“. . . to be considered sustainable by any rigorous definition, buildings must become energy neutral and either be built or converted to a “zero energy” building standard”
If we accept that “sustainable” takes its meaning from “sustainable agriculture“, or “the ability … to produce food indefinitely, without causing irreversible damage to ecosystem health”, then the definition of sustainable building must take on a more precise meaning. A more rigorous meaning that is quite different than what typically passes for a “green” building. (See Difference between Green and Sustainable)
What exactly might that mean? To answer that question it’s helpful to have a basic understanding of “carrying capacity”. Generally speaking, “carrying capacity” is the supportable population (animals, plants, trees, people, etc.) in a closed system, given the resources (food, water, energy, etc.) available within that system.
To simplify, for our built or human environment, the Carrying Capacity (human population) of earth can be described as a function of
- Standard of Living or Rate and Efficiency of Consumption
- Renewable Resources
- Non-Renewable Resources
- The Rate of Technical Innovation
- Timeframe considered
All of these factors are interrelated. If our timeframe is limited to a hundred years or so, then rapid depletion of non-renewable resources like oil is not a factor because the carrying capacity contributed by the oil’s energy content will not collapse within that timeframe. However, if we want to create a sustainable environment lasting many thousands or even hundreds of years, then that environment cannot be based on non-renewable resources.
In truth, due to the vast array of interactions between innumerable inputs and outputs the carrying capacity of earth in absolute terms in unknowable, but it doesn’t take much imagination to accept that there must be limits. For example, the U.S. represents about 5% of the earth’s population of over 6 billion people and consumes 25% of the worlds non-renewable oil resources. So it’s a fair assumption that it would far exceed the earth’s carrying capacity if the balance of world’s population were to attempt to achieve our “Rate and Efficiency of Consumption” (Standard of Living). However, that’s exactly what is happening today in China and India.
Within any closed system like planet earth, there are critical constraints to carrying capacity. For most of human history food, water, and biomass energy sources like wood where the primary constraints, however technical innovation and an apparently “endless” supply of fossil fuel (coal, oil, gas) energy greatly expanded the world’s food supply. In fact, since the discovery and exploitation of oil’s energy dense chemistry a little over a hundred years ago, the world’s population has increased from about 1.3 billion to 6.6 billion. Of the current population, about half live in poverty, one fifth are severely undernourished, and the balance of us live in comparative comfort and health.
Even those of us living in comparative comfort and health are now facing two serious constraints to the apparent safety of our carrying capacity. Both of these constraints are related to our use of fossils fuels.
Global warming threatens both the world food supply with rising temperatures and our coastal built environment with rising sea levels. Declining crude oil production after we reach “peak oil” will essentially collapse the contribution to carrying capacity provided by cheap oil over the last 100 hundred years. Without rapid and sustained technical innovation, the end result will be either a drop in population or standard of living or both.
What does this have to do with sustainable building? Since buildings are both a major consumer of energy and responsible for approximately 50% of the world’s green house gas emissions, to be considered sustainable by any rigorous definition, buildings must become energy neutral and either be built or converted to a “zero energy” building standard. Anything less may be marketed as “green” but cannot be considered sustainable.