Planetary Boundaries and Absolute Sustainability in Life Cycle Assessment - past, present, and future.

Efforts to steer the social metabolism (i.e., how human societies interact with nature) towards sustainability are pulling assessment practice in two directions: "relative" product-to-product comparisons and "absolute" product-to-limit benchmarking, where the limit works as benchmark for the assessment. Within approaches such as Safe and Sustainable by Design, inherently threshold-based "absolute" chemical Risk Assessment is coupled to Life Cycle Assessment, which has traditionally been used as a tool for comparing alternatives. This coupling is driving the development of methodologies for comparative assessment of (eco)toxicological impact potentials (e.g., chemical footprinting). In parallel, the emergence of the Planetary Boundaries framework has renewed the interest in shifting from comparative assessments towards absolute benchmarking against global environmental limits, leading to Absolute Environmental Sustainability Assessment. This paper reviews how environmental limits are incorporated into Life Cycle Assessment through early distance to target methods (such as Ecological Scarcity, Environmental Themes, Eco-Indicator 95) to support business-driven eco-efficiency decisions and how impacts were anchored in national or regional targets and critical loads. We then discuss how the Planetary Boundaries framework reorganizes limits around Earth-system processes and how Planetary Boundaries based Life Cycle Assessment translates global boundaries into Life Cycle Assessment indicators and Planetary Boundaries based "budgets" for products and sectors. Across these developments, we show that setting and allocating thresholds reflects conflicting notions of "weak" and "strong" sustainability and is unavoidably value-laden and argue that making this "valuesphere" explicit is crucial if Planetary Boundaries based Life Cycle Assessment is to inform credible, product-level Safe and Sustainable by Design decisions on absolute sustainability.