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Sustainability - Definitions and Criteria

By Danny Harvey, Department of Geography, University of Toronto

Executive Summary

Human civilization and societies, with their accompanying needs for energy, water, materials and nutrients, can be regarded as “sustainable” if they can continue indefinitely, the ultimate limit being set by the expected remaining lifespan of a habitable Earth, which is about 1 billion years. In practical terms, the changes in practices and attitudes that would be required to sustain human civilization over a period of only 1000 years are so large that they could very well be sufficient to sustain human civilization indefinitely.

The specific requirements for sustainability with regard to the construction, operation and eventual demolition of buildings include:

  • Reducing energy demand to levels that can be supplied from truly renewable energy sources
  • Reducing the net consumption of non-renewable materials over the entire lifecycle of the building to essentially zero
  • Minimizing the consumption of water in buildings and facilitating the infiltration into the ground of rainwater falling on the building and its property
  • Facilitating the complete recycling of all nutrients in human and food wastes


Human civilization and societies, with their accompanying needs for energy, water, materials and nutrients, can be regarded “sustainable” if they can continue indefinitely. How long is indefinite? The ultimate limit is the expected remaining lifespan of a habitable Earth, which is limited to about 1 billion years due to the inevitable expansion of our sun as it approaches its own spectacular end. In a fascinating article on what could be termed “deep” sustainability, Bruce Tonn provides two alternative criteria for sustainability of human civilization.1 The first, and more stringent criterion, is the ability to sustain intelligent life on Earth to the point where it develops the knowledge, and retains the capability, to find and colonize another planet orbiting a younger star, in another solar system, before the eventual oblivion of the Earth. A less stringent criterion is the ability to sustain intelligent life on Earth that acquires the knowledge, and retains the capability, to prevent that next collision of a comet, asteroid or other large object with the Earth and the ensuing mass-extinction that would otherwise occur (as has occurred every few 100 million years or less in the past, throughout the Earth’s history). Needless to say, these concepts of sustainability involve very, very long time frames.

Elsewhere,2 Tonn has argued that adopting a 1000-year planning horizon provides a sufficiently long time frame to ensure sustainability, because this time horizon is long enough to unmask the big picture problems that are hidden at shorter time scales, and because, if can manage to sustain our current civilization for 1000 years, we can probably sustain it indefinitely. Thinking on a 1000 year time horizon forces the development of a whole new mindset, particularly regarding growth on a finite planet and the conservation of resources.

Criteria for Sustainability

The most important condition for the longterm sustainability of human civilization is to maintain the integrity of the life-support systems on which we depend. We are part of a complex, interconnected web of life that has evolved over the course of the Earth’s history. One of the keys to maintaining life-support systems is to maintain biological (and genetic) diversity. Advanced civilizations also require energy and resources, which imposes additional constraints. As circumstances change over the coming decades, centuries and millennia, human social and political systems will need to be able to adapt and adjust in a timely manner. It is not clear, now, which of the many social and political systems, and cultural frameworks found in the world today, will be most adaptable in the long run. Thus, our prospects for long term survival are much greater if we maintain cultural and political diversity among human societies, as well the biological diversity of the natural world.

In light of these and other considerations, we propose the following over-arching, strategic-level conditions that must be satisfied in order to ensure longterm sustainability:

  • Protect global biodiversity and maintain functioning ecosystems
  • Maintain the composition of the atmosphere and oceans within ranges suitable for the longterm thriving of life
  • Maintain cultural and political diversity
  • Manage the human population – at the least, limiting further growth, and possibly managing a slow decline over the next few centuries.
  • Manage nuclear and hazardous wastes (ensuring their long term isolation from the biosphere)
  • Prevent the accumulation of toxic materials in the biosphere
  • Maintain adequate access to all of the elements needed to maintain an advanced civilization, including metals used in a wide array of technologies and the nutrients needed to maintain food production at levels sufficient to support the longterm human population.

To protect global biodiversity requires that we reserve sufficient space for natural ecosystems to thrive and to be able to adjust to changing climate (which, over the next few centuries, will be dominated by human emissions of greenhouse gases). This requires, among other things, building compact cities and using materials that are not extracted from natural ecosystems in a destructive way or in ways that undermine their longterm viability. To maintain a suitable composition of the atmosphere and oceans requires severely limiting the further buildup of greenhouse gases in the atmosphere and most likely reversing some of the increase that has already occurred by the end of this century. This requires a rapid (within decades) shift away from fossil fuels to a completely carbon-free energy system. Almost all of the CO2 that we emit into the atmosphere will ultimately by absorbed by the oceans, provoking acidification of the oceans and threatening mass extinction of marine life (as occurred repeatedly in association with large CO2 injections into the atmosphere in the Earth’s geological past), so restricting CO2 emissions also protects the composition of the oceans and the integrity of marine life, which in turn influences our own atmosphere and climate.

Implications for Architects and the Design of Buildings and Communities

Specific conditions that would have to be satisfied for a building or community to be regarded as “sustainable” include:

  • Reducing energy demand to levels that can be supplied from truly renewable energy sources.
  • Reducing the net consumption of non-renewable materials over the entire lifecycle of the building to essentially zero
  • Minimizing the consumption of water in buildings and facilitating the infiltration into the ground of rainwater falling on the building and its property
  • Facilitating the complete recycling of all nutrients in human and food wastes

To elaborate briefly, a measure of energy use in buildings is the energy intensity – MJ or kWh of energy use per square meter of building floor area per year (MJ/m2yr or kWh/m2yr). The product of energy intensity and floor area gives the total energy requirement. Some of this energy could be satisfied through on-site renewable energy systems, such as photovoltaic modules or building-integrated wind turbines, but the balance will have to be supplied from off-site renewable energy that is transmitted to the building site. The building energy intensity that can be regarded as “sustainable” on a regional basis therefore depends on the ultimate building floor area, which in turn depends on the longterm regional population and floor area per person, and on the capacity to supply energy from sources such as wind, hydro, centralized solar electricity generation, and biomass. As population and per capita floor area in particular are subject to change, and the truly sustainable energy supply is uncertain, there is not a hard and fast definition of what would constitute a “sustainable” energy demand. However, it is clear that any sustainable energy intensity will be several times smaller than that which is typical of new buildings today. With regard to heating energy use – which is by far the largest share of building energy use in Canada – a good benchmark is the internationally recognized Passive House Standard – which is a heating load of 15 kWh/m2yr. By contrast, many new buildings in Ontario – especially the largely glass condominiums that are so popular now – have heating loads in the range of 120-150 kWm/m2yr.

Modern buildings are also highly dependent on mechanical systems for cooling and ventilation and electric lighting systems even during daylight hours. Truly sustainable buildings, in contrast, will largely rely on passive solar heating, passive ventilation, natural and low-energy cooling techniques, and daylighting. As transportation energy use is a significant share of our total energy use, appropriate site selection – and in particular, the proximity to urban transit systems – is another element of truly sustainable buildings.

Buildings are a major consumer of most of the key materials that we produce or extract from the Earth, including cement, steel, copper, aluminum, plastics and wood. We need to minimize the use of these materials in new buildings, but also design buildings to be as long-lasting as possible (by designing for adaptive re-use as needs change) and to make it possible to easily separate and recycle all of the building materials (except cement, which cannot be recycled) when the building finally reaches the end of its life. In this way, the net material use – from construction to demolition and recycling – will approach zero.

The most significant use of water in Ontario, at 83% of total intake, is for cooling of thermal power plants used to generate electricity, the majority of which is used in buildings.3

The second and third largest uses are for manufacturing (at 10%) and municipal water supply (at 5.2%). Although water use in buildings is not a large use of water in Ontario, the provision of water to buildings does have implications for energy supply and for the cost of infrastructure to provide that water. In some locations in Ontario, and more-so in other parts of the world, water supplies are critical, and every effort needs to be made to use water efficiently. Measures to reduce the demand for electricity – especially at times of peak demand, which is met by thermal power plants – have significantly implications for the demand for water. Ultimately, our rate of consumption of groundwater must balance the longterm rate of replenishment of groundwater supplies, and our extraction of water from rivers and lakes must leave adequate water supplies for other species and ecosystems to thrive – with due allowance for diminished water supplies in the near future due to global warming. The generation of storm water often overloads storm water systems, and much of the rainwater that could be used to regenerate local groundwater supplies – necessary for the maintenance of healthy tree canopy during the hot and dry spells that are expected to become more frequent in the coming decades – is lost.

Finally, long-term sustainability of the human population requires maintaining adequate food production, and this in turn requires that all nutrients taken from the soil are ultimately replaced or returned to the soil. As the supplies of minable nutrients (phosphorus in particular) are limited, this ultimately requires close to 100% recycling of nutrients back to the soil. This has massive implications for the entire food production system, but also has implications for the design of human waste systems in buildings. In particular, toilets and plumbing systems designed to separate urine (where most of the excreted nutrients end up) and solid matter will have to become standard, with minimal dilution of liquid wastes so as to facilitate extraction of nutrients with minimal energy expenditure. This will have to be integrated into much larger nutrient recycling systems that are yet to be designed.

Prepared by: Danny Harvey, Department of Geography, University of Toronto (harvey@geog.utoronto.ca) 

1 Bruce Tonn, “Futures sustainability”, Futures vol 39, pp1097-1116, 2007.
2 Bruce Tonn, “Integrated 1000-year planning”, Futures vol 36, pp91-108, 2004.
3 Environment Canada: Water, Water Uses, Withdrawal Uses (http://www.ec.gc.ca/eau-water/default.asp?lang=En&n=851B096C-1