How big is humankind’s "footprint" on the planet? That depends on how you measure it.

Since the mid-1990s environmentalists, politicians, researchers and others have often used a concept called the ecological footprint to quantify the relative health of the planet under the influence of human activity and industry. According to that measure, our footprint has outgrown the planet on which we tread: humans now use 1.5 Earths to support our well-being. The concept has even given rise to a mock holiday, called "overshoot day," which marks the point humans exhaust the renewable natural resources that should have sustained us for the entire year. This year it fell on August 20.

But a new analysis suggests that the size of our seven-billion-person footprint has been mismeasured. "The original ecological footprint is a good metaphor and a good way of getting us thinking about overusing the planet, but what you really want a footprint to do is to be a management tool," says Peter Kareiva, co-author of the study, published November 5 in PLoS Biology. Kareiva is chief scientist at The Nature Conservancy, an environmental group that has used the ecological footprint concept from time to time. But now he is advocating for a better metric. "I would like people not to be satisfied with the current ecological footprint and try to come up with measures that really track the water, soil and all the ways we degrade ecosystems in a way that would become management metrics," Kareiva says.

At the most fundamental level, the ecological footprint incorporates six measurements—city cover, carbon dioxide pollution, farm fields, fisheries, forests and rangeland—to reveal "the aggregate area of land and water ecosystems required by specified human populations to produce the ecosystem goods and services they consume and to assimilate their carbon waste." Or so goes the definition from William Rees, an urban planning researcher at the University of British Columbia, and Mathis Wackernagel, head of the Global Footprint Network, who teamed up to develop the metric.

The essence of the case against the footprint is that, on a global level at least, the measurement all boils down to the assimilation of CO2. That is because, by definition, cropland, grazing land and other metrics of land and ocean use cannot exceed the planet’s size, as even Rees and Wackernagel acknowledge. "Unlike nations and regions, Earth cannot 'import' cropland biocapacity and therefore cannot show a deficit," Rees and Wackernagel wrote in a rebuttal to the critique.

In the global view, then, the accumulation of CO2 in the atmosphere is the sole reason that humankind’s ecological footprint is larger than Earth itself. What the outsize footprint means in physical terms is that the world does not have enough forests to soak up all the excess CO2 from human fossil fuel–burning and other activities. "It tells us that forests are not absorbing all of industrial emissions, but we all knew that before," says Linus Blomqvist, director of the Conservation and Development Program at the Breakthrough Institute, a neoenvironmental think tank. The ecological footprint, he says, is a "failed attempt at measuring carrying capacity.”

Humankind could shrink its footprint down to size by dramatically boosting the carbon uptake of the world’s trees—by, for instance, replacing forests with plantations of fast-growing trees like eucalyptus. But that would hardly be good for the planet, Kareiva notes, because natural forests deliver other benefits, such as fostering a diversity of animals, fungi, insects, microbes and plants. Nor does the footprint reveal anything specific about the potential overuse of cropland or grazing land, global deforestation or even the impact of sprawling cities.

The alternative that Kareiva prefers is what he calls an "Earth genome project"—a compilation of data that gets down to the local level on water use, soil degradation and, yes, greenhouse gas and other air pollution. Such a system would reveal whether the local water table was falling or if grazing was too intensive on a given landscape—exactly the type of judgments that the global ecological footprint is ill-suited to make. "You could overgraze an arid land and convert it permanently to desert—that's a local threshold," Kareiva explains. "We need to look for thresholds because they tell us the risk of the next degree of degradation."

The ecological footprint can, however, reveal important connections on the national and international levels, Rees and Wackernagel point out. So, for example, even though Canadians have a small footprint, Canada's excesses of cropland, forest and fisheries are essentially exported to countries like the U.S. and China, which have oversize footprints. "Most countries are in ecological deficit, increasingly dependent on potentially unreliable trade in biocapacity," Rees and Wackernagel wrote. "What could possibly be gained from ignoring footprint assessments?"