Abstract
Abstract. Many metrics for comparing greenhouse gas emissions can be expressed as an instantaneous global warming potential multiplied by the ratio of airborne fractions calculated 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 overestimates 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 compromise. Analysis of the timescale dependence of metrics expressed as Laplace transforms leads to an alternative metric that gives closer agreement with FEI at the expense of considering methane over longer time periods. The short-term behaviour, which is important when metrics are used for emissions trading, is illustrated with simple examples for the four metrics.
Highlights
Anthropogenic contributions to global climate change come from a range of greenhouse gases
The approach is applied to the consideration of CH4 emissions in terms of various definitions of their CO2-equivalent emissions
In the special case of exponentially growing emissions, forcing equivalent index (FEI) equivalence can be achieved when the emissions are scaled by the instantaneous (0 time horizon) global warming potential (GWP) multiplied by the ratio of the asymptotic airborne fractions
Summary
Anthropogenic contributions to global climate change come from a range of greenhouse gases. 2 we show how the prescription by Wigley (1998), which gives exact equivalence in radiative forcing between different time histories of emissions, may be elegantly expressed in terms of Laplace transforms. For FEI equivalence of emissions growing exponentially at rate α one has cX = aY RY (α) cY Interpreting these relations in terms of Laplace transforms can help clarify the different forms of equivalence metrics in the general case. For arbitrary emission perturbations the condition for FEI equivalence is defined by the Laplace transform of Eq (3): aY SY (p) RY (p) = aX SX(p) RX(p), In this expression FEI(p) = RY (p)/RX(p) is the Laplace transform of an integro-differential operator that acts on SY (t) in the time domain. In these last two cases, the FEI equivalence is achieved by scaling by aCH4 /aCO2
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
