Conceptual issue of the dynamic GWP indicator and solution

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PurposeDynamic LCIA has been developed in the past 12 years almost exclusively for climate change impact assessment, and especially the Global Warming Potential (GWP) indicator. Recently, discussions have taken place in France on the issue of integrating, a dynamic GWP indicator in the 2020 regulation (noted RE2020) on new buildings. This regulation uses a combination of static and dynamic indicators. Discussions to produce a standard on a dynamic GWP indicator could be soon launched at European level. This close perspective of using dynamic indicator into regulation recalls the importance using scientifically sound indicators. Therefore, this article re-examines the fundamentals concepts of the dynamic GWP indicator on which the future regulation is based.MethodThree basic principles of LCA are recalled: (i) no inventory flow should be omitted from an assessment, (ii) all impacts of all substances contributing to an impact category should have the same Time Horizon of the Impact (THI) in the indicator used for that impact category, and (iii) the time horizon of the impact has no reason to change as a function of time. It is then shown that the current and common version of the dynamic GWP (Levasseur et al. 2010) does not respect these principles. A new expression of dynamic GWP is provided in order to respect the three basic principles.ResultsBoth dynamic GWP indicators are compared according to their meaning and according to the decrease that they quantify compared to static GWP. The initial dynamic GWP indicator (Levasseur et al. 2010) sets a fixed Time of Observation Duration (TOD), chosen subjectively, after which nor flows nor impacts on climate change are assessed. The new dynamic GWP sets a TOD according to the life cycle duration (LCD) and the THI that avoids any flow omission and guarantees the full account of the impact of all flows. THI and LCD have independent values, but both time entities are related chronologically: THI has to be considered to account for the full effect for the very last emission of the product’s life cycle. Thus, the total observation duration (TOD) to consider for any dynamic approach is TOD = LCD + THI. The initial dynamic indicator overestimates the effect of temporary carbon storage of around 25% at the time 50 years. The raised issue is not only an implementation issue due to the choice of TOD, it is a conceptual issue, because even in the case of TOD > LCD, the value of THI would still be variable according to the moment of emissions. Finally, the notation between dynamic and static GWP is not harmonised which can lead to confusion.ConclusionThe initial GWP indicator (Levasseur et al. 2010) represents an assessment of the impact within a subjectively chosen observation period and neither the flows nor the effects of these flows on climate change are taken into account beyond this limit. The new equation provided for the dynamic indicator takes into account the totality of the flows and the entireness of their impact according to the value of THI; it thus represents only and exclusively the effect of delaying emissions compared to the static GWP. This new method is totally applicable for the regulation, as only new FRE2020(t) coefficients, produced in this paper, have to be changed.At a time when the dynamic GWP indicator is being considered as a regulatory tool on a French or even European scale, it seems crucial to consider its scientific relevance, because using the wrong method could lead to an overestimation of the possible beneficial effects of temporary carbon storage in the construction sector as a whole.