Over the last two decades, consumers have become increasingly aware and desiring of sustainable products. However, little attention has been paid to developing conceptual design methods that explicitly take into account environmental impact. This paper contributes a method of automated function component generation, and guided down-selection and decision-making based upon environmental impact. The environmental impact of functions has been calculated for 17 of the products found in the Design Repository using ReCiPe scoring in SimaPRO. A hierarchical Bayesian approach is used to estimate the potential environmental impacts of specific functions when realized into components. Previously, product environmental impacts were calculated after a product was developed to the component design stage. The method developed in this paper could be used to provide a criticality ranking based on which functional solutions historically have the greatest risk of causing high environmental impact. The method is demonstrated using a simple clock system as an example. A comparative case study of two phone chargers for use in third-world countries demonstrates the decision-making capabilities of this method, and shows that it is possible to compare the environmental impact of alternative function structures during the conceptual stage of design. With the method presented in this paper, it is now possible to make early functional modeling design decisions specifically taking into account historical environmental impact of functionally similar products.