A sustainability gap is the difference between the resources a person or population consumes and the resources available to support them. If demand exceeds supply, the sustainability gap measures by how much. If supply exceeds demand then no gap exists.

The concept can be applied to different resources including water, food, energy, or it can be aggregated in a measure such as the ecological footprint. (An ecological footprint measures the ecologically productive land and sea areas required to support a given population over the course of a year at their current rate of technology and affluence.)

Being able to assess what resources are available, for example on an annually renewable basis, and whether an individual or group is consuming within renewable limits is essential for living and developing sustainably. While this may seem self-evident, it is surprising how few cities are measuring their sustainability gap.

The International Ecocity Standards (www.ecocitystandards.org) supports cities that exist in balance with nature. It includes measures that inform cities about the amount of resources available on an annually renewable basis and the amount being consumed. This informs whether there is a sustainability gap.

Almost any city of significant size will exceed the resources that are available on an annually renewable basis in its bioregion. (A bioregion is defined by the topography that surrounds a city and directs its water flow, i.e., its watershed.) However, the degree to which this happens will be determined in part by the inherent ecological capacity of the city’s bioregion and in part by the level of consumption of the population that lives there.

Globally, one can get a sense of the world’s average consumption relative to annually available, per capita renewable supply by using the Global Footprint Network’s measure of ecological overshoot. Ecological overshoot is measured using the ecological footprint. However, it can be difficult to understand how the ecological footprint (measured in hectares of ecologically productive land and sea area) compares to the actual consumption of food, energy and water we experience in our daily lives.

The attached table helps provide some insights into how the sustainability gap can be represented in terms of a fair earth share (no gap), the global world average (with a sustainability gap equivalent to the resources produced by ½ of planet earth), and in comparison to a high-consuming culture such as North America (with a sustainability gap equivalent to the resources produced by 2 planet earths). In other words, if everyone consumed at a rate equivalent to the world average, it would require 1 and a half planet earth’s to produce sufficient resources and waste assimilative services. This means that the sustainability gap is equivalent to half a planet (because we only have one planet earth to share). Similarly, if everyone in the world consumed at a rate equivalent to an average North American, it would take three planet earth’s to produce sufficient resources and waste assimilative services. This means that the sustainability gap is equivalent to two planets (because we only have one planet earth to share).