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.
Differences in net global warming potential and greenhouse gas intensity between major rice-based cropping systems in China
