The United Nations Framework Convention on Climate Change (UNFCCC) uses fixed reference values to convert a greenhouse gas emission to its CO2 equivalent. However, researchers emphasize the value of flexibility in these conversions to lessen the economic costs of limiting global warming.
For example, in the 20 years following its emission, one kg of methane generates a cumulative greenhouse effect 84 times greater than that of one kg of CO2. But over a century, this conversion factor falls to 28 because of the faster disappearance of methane. In order to act on the greenhouse effect over a relatively short period of time, it may therefore be advantageous to take significant action against methane.
The scientists calculated the economic cost of different greenhouse gas "trajectories" over the 21st century, using conversion factors that vary over time. They examined scenarios consistent with the Paris Agreement in which warming is stabilized at 2°C or 1.5°C and so-called overshoot scenarios, in which this target is substantially exceeded before being returned to target values. In a scenario without any overshoot, taking variable durations into account reduces the economic cost by 2% when calculated with a fixed duration. This reduction rises to 5% for a scenario with overshoot.
The researchers have proposed simple and realistic combinations of conversion factors that minimize the costs of climate change mitigation. They suggest that the UNFCCC and the Parties to the Paris Agreement periodically re-evaluate the conversion factors, as the greenhouse gas trajectory reveals itself, in order to put in place the most economically advantageous emission reduction policies. This issue could be included in the UNFCCC global stocktake, which are scheduled every 5 years to measure the progress made by the various countries.
This work was conducted in collaboration with the Institut Pierre Simon Laplace, Chalmers University of Technology (Sweden), and the National Institute for Environmental Studies (Japan).