A synthesis of climate-based emission metrics with applications

Abstract

Abstract. In the context of climate change, emissions of different species (e.g. carbon dioxide and methane) are not directly comparable since they have different radiative efficiencies and lifetimes. Since comparisons via detailed climate models are computationally expensive and complex, emission metrics were developed to allow a simple and straight forward comparison of the estimated climate impacts of the emissions of different species. Because emission metrics depend on a variety of choices, a variety of different metrics may be used and with different time-horizons. In this paper, we present analytical expressions and describe how to calculate common emission metrics for different species. We include the climate metrics radiative forcing, integrated radiative forcing, temperature change, and integrated temperature change in both absolute form and normalized to a reference gas. We consider pulse emissions, sustained emissions, and emission scenarios. The species are separated into three types: species with a simple exponential decay, CO2 which has a complex decay over time, and ozone pre-cursors (NOx, CO, VOC). Related issues are also discussed, such as deriving Impulse Response Functions, simple modifications to metrics, and regional dependencies. We perform various applications to highlight key applications of simple emission metrics, which show that emissions of CO2 are important regardless of what metric and time horizon is used, but that the importance of SLCFs varies greatly depending on the metric choices made.