Simple emission metrics for climate impacts

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 straightforward comparison of the estimated climate impacts of emissions of different species. Emission metrics are not unique and variety of different emission metrics has been proposed, with key choices being the climate impacts and time horizon to use for comparisons. 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 normalised to a reference gas. We consider pulse emissions, sustained emissions and emission scenarios. The species are separated into three types: CO₂ which has a complex decay over time, species with a simple exponential decay, and ozone precursors (NO_x, CO, VOC) which indirectly effect climate via various chemical interactions. We also discuss deriving Impulse Response Functions, radiative efficiency, regional dependencies, consistency within and between metrics and uncertainties. We perform various applications to highlight key applications of emission metrics, which show that emissions of CO₂ are important regardless of what metric and time horizon is used, but that the importance of short lived climate forcers varies greatly depending on the metric choices made. Further, the ranking of countries by emissions changes very little with different metrics despite large differences in metric values, except for the shortest time horizons (GWP20)

Variation in emission metrics due to variation in CO<sub>2</sub> and temperature impulse response functions

“…1), and A X the radiative efficiency of species X. The radiative efficiency can depend on the background concentration, but we assume a constant background as is common for emission metrics (Joos et al, 2013;Aamaas et al, 2013). The radiative efficiency values we use are given in Table 1.…”

“…If one additionally assumes that the RF is a linear function of the atmospheric burden, then the evolution of the RF as a function of time can be expressed by a simple multiplication of the radiative efficiency and the IRF (Aamaas et al, 2013). In general linearity does not hold for CO 2 , CH 4 , or N 2 O where the RF shows a non-linear dependence on their burden -moreover N 2 O and CH 4 have a spectral overlap (Ramaswamy et al, 2001, Table 6.2).…”

“…The most common emission metric is the global warming potential (GWP), but the global temperature change potential (GTP) has received considerable attention more recently (Fuglestvedt et al, 2003;Aamaas et al, 2013). Both these metrics compare the climate impact of the pulse emission of a certain species with the impact of the pulse emission of the same amount of carbon dioxide (CO 2 ).…”

“…Secondly, IRFs are also used to characterize the global temperature change induced by a pulse radiative forcing (Hasselmann et al, 1993;Sausen and Schumann, 2000). If one additionally linearizes the expression for the radiative forcing to obtain the radiative efficiency (Aamaas et al, 2013), one obtains a simple and useful description of the atmospheric response to the emissions of radiatively active species through a simple combination of the radiative efficiency and IRFs.…”

“…Together with the climate impact to be evaluated, the time horizon is an important quantity affecting the metric values (Fuglestvedt et al, 2003;Aamaas et al, 2013). Frequently used time horizons are 20, 100, and 500 yr for GWP, and 20, 50, and 100 yr for GTP (Fuglestvedt et al, 2003Shine et al, 2005Shine et al, , 2007.…”