The environmental carrying capacity (CC) is defined as the capacity of the earth to absorb or tolerate potentially stressful burdens imparted at various scales and locations, that is, to accommodate the ecological stresses without showing permanent damage. The CC can be used as a reference dataset for Life Cycle Assessment (LCA) purposes and as a baseline for other environmental studies.

In this research, a set of impact-oriented U.S. CC is developed for both input- and output-related impacts. CC for eight common impact categories is evaluated: resource depletion, global warming, ozone depletion, acidification, eutrophication, photochemical ozone formation, human toxicity, and eco-toxicity. Numerous sources of information and various environmental models are used to estimate the CC at the appropriate scales. The CC for output-related impacts is mostly based on the threshold-oriented technique using threshold concentrations in environments. A CC is basically determined from the emission that causes the environmental conditions not exceeding the threshold levels.

The CC estimates are applied as the baseline reference for the Sustainability Target Method (STM), a Life Cycle Impact Assessment (LCIA) method, in three LCA case studies. The STM is a single-score LCIA method which offers an absolute metric for environmental performance evaluation. The STM not only compares alternatives in terms of environmental performance, but also evaluates the performance by identifying the significance of impact in relation to the earth's carrying capacity. The case studies presented are the LCA of electrical energy generation using various fossil fuels, the production of various basic materials, and the production of a coffee maker. The results are compared with those of other LCIA methods: Eco-Indicator 95, Eco-Indicator 99, BPS, and EDIP.

The advantages of using the STM in conjunction with the CC estimates are that: it provides an absolute metric related to environmental sustainability; it allows economic consideration; it eliminates the subjective weighting procedure inherent in other LCIA methods; it deals with the temporal and spatial variations in life cycle stages; and it is flexible and not limited to the selection of impacts.