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An environmental impact is a change in environmental conditions attributable to an activity or intervention. The change can concern biodiversity, water quality, carbon storage or another specified property of an ecosystem. Its sign and magnitude depend on the chosen outcome, spatial boundary and observation interval.

Causal interpretation requires a comparison: a change observed after an intervention might also reflect climate variation. A long-term grassland experiment directly varied species richness and followed annual aboveground biomass production. Higher species richness increased the temporal stability of total production, even though individual species could fluctuate more strongly.^[ref] The example shows why measuring an environmental impact requires identifying both the affected property and a credible comparison, rather than treating every environmental measurement as evidence of a particular cause.

John Holdren and Paul R. Ehrlich analyzed environmental impact by separating the number of people from impact per person. Their 1971 paper writes an opening decomposition of the form $I=PF$, while allowing the per-person factor $F$ to depend on population size. The identity becomes predictive only when that factor is measured or modeled. This makes the assumptions visible when comparing changes in the number of people with changes in resource pressure per person, without presuming that the two factors vary independently.^[ref]

Physicist and coauthor of Impact of Population Growth by Paul R. Ehrlich and John P. Holdren.