Design criteria and performance of emissions equivalency metrics

Assessments of the life cycle climate impacts of technologies and policies can depend strongly on the equivalency metric used to compare short-lived greenhouse gases (e.g., methane) to long-lived carbon dioxide. However, the... [ view full abstract ]

Assessments of the life cycle climate impacts of technologies and policies can depend strongly on the equivalency metric used to compare short-lived greenhouse gases (e.g., methane) to long-lived carbon dioxide. However, the tradeoffs among metrics are not well understood, and in practice only a small subset of possible metrics (whose values are readily available) receive much use. Previous research suggests that the popular global warming potential (GWP) fails to represent the time-dependent climate impacts of technology emissions, leading to sub-optimal technology choices and, when applied globally, to deviations from climate policy targets. Here we propose a new approach to designing equivalency metrics, which we refer to as testing-inspired design. First, we identify the fundamental design choices that give rise to the diversity of metrics proposed in the literature. Results suggest that metric values are relatively similar across impact measures (e.g., radiative forcing, temperature, and sea level rise).  However, the choice of weighting scheme can dramatically change metric values. We apply our framework to generate a comprehensive set of metrics, including both novel and previously-proposed formulations. These metrics vary in terms of their shape, maximum value, and (for time-dependent metrics) slope.

Second, we use a testing procedure to simulate the outcomes of applying candidate metrics on climate change and the costs of mitigation. There are tradeoffs across metrics in terms of the peak climate impact, rate of climate change, long term climate impact, and the level economic activity that result from their use. For example, high metric values for short-lived greenhouse gases reduce the risk of large near-term climate changes but may lead to higher mitigation costs. The preferred metric depends on how stakeholders balance economic and climate risks over time. We develop visualizations to explore these tradeoffs and present an example set of metrics selected to meet a commonly-cited climate policy targets at minimal cost. Results suggest that the appropriate metric depends on the ease of substitution between gases, which is a function of the technologies available and the scale and structure of the climate policy. Static and dynamic metrics whose values are anchored to broader climate policy targets outperform the standard GWP.  A new class of time-dependent metrics whose values increase initially and later decrease perform particularly well across a range of situations. We discuss implications for the design of equivalency metrics for technology evaluation, infrastructure planning, and emissions regulation.