Impact de l'utilisation des ressources minérales et métalliques dans un contexte cycle de vie : une approche fonctionnelle

Humans have used mineral and metallic resources for millennia. As long as resources are useful to us, we will continue to use them to fulfill many functions. Mineral and metallic resources are mainly non-renewable, and the question of depletion—depriving humans from benefitting from these functions—arises. Life cycle assessment (LCA) is a decision-making tool that takes multiple potential environmental impacts into account, including the impacts of resource use. To do so, life cycle impact assessment (LCIA) methods are based on characterization factors. A characterization factor is the mathematical translation of a cause-effect chain. Characterization factors can be obtained using two approaches: a midpoint approach, which characterizes the impact earlier in the cause-effect chain and has the advantage of being robust, and an endpoint approach, for which the impact is modeled to the end of the chain, yielding environmentally relevant impact characterization with the drawbacks of uncertainties. The characterization factors required to evaluate the impacts of mineral and metallic resource use differ from one impact evaluation method to another, on both the mid- and endpoint level. There is no unanimous characterization factor for these impacts, and the scientific community has recommended improvements to the characterization factors to fill the gaps of current methods. (i) There is a need to clarify the nature of resource use impacts. Indeed, each method assesses the impacts of resource depletion differently (through exergy considerations, time before depletion, additional energy, costs required to continue extracting a resource that is becoming scarcer, etc.) and may be challenged on its relevance in assessing resource depletion impacts. Since a resource is only used when it is functional to humans, the environmental nature of the impacts of resource depletion is arguable. It is recommended to clarify the nature of the effect of the depletion of mineral and metallic resources. (ii) A resource only has value when it is used within a function for which it is useful to develop it, and a resource should be studied based on its functional value to society and not on its intrinsic value, as is currently the case with impact characterization models. A functional approach is therefore required. (iii) The functional approach implies that multiple resources can fulfill a single functionality. As such, resources are substitutable for a given function, and the substitutability of a resource by another should to be taken into account when using a functional approach to evaluate the impacts of mineral and metallic resources. (iv) Although only a few LCIA methods consider recycling or reuse, a distinction must be made between resources that are extracted and resources that are dissipated. Indeed, a resource that is extracted but recovered in its end of life is not used in a dissipative way and can be considered as a displaced stock. Therefore, it is recommended to consider recycling and reuse when evaluating the impacts of mineral and metallic resource use. Based on the literature, the main objective of this research is to develop a life cycle impact evaluation method to assess mineral and metallic resource depletion impacts according to resource functionalities, distinguishing extracted and dissipated resources and considering the adaptation of the users competing for the functionalities of these resources (through substitutability) in order to define the effect of mineral and metallic resource depletion on the users who are unable to adapt before depletion. To meet the main objective, three sub-objectives were set out: 1. Measure the competition among mineral and metallic resource users considering resource functionality, the resource dissipation rate (rather than extraction rate) and the substitutability of the mineral and metallic resources by functionally equivalent resources. 2. Define the effect of mineral and metallic resource depletion on competing users in order to establish a endpoint characterization factor for the resources that accounts for resource competition and the consequences of dissipating easily accessible stocks on the users who are not able to adapt before depletion. 3. Test the operationalization of the functional competition model through a case study and verify the robustness of the competition factors when a major technological breakthrough, such as the introduction of a global electric vehicle fleet, is implemented. A methodological framework that accounts for all the indicators relevant to impact assessment was therefore developed. The causality chain was first modeled to generate a midpoint indicator that quantifies the competition that exists between users. The competition evaluation model is based on two parameters: the number of years of resource availability and the potential adaption of users facing resource depletion. The number of years of availability is a ratio of accessible reserves over the annual dissipation rate of resources. Using the dissipation rate makes it possible to discriminate between resources that are being extracted and those that are actually lost to future users. Adaptation expresses the users' ability to substitute their resource by a functionally equivalent one. Hence, the longer resources are used, the more the fraction of initial resource users decreases. These decrease in total users, and therefore of the dissipation rate, make it possible to adjust the number of years of resource availability for the remaining users and generate a dynamic depletion index (dDI). The MACSI (material competition scarcity index) competition index, which is a midpoint characterization factor, expresses the fraction of initial users who are potentially unable to substitute their resource by another when the easily accessible reserves have depleted (i.e. when dDI is reached). Once the midpoint characterization factors were obtained, the causality chain was modeled further to determine the effect of resource use on future users. This effect makes it possible to generate endpoint characterization factors. While the reserves deplete, users try to adapt based on resource competition. Three different types of users were identified: (i) users who do not have access to a substituting resource and who will always require the initial resource provided by an ultimate technology (back-up technology) to extract it at higher price, (ii) users who do not have access to a substituting resource but who could potentially substitute the resource in the future and (iii) users who have access to an alternative resource today. The cost of type (ii) and type (iii) user adaptation is supposed to be null in the proposed model, considering that users will adapt once the resource is substituted at the same cost as the initial resource. This is consistent with the law of supply and demand. For type (i) users, the price of acquiring the resource using a back-up technology was evaluated. The underlying model hypothesis is that, for each kilogram of additional resource used, a fraction of the kilogram required by the unadapted users (MACSI) will be extracted by the back-up technology. The effect factor is therefore expressed in the difference in resource acquisition prices between today's prices and future prices when the back-up technology is required. The endpoint indicator was measured by multiplying the MACSI with the difference in extraction costs. This indicator is the additional price society incurs to access a less easily available resource. Modelling mineral and metallic resource use impacts led to midpoint characterization factors for 65 minerals and metals. At the endpoint level, characterization factors were generated for seven resources used in electric vehicle batteries. The major challenge of modeling to the endpoint level lies in the determination of back-up extraction technologies. Midpoint characterization factors are operational, and a ranking of the resources based on their competition factors is provided. These competition factors are integrated into the IMPACT World+ life cycle impact assessment method. Finally, the fact that the method relies on current resource use profiles to predict future depletion in a world in which technologies evolve at fast pace and resource needs fluctuate constitutes a challenge. The robustness of the model to a major technological breakthrough was tested. A perturbation in the data was introduced to verify the stability of the midpoint characterization factors that were developed. Indeed, the competition factors were recalculated assuming that the various goals of international transport electrification policies were met and sustained. Therefore, an introduction of 3.8 million and 15 million electric vehicles is supposed by 2020 and 2030, respectively. A lifetime expectancy of 10.8 years was supposed for light-duty vehicles. Metal and mineral requirements to maintain these fleets were analyzed, and the impacts of these fleets on metal availability and the competition among users were studied. Although competition factors and the number of years of resource availability change, the variations are relatively low and do not influence decision-making since the resource ranking remains unchanged. The MACSI is therefore quite robust, although regular updates are recommended to take technology developments and the discovery of new reserves into account. Such a large-scale transport electrification project is feasible but it shifts the burden from climate change to resource depletion. However, the burden shift does not generate notable variations in competition. Nevertheless, if the current worldwide light-duty fleet were entirely replaced by electric vehicles, resource depletion would become a major issue with the potential depletion of the world's lithium reserve bases within the next 40 years.