New Paper Challenges Metrics of Ecological Overshoot

Is humanity really using 1.5 Earths? That is the central finding of the Ecological Footprint (EF), a widely cited global sustainability indicator used by the United Nations and major NGOs around the world to estimate the impact of human activity on the biosphere. But an open access paper published today in PLoS Biology finds the method behind the Ecological Footprint "so misleading as to preclude its use in any serious scientific or policy context."

"Does the Shoe Fit? Real versus Imagined Ecological Footprints," authored by Linus Blomqvist (Breakthrough Institute), Barry Brook (University of Adelaide), Erle Ellis (University of Maryland, Baltimore County), Peter Kareiva (The Nature Conservancy), Ted Nordhaus & Michael Shellenberger (Breakthrough Institute), decomposes the six metrics that make up the Ecological Footprint and finds that five of the six – cropland, grazing land, built-up land, forests, and fishing grounds – are either in balance or surplus, suggesting that the Earth's renewable capacity in these categories meets or exceeds human demand for these resources.

The entirety of the so-called "ecological overshoot" is attributable to the sixth metric, human emissions of carbon dioxide. As such, there is much less than meets the eye to the EF claim that it documents a substantial overshoot of the Earth's biocapacity by human societies. Yet even as a measure of the human carbon footprint, the EF uses an arbitrary methodology that is highly sensitive to its authors' assumptions. The EF methodology measures carbon footprint by calculating the hypothetical forest area needed to absorb all industrial carbon emissions, after discounting the fraction absorbed by oceans. In so doing, the EF attempts to convert atmospheric emissions of carbon dioxide into a land use category, comparable to the other five metrics tracked ­– ignoring many other means of mitigating or absorbing CO2, including carbon capture and sequestration, geoengineering, and switching to lower-carbon energy, transportation, and agricultural systems.

The EF carbon footprint measure is also highly susceptible to assumptions about the carbon absorptive capacity of forests. The size of the EF carbon footprint, and hence the entirety of the ecological overshoot claimed by the EF methodology, comes down to a single number representing the assumed rate of carbon uptake in forests. Double that rate and the carbon EF halves; halve it and the footprint doubles. Actual carbon uptake by global forests has fluctuated significantly in recent decades, between zero and 6 gigatons per year. Carbon uptake from individual forest samples varies even more substantially, making extrapolation to the global scale fraught with uncertainty.

Using a range of plausible assumptions about carbon uptake from forests results in wildly different measurements of the carbon footprint. If forests globally were to become a net source of carbon to the atmosphere in the future – an all-too-plausible scenario under climate change – the EF would approach infinity, since additional forest would augment human carbon emissions rather than offset them. Conversely, using young forests – which grow faster than more mature ones – as the carbon offset mechanism in the EF balance sheets eliminates the entire global ecological overshoot.

This makes the Ecological Footprint a highly unreliable, even misleading, indicator of ecological sustainability. Taken at face value and with a proper understanding of how it has been constructed, the EF suggests that the easiest way to avoid overshooting the planet's biological carrying capacity would be to convert huge land areas to eucalyptus plantations. We suggest that this represents neither a particularly useful guide for policy-makers nor a credible path to sustainability.

Simple and practical indicators of how well humanity is managing Earth's biological resources and ecosystem services are essential. However, these indicators must meet basic criteria for scientific soundness, something the Ecological Footprint fails to do. As ecologists and environmental scientists go back to the drawing board, we suggest a number of practical guidelines for selecting and improving ecological indicators. Key among these is the suggestion that a set of indicators is likely to be more accurate and useful than a single aggregate index.

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