A research report from scientists at the Department of Global Ecology, Carnegie Institution for Science, Stanford, California and Intellectual Ventures, a patent consolidator and research house in Bellevue, Washington concludes that “if there is substantial methane leakage, natural gas [power] plants can produce greater near-term [global] warming than coal plants with the same power output. However, if methane leakage rates are low and power plant efficiency is high, natural gas plants can produce some reduction in near-term warming. In the long term, natural gas power plants produce less warming than would occur with coal power plants. However, without carbon capture and storage natural gas power plants cannot achieve the deep reductions that would be required to avoid substantial contribution to additional global warming.”

The finding is not surprising to many in the environmental science community but it should be cautionary to some in the environmental ngo and political communities who have been pushing natural gas at least as an interim replacement for coal.

The methodology for the Carnegie study is interesting. The researchers developed a power plant GHG emissions model, and used a schematic climate model  to investigate the radiative forcing and resulting global mean temperature changes (ΔT). This model considers the pathway from emissions of GHGs to global mean temperature response. The generating plants included in this study are single natural gas and coal fired power plants with capacity of 1 GW. The major emissions from power plants occur during operations; construction emissions are relatively small in total life-cycle emissions, and relatively similar for the two types of plants, so they are ignored.

The analysis is complex with several key variables. For example, both natural gas and coal can produce significant methane emissions, the former from avoidable, but not always avoided, leaks in the distribution system and the latter from much ore difficult to avoid leaks directly from the coal bed. The study makes numerous conclusions but overall indicates that “if there is substantial methane leakage associated with natural gas supply, then natural gas plants can produce more warming than a comparable coal plant during the period of operation’. In the best case, if the methane leaks are addressed, then “the best natural gas plant with a zero methane leakage rate still produces about two-thirds of the century-integrated warming [ie over a hundred year time period] as does the best coal power plant”.

The researchers note that “many well-publicized GHG emission targets require much deeper cuts in emissions than can be provided by natural gas. For example, California’s AB32 regulation calls for 80% reductions in emissions below 1990 levels by mid-century. Many power plants built today could still be operational in mid-century; this raises the question of the extent to which natural gas can help in achieving these policy objectives. If natural gas is to play a long-term role in electricity production in a world with greatly constrained carbon emissions, then carbon capture and storage may be an essential component of future natural gas systems”.

The article concludes “natural gas is thought of as a ‘bridge’ fuel by some policy makers—a temporary fuel to be used until a transition to near-zero emission technologies becomes more feasible. Thus, natural gas is promoted as a way to decrease near-term emissions as we make a transition to energy systems that deeply cut longterm emissions. However, if methane leakage rates cannot be maintained at very low values, near-term climate benefits may be small or non-existent. There is potential that, relative to coal, the deployment of natural gas power plants could both produce excess near-term warming (if methane leakage rates are high) and produce excess long-term warming (if the deployment of natural gas plants today delays the transition to near-zero emission technologies. Thus, achieving climate benefits from the use of natural gas depends on building high efficiency natural gas plants, controlling methane leakage, and on developing a policy environment that assures a transition to future lower-emission technologies.”

The article is published in Environmental Research Letters, a peer-reviewed, scientific journal, and can be found at http://iopscience.iop.org/1748-9326/9/11/114022?fromSearchPage=true