The growth in natural gas production in the United States over the past decade has led to a shift in thinking about domestic electricity generation. The sheer volume of economically recoverable natural gas may signal the end of coal’s reign as the primary source of fuel for electricity generation. Given that lower commodity prices and resource abundance certainly generate financial incentives for natural gas use in power plants, society must also consider the emissions and potential climate impacts when deciding between natural gas and coal. Secretary Moniz said in May 2013:
“What I would argue is that the way to look at it [is] as kind of a bridge to a very low carbon future – is that it affords us a little bit more time to develop the technologies, to lower the costs of the alternative technologies, to get the market penetration of these new technologies.”
The question is – How much time?
The Metrics
In this ongoing debate about the climate benefits of fuel switching from coal to natural gas for power generation, we can use a variety of climate change metrics. One metric is radiative forcing. Radiative Forcing (RF) is the measurement of the capacity of a gas or other forcing agents to affect that energy balance, thereby contributing to climate change. Put more simply, RF expresses the change in energy in the atmosphere due to GHG emissions. RF can be easily calculated at any point in time. Cumulative radiative forcing is the amount of radiative forcing over a time period.
A metric based on cumulative radiative forcing is, global warming potential (GWP). GWP compares the ratio of the cumulative radiative forcing from year 0 to N of 1 kg of the gas as compared to the cumulative radiative forcing from year 0 to N of 1 kg of CO2. GWP has arbitrary time frames (20 & 100 years) with no relevance to gas lifetimes.
Technology warming potential (TWP) is another climate impact metric based on cumulative radiative forcing. First introduced in 2012, TWP compares the climate impacts caused by emissions from two different technologies using a ratio of the radiative forcing of both technologies over a period of time. The results are time dependent and allow for an analysis that illustrates when, and if, a competing technology will produce lower cumulative radiative forcing than a reference technology. Values less than 1 indicate that the alternate technology results in lower cumulative radiative forcing than the reference technology, while values greater than 1 indicate that the reference technology produces less cumulative radiative forcing. If the TWP is equal to 1, there is not a preferred technology on the basis of cumulative radiative forcing.
In addition to metrics based on cumulative radiative forcing, there are some metrics based on temperature. Temperature is related to radiative forcing by a parameter called the climate sensitivity. This parameter is highly uncertain, which leads to temperature changes being more uncertain than radiative forcing changes.
For instance, Global Temperature Change Potential (GTP) compares the temperature impact in year N. Unfortunately, GTP does not allow for the evaluation of a sustained emission of a finite length further into the future than the length of that sustained emission (i.e. we cannot see the impacts of a power plant that was operational for 30 years, 100 years into the future).
Results
Clearly the metrics used to model climate impacts are important, as different metrics may lead to different conclusions. In a paper shared at the International Symposium on Sustainable Systems and Technology (ISSST), and currently in review at a peer-reviewed journal, we evaluate the life cycle greenhouse gas (GHG) emissions of coal and natural gas-based electricity using a broad set of available climate metrics in order to test the robustness of results comparing coal and natural gas used in advanced power plants. We find that all climate metrics suggest a natural gas combined cycle plant offers life cycle climate benefits over 100 years compared to a pulverized coal plant, even if the life cycle methane leakage rate for natural gas reached 5%. Over shorter timeframes (i.e. 20 years) natural gas with 4% leakage rate has similar climate impacts than coal, but is no worse than coal. If carbon capture and sequestration becomes available for both types of power plants, natural gas still offers climate benefits over coal as long as the life cycle leakage rate remains below 2%. These results are consistent across climate metrics and the MAGICC model. While there is no clear metric better suited for life cycle assessment, given the recent criticism of global warming potential, we recommend the use of alternative metrics to assess the robustness of life cycle results.
Author Biography: This blog is authored by Dr. Kelly Klima, and is partially adapted from two papers in review: 1) A paper by DeVynne Farquharson, Paulina Jaramillo, Greg Schivley, Kelly Klima,Derrick Carlson, and Constantine Samaras on comparing greenhouse gas metrics, and 2) A paper by Roger Lueken, W. Michael Griffin, Kelly Klima, and Jay Apt on a coal to gas switch. Dr. Klima is a Research Scientist at the Department of Engineering and Public Policy of Carnegie Mellon University with over ten years of research experience on adaptation, hazard mitigation, climate, extreme weather, and risk communication. Her research work supports community resilience throughout the world, and has been applied in the City of Pittsburgh and counties in New Jersey. Previously, Dr. Klima worked at the Center for Clean Air Policy (CCAP), where she helped New York and Washington D.C. advance adaptation planning. Dr. Klima completed her doctoral research in the Department of Engineering and Public Policy (EPP) at Carnegie Mellon University where she used physics, economics, and social sciences to conduct a decision analytic assessment of different methods to reduce hurricane damages. She has published several journal articles, is an active member of 9 professional societies, and serves on the Natural Hazard Mitigation Association (NHMA) Board of Directors and the American Geophysical Union (AGU) Board of Directors. Dr. Klima also has an M.S. in Earth, Atmosphere, and Planetary Science (MIT), an M.S. in Aeronautics and Astronautics (MIT), a B.S. in Mechanical Engineering (Caltech), and a CFM from the Association of State Floodplain Managers.