  Greenhouse gas (GHG) metrics, that is, conversion factors to evaluate the
emissions of non-CO2 climate forcers on a common scale with CO2, serve crucial
functions upon the implementation of the Paris Agreement. While different
metrics have been proposed, their economic cost-effectiveness has not been
investigated under a range of pathways, including those temporarily missing or
significantly overshooting the temperature targets of the Paris Agreement. Here
we show that cost-effective metrics for methane that minimize the overall cost
of climate mitigation are time-dependent, primarily determined by the pathway,
and strongly influenced by temperature overshoot. The Paris Agreement will
implement the conventional 100-year Global Warming Potential (GWP100), a good
approximation of cost-effective metrics for the coming decades. In the longer
term, however, we suggest that parties consider adapting the choice of common
metrics to the future pathway as it unfolds, as part of the global stocktake,
if cost-effectiveness is a key consideration.
