Greenhouse gas equivalency metrics for evaluating energy technologies

Thesis: Ph. D. in Engineering Systems, Massachusetts Institute of Technology, School of Engineering, Institute for Data, Systems, and Society, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 127-137).This thesis addresses a long-standing question about how to compare energy technologies that emit different types of climate forcers during their life cycles. This problem is challenging because these forcers have dissimilar lifetimes in the atmosphere, ranging from days (black carbon) to decades (methane, CH 4) to centuries or more (carbon dioxide, CO2 ). Efforts to reduce the climate impacts of energy use may involve a tradeoff between these short-and long-lived emissions. Equivalency metrics, which express emissions of one forcer (e.g., CH4 ) in units of another (typically CO2), are widely-used tools for comparing the climate impacts of emissions. These metrics allow climate impacts to be expressed on a single scale, but they require assigning a relative value to short- versus long-lived climate forcing. The equivalency metric approach is used in a large variety of applications, from technology evaluation to emissions trading. These applications almost universally rely on a single metric, developed as a placeholder over twenty-five years ago. This metric, the global warming potential (GWP), compares gases based on their radiative forcing impacts over a fixed time horizon (usually 100 years). The design of the GWP, including critically the time horizon over which emissions are compared, is largely arbitrary, yet it has enormous implications for comparing the climate impacts of energy technologies and other emissions sources. Despite the practical and political importance of equivalency metrics, the scientific literature has not produced a consensus on how to design or choose these metrics. To address this gap in the literature, this thesis develops a new conceptual and quantitative modeling approach to link equivalency metric design to global climate policy goals. This procedure involves (a) formulating a set of goal-inspired equivalency metrics, (b) testing metrics by simulating the results when they are applied in real-world contexts, and (c) selecting metrics based on multiple performance criteria. We highlight two dimensions of metric performance: climate performance (i.e., whether metric-based decisions meet climate policy goals) and energy performance (i.e., whether these decisions support energy use, for example during a technology transition). No metric performs optimally across all criteria, and this approach allows us to quantify these performance tradeoffs. The central result of the thesis is that climate policy goals can be used to inspire equivalency metric design, and these goal-inspired metrics address key shortcomings of the GWP(100). Specifically, under a policy to limit global temperature change to 2°C (where radiative forcing levels stabilize around mid-century), a shorter time horizon is essential. We find that applying the GWP(100) in this policy context can lead to radiative forcing overshoots in excess of two thirds of the remaining budget. One set of goal-inspired metrics addresses this concern by reducing the time horizon over which emissions are compared as a radiative forcing limit is approached (Chapter 2). These metrics increase the impact value placed on short-lived CH4 (relative to long-lived CO2 ) over time. We find that this design reduces the risks of overshooting radiative forcing limits, despite inherent uncertainty in the timeline for reaching these limits (Chapter 3). Relative to other metrics that lead to similar peak radiative forcing outcomes, these goal-inspired metrics allow more energy use early on, which can help enable technology transitions (Chapter 4). Applying these goal-inspired metrics to evaluate natural gas suggests that the mitigation benefits of this high-CH 4-emitting fuel will decrease significantly in the coming years. For example, under a radiative forcing limit consistent with a 2°C temperature change policy, the climate impact of natural gas electricity increases from 50% that of coal to 80% by mid-century (Chapter 2). Similar results apply to transportation fuels with high CH4 (or black carbon) emissions (Chapter 2, Chapter 5). This result draws into question large investments in technologies and long-lived infrastructure with high life cycle CH4 emissions - and provides a quantitative basis for calculating timelines to reduce the CH4 intensity of these technologies or transition to lower-emitting technologies. A bridging strategy, where technologies with high CH4 emissions are followed by those with lower emissions, permits greater overall energy consumption while meeting climate policy targets (Chapter 5).by Morgan R. Edwards.Ph. D. in Engineering System