On the cover of Our Ecological Footprint, published in 1996, a giant foot stomps on the Western hemisphere, carrying the weight of cars, overpasses and skyscrapers. William Rees, a population ecologist at the University of British Columbia, first thought of the footprint metaphor while boasting to a graduate student about the “small footprint” of his new computer tower in 1992. Linguists trace the use of footprint to mean “space occupied” to 1965 when astronomers described the landing area for a spacecraft. It would be another fourteen years before a Senate committee first uttered “environmental footprint.” But is this the best metaphor for humanity’s impact on the natural world?
Today Our Ecological Footprint is a classic text among biologists, and “ecological footprint” and “carbon footprint” are terms as familiar as “Googling” and “selfie.” A number of NGOs offer ecological footprint calculators, including the Global Footprint Network, the WWF footprint calculator and the Earth Day Network Footprint Calculator. The calculators approximate the amount of biologically productive land required to generate the resources an individual or a population consumes and to absorb the waste that the individual or population leaves behind. The results of footprint calculators are often used to highlight global disparities. In 2007, for example, the U.S. footprint per capita was 9.0 global hectares while China’s was just 1.8. Global estimates of human impact on natural systems are bleak. Species are going extinct 1,000 to 10,000 times more rapidly than they historically have between major extinction events. There are as many introduced plant species on oceanic islands as native plants. Humans consume about one-third of all solar energy converted to plant matter through photosynthesis, and their actions directly impact 75 percent of terrestrial Earth – or, if we take climate change into account, the entire Earth. The magnitude of these changes has prompted some geologists and ecologists to favor the term “Anthropocene” when referring to the present geological age, even though the International Union of Geological Sciences still places us in the Holocene. Given the picture painted by such statistics, it is no surprise that the footprint metaphor has caught on. A footprint is a mark one never meant to leave: a revealing clue in a garden, a blemish on an otherwise sparkling floor. It evokes both the weight of whoever left it and that being’s ominous absence. Verbs popular among environmentalists include trample, trod, oppress and dominate. Heavy feet imply antagonism and the lack of intimacy between humans and the non-human world. Are we waiting for the other shoe to drop?