Five researchers shed new light on a key argument for reducing greenhouse gases (GHGs): they provided the first economic analysis of the conversion factors of other GHGs such as methane into their CO2 equivalent in exceedance scenarios . Although the United Nations Framework Convention on Climate Change (UNFCCC) plans to be satisfied with a reference value (called “common metric”) to perform this conversion between the Paris Agreement, the models presented here show the economic advantage of flexibility between different conversion factors. “A key notion of the UNFCCC is to reduce GHG emissions in the least costly way in order to ensure global benefits” underlines Katsumasa Tanaka, lead author of the Scientific progress study.
The research provides series of dynamic variations of the conversion factors according to the possible trajectories of global warming to decrease the economic cost while maintaining a certain stability to anticipate the implementation of policies. They took into account different scenarios, one in which we achieve the Paris Agreement stabilization targets at 2 ° C and 1.5 ° C and others in which we would exceed those targets and need to step up efforts. thereafter. These exceedance scenarios violate the Paris Agreement, but the authors argued that such possibilities cannot be ruled out, given current short-term climate policies. They further noted that the feasibility of these scenarios still depends on very deep mitigation measures needed later in this century. They applied conversion factors in the numerical model and simulated the additional mitigation costs in all of these scenarios to target the most favorable values.
The choice of a common conversion factor
A CO2 equivalent conversion system makes it possible to determine the participation of different GHGs at a given time in order to prioritize actions. A well-known example is the global warming potential (GWP). To allow comparison between parties to the Paris Agreement, the 100-year global warming potential (GWP100) has been chosen as a benchmark. Since greenhouse gases have a very different lifespan and radiative impact, this conversion system depends on the choice of a time horizon.
“With GWP100, we look at the greenhouse effect accumulated over a period of 100 years, which for methane gives a conversion factor of 28. This means that a kilogram of methane is 28 times more powerful than a kilogram of CO2 ”explains Johannes Morfeldt, a collaborator of this study joined from Sweden. However, since methane has a shorter lifespan and a higher radiative impact than CO2, the cumulative effect over 20 years (GWP20) is much greater: 84 times more than 1 kilogram of CO2.
The modification of the time horizon modifies the conversion factor and therefore influences the gas which occupies a priority place in the agenda. If a kilogram of methane is 84 times larger than a kilogram of CO2, it will be more effective to reduce global emissions by reducing methane. A debate has been underway since the 90s on the conversion factor to use, and this team of researchers intends to provide additional information on their economic cost in light of the possible routes of global warming.
“We realized with our model that the GWP100 is good for decades to come, but far from ideal in the long term”, explains Philippe Ciais, one of the co-authors of the study. “We don’t see much variation in an optimal 2 ° C stabilization scenario. But in an overshoot scenario, we see a large gap between the ideal conversion factors for today and when we reach 2 ° C of If we do not have a dynamic approach to change these values along the way, then society will bear an additional cost to mitigate climate change, ”adds Olivier Boucher, another co-author.
An optimal agenda
The researchers then modeled these additional costs to estimate which conversion factor would be ideal at any given time for different temperature trajectories. They showed that embedding the GWP100 would lead to additional mitigation costs that could be avoided by switching to a dynamic factor. In a 2 ° C stabilization scenario, these additional costs round to almost 2%, but in high overrun scenarios, they go up to 5%. “This shows that the ideal conversion factors depend on a time horizon but are also mainly determined by the trajectory, and strongly influenced by a temperature overshoot” underlines Daniel Johansson, Swedish co-author.
This study shows that adapting to possible trajectories by switching from the GWP100 to shorter time horizons in the future could save additional mitigation costs compared to using the GWP100 alone. Researchers also understand that these values cannot continually change in order to anticipate and implement policies. Thus, they propose series of simple combinations of profitable conversion factors according to the possible routes. The authors suggest that “the UNFCCC and the Parties to the Paris Agreement consider adapting the choice of conversion factors to the future trajectory as it unfolds, in order to implement the lesser options. expensive to reduce greenhouse gas emissions ”.
As we do not yet know the long-term path, the issue of cost-effectiveness could be included in the technical assessment supporting the global stocktake within the UNFCCC. This key element of the Paris Agreement assesses countries’ collective progress towards long-term goals every five years and aims to increase the level of ambition of national policies. Including the cost-effectiveness of conversion factors in this recurring inventory process could allow the necessary assessment in time to inform subsequent sessions as the long-term path unfolds.
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