Identifying Optimal Energy Sources
Dr. Hamburg suggested that moving forward, it is imperative to determine which energy sources are optimal in which situations. Achieving this requires more detail assessments. For example, electricity generated from coal combustion can be compared with electricity generated from natural gas combustion. Historically, only the emissions from smokestacks at the different power facilities were examined in this comparison. Now it is recognized that this is insufficient. Instead, how these fuels affect the environment and society throughout the supply chain must be considered. This includes the mining, production, and transport of coal and natural gas, as well as end use of the electricity generated, which for natural gas, includes use by industry and residential homes for heating.
He pointed out that it is necessary to understand the implications that a transition from coal to natural gas will have on climate change. Methane is the main constituent of natural gas, and there is methane leakage at each point along the natural gas supply chain—production, processing, transport, and combustion. Methane is also a greenhouse gas, and its leakage should be accounted for in the comparison of natural gas and coal. The challenge is how to make methane leakage equivalent to carbon dioxide emissions—which persist in the atmosphere for very different lengths of time—for a true comparison. It is possible to use the 100-year global warming potential (GWP) of methane,1 which is 212 (EPA, 2013b), but GWP assumes that the short term is not relevant because the outcome is assessed 100 years from now. More specifically, GWP is the impact of a single pulse emission 100 years after it is released. A useful analogy for GWP is worrying about the impacts of renting a car today 100 years in the future. What are the implications 100 years from now of a pulse of emissions from a power plant? But a hundred years from now is an abstraction. There is a lot of time between now and then, and most people care about what happens between now and 100 years from now. What we really care about are the implications of owning a particular type of car for its lifetime, or even more important what are the implications of changing the characteristics of an entire fleet of cars (e.g., Corporate Average Fuel Economy [CAFE] standards). GWP, as traditionally applied, does not reveal much about the impacts of a power plant over time. Most people care about what the power plant does over its functional life or the effects of its emissions over the next 20 years, as well as its impacts over the longer term.
1 Global warming potential was developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to carbon dioxide.
2 It should be noted that the Intergovernmental Panel on Climate Change uses 25 for the GWP (100-year) of methane; for a series of reasons, the U.S. Environmental Protection Agency uses an outdated factor.