Résumé
Bioenergy systems are promoted in an effort to mitigate climate change, and policies are defined accordingly to be implemented in the coming decades. Life Cycle Assessment (LCA) is used to assess the environmental performance of bioenergy systems, yet subject to the limitations of static approaches. In classical LCA, no temporal differentiation is undertaken: all inventoried instant to long-term greenhouse gases emissions (GHG) are aggregated and characterised in the same way, over a fixed time horizon, by means of fixed characterisation factors. Positive and negative impact contributions of dynamic biogenic carbon (Cbio) sum up to zero, yielding the same result as carbon neutral estimates. Climate mitigation results are biased without the temporal consideration of these flows. The purpose of the study is to highlight the time-sensitive potential climatic consequences of policy-driven transport strategies for metropolitan France, in the specific context of the dynamic LCA framework and climate change mitigation. We therefore propose a dynamic approach coupling a partial-equilibrium model (PEM) with dynamic Cbio models. The PEM analyses in detail the techno-economic performance of the metropolitan French energy-transport sector. It explores prospective optimization options (supply-demand equilibrium) of emerging commodity and energy process pathways in response to a policy in question. The Cbio model generates dynamic inventories of the Cbio embedded in the primary renewable biomass outputs of the PEM. It captures the dynamic Cbio exchange flows between the atmosphere and the technosphere over time: negative emissions from fixation (sequestration) and positive emissions from release (e.g. combustion or decay). A dynamic impact method is applied to evaluate the mitigation effects of Cbio from forest wood residues by comparing the climate change impacts from complete carbon (fossil + biogenic) with carbon neutral inventories across scenarios. Two sets of results are computed concerning the overall transport (all emissions) and bioethanol (wood-to-fuel emissions) systems. The mitigation effect from long-term historic sequestration allocated to bioethanol (462%) is significantly larger than for transport (3%), expressed as the difference with carbon neutral estimates. The fossil-sourced emissions from bioethanol production represents only 5.4%. In contrast, a comparison with an alternative reference scenario involving wood decay demonstrated higher impacts (i.e. an increase of 316%) than carbon neutral estimates. The representation of the actual climatic consequences depends on the chosen fixed end-year of the dynamic impact assessment. Moreover, the mitigation effect is proven sensitive to the rotation length of forestry wood: the shorter the length the lower the mitigation from using renewable forest resources. Other energy-policy scenarios, Cbio modelling approaches and consequences of indirect effects should be further studied and contrasted.