### An Introduction to Life Cycle Assessment  
Life cycle assessment (LCA) is an approach for quantifying the environmental impacts of products and services, standardized by the International Organization of Standardization [1]. LCA takes a 'cradle-to-grave' perspective by considering all phases of production, use, and end-of-life. An LCA of a product considers extraction of raw materials, transformation of materials to products, distribution and use by a consumer, and the recycling and disposal of the product. This includes the transportation, the heat and energy inputs, infrastructure, and services, and most importantly the emissions at every life cycle stage. Combining all the emissions into a single inventory, normalized to a functional unit, allows for comparisons with other technologies that perform the same function. Life cycle impact assessment aggregates and characterizes the final emissions profile to potential impacts of interest (greenhouse gas contributions to global warming potential is the best-known example, but eco-toxicity, photochemical smog, and human health effects are also common impacts estimated in LCA). LCA provides a holistic picture of the total emissions and potential impact of an economic activity, including those outside the immediate control or scope of a decision maker. It parses the total emissions down to contributions from individual activities allowing for the identification of 'hotspots'--opportunities to improve efficiency, reduce impact, and improve the overall performance of the system. In practice, an LCA is produced in four phases: goal and scope definition, life cycle inventory, life cycle impact analysis, and interpretation (see Figure 1). The goal and scope definition establishes the intention of the LCA, the study stakeholders, the processes and technologies of interest, and the expected application of the LCA results. The goal and scope definition sets the requirements for data collection regarding which processes are under consideration, and which flows to and from nature, must be measured to quantify impacts of concern (such as global warming, water quality, human health, etc.). During goal and scope definition, influential values choices are specified according to the intention of the LCA. Examples of value choices include: identifying critical life cycle processes requiring primary data collection, identifying general background data sources such as electricity and waste management models, choosing an impact method to characterize emissions into impacts, setting cut-off criteria for excluding minor or non-influential contributions, and setting rules for aggregation, co-product management, and other LCA specific modeling decisions. The requirements outlined in the goal and scope phase directly inform the data collection, organization, and modeling work, known as the life cycle inventory phase. The data intensive nature of the modeling approach often makes the life cycle inventory phase the most expensive part of performing LCA. Primary data for LCA is collected for key processes identified during goal and scope definition. Secondary data in the form of past  
Figure 1: The four phases of life cycle assessment. LCA work, engineering models, and interviews with subject matter experts is collected to fill out the rest of the supply chain. The modeling decisions and value choices documented during goal and scope definition are implemented, and produce a life cycle inventory (LCI). An LCI is a single list of emissions and quantities representing the entire production life cycle of a product of interest, normalized to a functional unit. The LCI illustrates the component emissions that contribute to impacts (for example: the CO\({}_{2}\), methane, and nitrous oxides inventory all contribute to global warming potential). The LCIA result is the primary quantitative information product of an LCA study. The LCIA result is produced by characterizing the LCI by a life cycle impact assessment method (LCIA) into an impact of interest.