Defining and determining ecological footprints
Determining ecological footprint is essentially measuring supply and demand on a global scale for an entire planet's worth of commodities and services. Ecological footprint (EF) analysis does this by assessing the biologically productive land and marine area required to produce the resources a population consumes and absorb the corresponding waste, using prevailing technology. This approach can also be applied to an activity such as the manufacturing of a product or driving of a car, and everything considered in a given footprint analysis is converted into a normalized measure of land area called 'global hectares' (gha). On a large (countries, or continents, for example) scale, footprint analysis is useful for determining to what extent a nation uses more (or less) than is available within its borders, or to what extent the nation's lifestyle would be replicable worldwide. On a smaller scale, EF analysis can be a useful tool to educate people about how their lifestyle compares with the planet's ability to keep up and renew the resources used, with the goal of changing personal behavior to be more sustainable.

Average ecological footprints: where are we now?
Currently, according to the ecological footprint calculator at the Earth Day Network, if everyone in the world lived like the average U.S. citizen -- that is, had similar eating, transportation, living, and consumption habits -- we'd need 5 1/3 planets to support ourselves; the planet has about 4.5 biologically productive acres for each person in the world, and the average ecological footprint in the US is 24 acres. Yikes.

How should ecological footprints be calculated?
These numbers raise a bigger question: how should these footprints even be calculated? While there has been some disagreement about how EF should be calculated, there was a standard set of ecological footprint calculation standards put forth in 2006 that detail both communication and calculation procedures; these standards have helped clear up some sticking points, like how sea area should be counted, how to account for fossil fuels, how to account for nuclear power (many studies simply consider it to have the same ecological footprint as fossil fuels), which data sources used, when average global numbers or local numbers should be used when looking at a specific area, how space for biodiversity should be included, etc.

That doesn't mean that the method is perfect; it's a very macro-level analysis that casts a very wide net, and can't consider every variable the planet's systems have to offer. For example, if yields from organic agriculture are less than their conventional counterpart, the organic farms' footprint would be larger even though organic food is widely considered more planet-friendly than the alternative. Similarly, mechanized rural farmers in developed nations can easily consume more resources than urban inhabitants, due to transportation requirements and the unavailability of economies of scale.

Reducing our ecological footprints
Still, it's a valuable system for determining how humans are using the planet's resources (but, by no means, a silver bullet, or only necessary calculation system). For societies, EF analysis shows who's using the lion's share of resources (and who isn't); for individuals, calculating your footprint serves as another remind that everything we do must come from somewhere (and go somewhere when we're done with it) and that every little thing adds up. The Global Footprint Network has contextualized all of this for us with Ecological Debt Day, the point on the calendar each year where we've used up a year's worth of resources. In 1987, it was December 19; in 2007, it was October 6.

Ecological footprints: learn more in TreeHugger
In TreeHugger, learn more about the effect of meat in our diets on ecological footprints, check out how Radiohead is reducing theirs and read our interview with Mathis Wackernagel, who originated the concept.

Elsewhere, take the quiz at Earth Day Network, play American Public Media's Consumer Consequences game, read up on the finer points at the Global Footprint Network and learn more (and take another quiz) at Redefining Progress.

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