The Sustainability Metric



“You cannot manage what you do not measure.” So runs the business adage. Immeasurables, too, often need managing, but the point remains that metrics matter. Marketers and many of the rest of us blithely dub products, cities, activities, and almost anything else under the sun “sustainable” with no quantification that might allow independent verification. If we are to manage our way to a sound environment and a durable civilization, we’ll need to weigh rigorously our progress in ways scientists can support and the rest of us agree on.

Some sustainability metrics are straightforward. The atmosphere will stabilize when the mass of greenhouse gases that humanity emits is no greater than the mass the earth reabsorbs. Global progress toward emissions sustainability can be tracked, leaving only the harder task of devising ways to mark individual and national sustainability. Since we emit more almost every year, we know we are less “emissions-sustainable” with each passing hour. How, though, do we track progress in sustaining biological diversity? With so much uncertainty about causes and rates of extinction, it is much harder to find the set point for “biodiversity-sustainable.”

Developing sustainability metrics will be an evolutionary process, an objective to work toward and use for accountability in the long conversation ahead. The authors in this section ponder the task and its implications in a variety of environmental systems and natural resources. Carl Folke opens with an assessment of perhaps the broadest and most critical range of sustainability metrics: those defining literal boundary points on the planet that we pass only at peril to our future. Among these are the two systems just mentioned—climate and biodiversity—but also key mineral cycles and changes in land, oceans, and air. Marking these boundaries and our position relative to them sometimes requires subjective judgment, yet the process nonetheless contributes to better metrics. The concepts of planetary boundaries and of the Ecological Footprint, discussed here by Jennie Moore and William E. Rees, offer among the most influential sustainability metrics yet devised, and their implications are daunting.

Renewable freshwater especially lends itself to sustainability quantification. Hydrologists have carefully measured much of Earth’s water cycle. We will never run out of water, but some societies drive themselves into scarcity by using so much water that precipitation fails to maintain levels in rivers, lakes, and aquifers. Sandra Postel explores these metrics—and finds hope for future sustainability in the fact that so much freshwater is wasted through inefficient use. Covering 71 percent of Earth’s surface, salt water offers wide scope for sustainability metrics. As Antonia Sohns and Larry Crowder note, unsustainable human behaviors of many kinds ultimately leave their mark on the seas—in acidification, rising temperatures, declining oxygen content, the onset of red tides, and the ongoing decline of fisheries. More challenging is the task of connecting each of these trends and others with the metrics of the human activities that lead to them, but that too is part of our task.

On renewable energy, Shakuntala Makhijani and Alexander Ochs approach quantification from a different perspective, measuring the potential to expand access to “sustainable energy” to the point that this all-important sector no longer adds to the atmospheric burden of greenhouse gases. Eric Zencey develops metrics for energy-related principles such as Energy Return on Energy Invested (EROI), which like unforgiving physical laws may limit how much energy humanity can mobilize and for how long. Gary Gardner takes up EROI as well, in addressing quantification of natural resources that perhaps can only be used sustainably with perfect recycling—which of course excludes fossil fuels and other resources consumed entirely by use.

Kate Raworth tackles another kind of sustainability, that of the social sphere. She takes inspiration from the planetary boundaries work to explore metrics that might help us understand when our treatment of our fellow human beings exceeds the bounds of what is needed for long-term societal survival. Social sustainability may be the hardest type to submit to measurement, but without enduring societies, a supportive natural environment will matter to few human beings. The question of how we live together on a crowded planet that unravels even as we work to hold its strands in place may call forth the most important sustainability metric of all.

—Robert Engelman

Cartoon credit page 17: Giacomo Cardelli/Cartoon Movement