013.
Authors of the paper include: Ramón A. Alvarez (EDF), Stephen W. Pacala (Princeton University), James J. Winebrake (Rochester Institute of Technology), William L. Chameides (Duke University) and Steven P. Hamburg (EDF). The full text of the PNAS paper is available at http://www.pnas.org/content/early/2012/04/02/1202407109 and EDF’s methane leakage model is available at http://www.edf.org/methaneleakage.
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