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Preprints
https://doi.org/10.5194/acp-2020-996
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-996
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  19 Oct 2020

19 Oct 2020

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This preprint is currently under review for the journal ACP.

Technical note: On comparing greenhouse gas emission metrics

Ian Enting1 and Nathan Clisby2 Ian Enting and Nathan Clisby
  • 1CSIRO Climate Science Centre, Oceans and Atmosphere, Aspendale, Vic, Australia
  • 2Department of Mathematics, Swinburne University of Technology, PO Box 218, Hawthorn Vic, 3122, Australia

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.

Ian Enting and Nathan Clisby

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Ian Enting and Nathan Clisby

Ian Enting and Nathan Clisby

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Short summary
We provide a new framework for comparing short-lived greenhouse gases, using methane as an example, to long-lived greenhouse gases such as carbon dioxide. This can clarify the differences between various proposals that have been introduced in order to overcome the use of Global Warming Potentials as a measure of greenhouse gas equivalence.
We provide a new framework for comparing short-lived greenhouse gases, using methane as an...
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