It is instructive to ask a class of undergraduate engineering students what "sustainable engineering" means to them. Initial answers tend to reflect common assumptions: reduce greenhouse emissions as much as possible; extend the life of products as long as possible; use as little energy or materials as possible; reduce toxics to the extent possible.

These are reasonable heuristics, and if the world were simple -- if there were no tradeoffs involved as each were implemented -- sustainable engineering would be relatively trivial. Unless considered carefully, however, these approaches may confuse as much as they enlighten -- if, for example, 1960's muscle cars had been built to last, our air would be considerably dirtier.

More fundamentally, note that, reflecting the wider social tendency to speak in terms of sustainability, but to act in terms of environmentalism, these are all environmental, not sustainability, heuristics. To some extent this reflects the institutional reality that sustainability discourses are dominated by environmentalists. It also reflects the greater ease of quantifying environmental, rather than social or cultural, issues, and the greater importance of normative values in interpreting and weighing the latter.

Moreover, the sustainability discourse arose in large part from environmentalism. This historical bias is also present in engineering: the main methods and fields of study relied on by engineers interested in sustainability -- industrial ecology, Design for Environment, Life Cycle Assessment, material flow analysis, and the like -- clearly arose from, and have not outgrown, environmental roots.

But while environmental considerations are clearly key to sustainability, it is also axiomatic that "green engineering" is not sustainable engineering.

It is critical to understand that engineers are basically problem solvers; an engineer's primary responsibility is to produce a solution that works in the real world, with all its constraints -- competitive, economic, regulatory, time to market, ergonomic. In addition, most engineering activities are necessarily highly coupled to social and cultural domains, with globalization and its mix of differing cultures, technological infrastructures, and regulatory regimes, adding to the complexity.

To enable solutions in such complicated spaces, engineers are highly quantitative both by inclination and by methodological choice; design solutions arise out of algorithmic treatment of design objectives and constraints in highly structured methodologies. Thus, engineers do not have the luxury of constructive ambiguity, a hallmark of most other sustainability discourses.

Engineers thus operate in a very different world than that of the sustainability discourse. Indeed, the latter is perhaps best understood as a modern mythology -- not pejoratively, but in the sense of a foundational narrative that helps individuals make sense of a complex and unpredictable world.

Once sustainability is thus understood, its surrounding language and culture become more intelligible. Admonitions that "the planet" is "suffering," for example, make no sense to a scientist, for planets cannot suffer; but in a mythic structure, suffering of nature, the individual, and society are frequently coupled.

The apocalyptic language around global climate change offers another example: the idea that the end is near, and that only immediate, dramatic sacrifice can avoid it, is typical of mythic structures. But neither formulation helps design complicated modern technologies.

Understanding the dramatic difference in underlying worldviews between engineering and sustainability does not mean they are mutually exclusive; rather, it is the first step in achieving more environmentally, culturally, and socially preferable engineering solutions.

This understanding, for example, explains why no engineering solution could be "sustainable," for physical artifacts can, at best, gesture at such comprehensive and frequently internally inconsistent mythic structures. But the existence and popularity of the myth also pressures engineering to continue to make fundamental advances if it is to design for, and reflect, the reigning zeitgeist.

In particular, while efforts directed towards environmentally sensitive engineering must continue, they must be augmented by increasing consideration of the social and cultural dimensions, complicated and normative as they are. This is a far more fundamental challenge to engineering, for environmental considerations, although difficult, arise from scientific discourses and can generally be quantified and integrated into engineering education and processes.

When it comes to the highly normative social and cultural dimensions of technology, however, many engineers -- like environmentalists and most of the rest of us -- intuitively favor their own values, in many cases not recognizing the historical and cultural contingency of their belief systems. Indeed, many engineers embrace sustainability not as a powerful normative system but as self-evidently valid, and their failure to understand the contingent and mythic nature of their beliefs ironically becomes a major barrier to more integrated sustainable engineering practices.