Sustainability of the global energy system implies an eventual shift from primary dependence on oil, coal and natural gas to even more abundant or inexhaustible energy sources with lower environmental impacts.

However, such a drastic transformation of one of the main driving forces of the world economy presents major challenges to the engineering profession. The least-cost pathways for such a transformation of the energy system are poorly defined at the present time, although there is broad consensus that the likely endpoint will be electrification of most stationary energy uses, with non-fossil hydrogen becoming the preferred transportation fuel. Ongoing technology advances will keep not only coal, but also oil and natural gas cheap and plentiful for many decades.

Therefore, this transition will probably span a century and fossil fuel consumption will continue to grow over much of this period, pending the development and commercialization of cost-competitive renewable energy options in tandem with the rehabilitation of the nuclear fission option.

Minimization of the economic, social, environ-mental and technological obstacles to an orderly transition will require continuing improvements in the emission characteristics of petroleum-based transportation fuels; dramatic increases in efficiency of automotive transportation; substantial expansion of the global use of natural gas--the least polluting of all fossil fuels; further advances in clean coal technologies for power generation to make them more competitive in first cost and efficiency with gas-fired combined turbine cycle technologies; and restoration of nuclear power as a credible contributor to sustainable development.

Engineers capable of managing energy system development so as to optimize economic and social progress while preserving environmental values will play leading roles in meeting most of these challenges. Such engineers will also be deeply involved in the implementation of the industrial ecology paradigm that will minimize and eventually eliminate dissipative material flows into the biosphere, not only from the energy system, but from all industrial activities.

An important tactical element in achieving sustainability is the consideration of all energy supply and end-use options in the context of their economic, social and environmental impacts. This concept is now generally captured by the notation Energy/Environment/Economics, or E3 for short.

Henry Linden has been an early advocate of the E3 rationale for energy decisionmaking, which evolved from the fully-internalized, least-cost energy service strategy originally proposed by Roger W. Sant.*

*Roger W. Sant and Steven C. Carhart, "Eight Great Energy Myths: The Least-Cost Energy Strategy, 1978-2000," Energy Productivity Report No. 4, The Energy Productivity Center, Mellon Institute, Carnegie-Mellon University Press (1981).