Technical note: On comparing greenhouse gas emission metrics

Abstract

Many metrics for comparing greenhouse gas emis sions can be expressed as an instantaneous global warming potential multiplied by the ratio of airborne fractions calcu lated in various ways. The forcing equivalent index (FEI) provides a specification for equal radiative forcing at all times at the expense of generally precluding point-by-point equivalence over time. The FEI can be expressed in terms of asymptotic airborne fractions for exponentially growing emissions. This provides a reference against which other metrics can be compared. Four other equivalence metrics are evaluated in terms of how closely they match the timescale dependence of FEI, with methane referenced to carbon dioxide used as an example. The 100-year global warming potential overesti mates the long-term role of methane, while metrics based on rates of change overestimate the short-term contribution. A recently proposed metric based on differences between methane emissions 20 years apart provides a good compro mise. Analysis of the timescale dependence of metrics ex pressed as Laplace transforms leads to an alternative metric that gives closer agreement with FEI at the expense of con sidering methane over longer time periods. The short-term behaviour, which is important when met rics are used for emissions trading, is illustrated with simple examples for the four metrics.