The ecological footprint is a measure of our resource use, and indicates the extent to which we are overshooting the available biocapacity of the earth. If you total up all the biocapacity and divide it by the global population, you end up with a fair share of approximately 1.8 global hectares per person. Citizens in Europe generally consume so many resources that, if everyone were to live like us, we'd need three times the biocapacity of the earth to support us.

Biocapacity is divided into six main land types:

• Cropland - subdivided into primary and marginal land (e.g. wheat and olives), measured in tonnes per hectare per year of crop that you can harvest;
• Pasture - tonnes per hectare per year of meat/dairy, though the total footprint of the livestock will take into account the cropland and fishieries needed for animal feed;
• Forestry - metres cubed per hectare per year, there is no difference between natural or managed land;
• Fisheries - the maximum sustainably caught yield in tonnes per hectare per year;
• Carbon - the area of forestry required to sequester the tonnage of carbon dioxide, including nuclear which is considered to be equivalent to fossil fuels;
• Build up land - considered to be replacing primary cropland, though this assumption is obviously shaky, e.g. think of Dubai or Tibet!

The ecological footprint is measured in global hectares, an indication of the proportion of the earth's surface required to support a particular activity. This unit takes into account the different biocapacities of each land type, and for each country/area. If the footprint were just measured in hectares, it would be a bit meaningless; if I said "I need 2.4 hectares to support me", would I mean 2.4 hectares of UK forestry or Chinese cropland? The biocapacity of each is very different. Global hectares factor in different land types and locations, and average our their biocapacity. So one global hectare represents the average biocapacity of all hectares on earth. Saying "I need 2.4 global hectares to support me" allows you to translate that into proportions of each land type, and conversely to convert your different land type dependencies into a single comparable figure.

Biocapacity and ecological footprints are calculated as follows:

	First, you calculate the yield factor for a particular type of land for a particular country (e.g. cropland in the UK, or forestry in Japan). This shows the relative productivity of a particular bit of land, so for example you can determine that cropland in the UK is x times as productive as cropland in Tibet.
	Next, you need to calculate the biocapacity of that bit of land. Multiply the area (in hectares) by the yield factor, and by the equivalence factor. This latter factor represents the relative productivity of a particular type of land (e.g. cropland) to the world average productivity of all land. So you end up with an indication of the biocapacity that accounts for the area of land, the type of land, and the geographical location of the land.
	Finally, you can calculate the ecological footprint of a particular nation / community / product. You multiply the yield and equivalence factors by the ratio of tonnage to yield for each type of land being used. The tonnage is the mass of products being consumed, whilst the yield is the number of tonnes per hectare that you would normally get from that country's land type. This all cancels out into a figure in global hectares. You can then compare the global hectares of biocapacity (supply) to the global hectares of our footprint (demand), and estimate the extent of our ecological overshoot.

Footprinting a nation

For nations and other geographically defined groups, it's relatively easy to then calculate their ecological footprint. You know the levels of consumption, so you can calculate the impact that consumption has on the earth's resources.

The ecological footprint of a nation can then be calculated in one of two ways. The most simple is the mass balance approach: take data on the consumption in tonnes (or metres cubed for forestry) of all resources for the nation, and then run that through the equation above to arrive at the eco-footprint.

The more complex approach, input-output, takes economic data as a proxy for consumption levels. Different industrial sectors are matched up against resources they use in a matrix, so the economic intensity of each sector is converted into a resource intensity. For example, the cotton textiles sector uses 1 hectare of cropland and 3 hectares of carbon land to produce $1 worth of goods, so you can multiply the total income of the sector by those figures to arrive at their resource use. These resource consumption levels can then be used to calculate the footprint. Data comes mainly from the UN Food and Agriculture Organisation (FAO), and from national statistics offices.

Footprinting an organisation

For organisations such as businesses, the footprint is less clearly defined - where do you draw the boundaries? The diagram below shows a car manufacturer with the various steps that lead to the production and eventual use of their product (a car). Traditionally companies are happy to account for the operations and the final product, and increasingly the supply chain, external activities and the product use are accounted for as well. A further challenge is for an organisation to look at their influence, both on their own sector, their supply chain, and on the external infrastructure their business requires or leads to.

A completely different approach is required, more appropriate for products and small communities - you do it "bottom up". Start by conducting a life cycle analysis (LCA) of each product consumed, taking into account every resource used from cradle to grave, and then sum up the total resources used to support that social unit's consumption levels. The lifecycle of a car, for example, will include everything from the mining of metal ores and the energy used to process that into steel, to the crushing of the car and smelting the materials back into useful scrap metal. This is where the boundary problems illustrated above become particularly fraught - do you also account for the workers' commutes, the cars they need to commute, the production process for those cars, the workers who produced the cars, and so on into infinite regress?

Top-down or bottom-up?

The approaches yield similar, but significantly different answers to the question: what is this social unit's eco-footprint? The top-down approaches provide a complete picture, but the granularity is low so you can't pinpoint specific remedies very accurately. In contrast, the bottom-up approach has a very fine granularity, since you have calculated the footprint for every product used, but you are likely to have an incomplete picture due to the difficult of LCAs and accounting for every resource used.

The top-down method works very well for nations and geographical regions, and allows us to say that certain areas are generally consuming more or less than their fair share. We can also get a crude understanding of the kinds of activities that have the greatest impact, such as food, transport or housing. The bottom-up method works very well for organisations, and reasonably well for regions, and allows us to pinpoint specific problems and remedies.

However, we shouldn't get carried away with detailed analysis of footprints without understanding the tool's limitations.

Limitations of eco-footprinting

The ecological footprint is one indication of unsustainability. Because of the limitations below, you can say that "x is unsustainable because it's ecological footprint exceeds the fair share" but you cannot say "x is sustainable because it fits within the fair share"; you would then need to account for pollution, water use, toxicity, health, happiness, and so on.

Eco-footprints don't account for:

• Any economic, political or cultural factors such as well-being;
  
  
• 78% of the surface of the earth, which is deemed to lack any biocapacity (deep oceans, deserts, mountains);
  
  
• Water and waste, except insofar as they affect the biocapacity of a region and so show up by those proxies;
  
  
• Non-renewable resources and their depletion, only renewable resources in the biosphere. The exceptions are where they affect the biosphere, for example pollution from mineral mining reducing the biocapacity of a fishery;
  
  
• Biodiversity, toxicity, pollution and other traditional environmental concerns;
  
  
• Unsustainable management of the biosphere, for example clear-cutting a rainforest for agriculture would seem to increase biocapacity because the yield factor of cropland is higher than that of forestry;
  
  
• Related to the above point, destruction of biocapacity by long-term processes such as climate change;
  
  
• The true use and exchange value of different land types, for example forestry doesn't include the pharmaceutical potential of the species that live there;
  
  
• Methane and other greenhouse gases, only carbon dioxide;

The accuracy of any given footprint analysis is also constrained by the quality of the data. The granularity of most data is very low, and the error margins quite high, so in general footprints are deemed to have an error margin of around 20-30%. Whilst the UK has exceptionally good data on the input/output economic flows, breaking it down into over 160 categories, for most countries it's around 40 categories making translations from industry to materials very rough-and-ready.

The assumptions behind the data are also problematic. For example, there is data for a variety of different kinds of cropland in the UK, but this still misses regional variations, the crops we might need to fulfil dietary requirements, and the crops we need to meet a reasonable demand profile for a wider area including exports to the continent. Imagine Japan, which is completely dependent upon imports of most raw industrial materials, and one can see how a footprint might mislead us into thinking Japan is totally unsustainable even if the trade relations with surrounding countries is in fact genuinely sustainable.

Because of these limitations, ecological footprinting should be used as one tool amongst many. It is excellent at providing an overview of global, national and regional resource use, producing headline figures. Life cycle analysis can help us analyse products and practices in considerable detail. But it would be misleading to make apparently scientific claims about reductions in the ecological footprint for those products and practices.