Ecological Footprint, Environmental Impact Assessment and Measuring Sustainability

Why sustainability needs a number. Sustainability means living off natural income — the interest — without drawing down natural capital. That definition is useless for a decision until something is measured. Two tools dominate the syllabus: the ecological footprint, which measures demand on the biosphere, and the environmental impact assessment, which measures what one project will do to one place.

The ecological footprint. The footprint of a population is the area of land and water needed to supply all the resources it consumes and to absorb all the waste it produces, expressed in global hectares per person. Its counterpart is biocapacity: the productive area actually available. Footprint above biocapacity is a deficit that can only be sustained by importing biocapacity from elsewhere or by depleting stocks. Footprints rise with meat-heavy diets (a cow needs land for feed as well as itself), fossil-fuel energy (the largest component is the forest area needed to absorb carbon dioxide), unrecycled waste and long-distance imports; they fall with plant-based diets, renewable energy, waste reduction, local production and stable population. Humanity's total footprint is about 1.7 planets — not a forecast but a measure of how fast capital is being drawn down.

Reading footprints critically. A small footprint is not the same as sustainability: a densely populated country can be in deficit with a tiny per-person footprint because its biocapacity is tinier still, while a large, forested country runs a surplus that it is exporting as soybeans and beef. The index also has limits an evaluation should name: it converts everything to hectares, so it ignores water, toxic pollution and biodiversity; it hides inequality inside national averages; and it depends on assumptions about yields and carbon absorption. It is best used to compare and to track change, not as an absolute verdict.

Environmental impact assessment. An EIA is a report prepared before a major development — a dam, a road, a mine — that measures the baseline state of the environment, predicts the impacts of each option including doing nothing, proposes mitigation and monitoring, and is published for public comment before a decision. Its strengths: it forces impacts to be considered before money is committed, gives affected people a voice, and creates a baseline against which later change can be proved. Its weaknesses are the ones to name in an evaluation: it is usually paid for by the developer, so its scope and conclusions may be shaped; decision-makers may not be bound by it; indirect and cumulative effects (the road that opens a forest to settlers) are hard to predict; and monitoring after approval is often the first thing cut. An EIA is a tool that is only as good as the independence of those who write it and the willingness of those who read it.