Sustainability challenges confronting society in the 21st century include global climate change, declining fossil resources, persistent organic pollutants, freshwater scarcity, ecosystem degradation, biodiversity loss, overpopulation, and limited access to basic human necessities particuarly in developing countries. Ultimately, natural resource depletion and pollution are driven by material and energy flows associated with goods and services. The life cycle of a product system, which includes the material and energy flows across materials production, manufacturing, use and service, and end-of-life management stages, is a logical framework for understanding and improving the link between production and consumption activities and natural systems. It has become clear that significant changes in the production and consumption of goods and services are essential for maintaining the planet's life support system, which is increasingly threatened. Life cycle-based sustainability models and metrics play a key role in guiding the transformation of technology, consumption patterns, and corporate and governmental policies for achieving a more sustainable society.
Life cycle modeling represents a unique sustainability assessment framework for at least four reasons:
1) the life cycle of a product system encompasses all processes for addressing societal needs including materials production through end-of-life management.
2) the life cycle links production and consumption activities
3) the life cycle boundary enables a comprehensive accounting of sustainability performance including environmental, social, and economic metrics
4) metrics can be used by key stakeholders that manage and control the life cycle supply chains to guide their improvement
A wide set of analytical methods and tools have been developed around a life of cycle system boundary. Table 1 provides a list of life cycle based techniques and examples of metrics that have merged over the last three decades.
These tools yield a wide array of metrics that can contribute to the understanding and assessment of environmental, social, and economic sustainability of goods and services. Life cycle methods serve to help operationalize the broader concepts of sustainable development as articulated in the Brundtland Commission definition: development which "...meets the needs of the present without compromising the ability of future generations to meet their own needs."
The objective of this chapter is to review the range of life cycle methods and metrics for evaluating the sustainability of products and technology. This review will highlight the relevant aspects of sustainability that each method addresses. In addition to analyzing these tools, this chapter will demonstrate the application of life cycle models and metrics for diverse sectors including transportation, buildings, renewable energy, and consumer products. Strengths and limitations of these methods and metrics for assessing sustainability will also be discussed. The authors envision that life cycle metrics and indicators will continue to evolve in the decades ahead and in the process provide more explicit meaning to the term sustainability.