Habitability Limit

Before humans had any significance influence on our planet, the population of a typical species was about two-thirds higher than in 1980, when humans began routinely consuming all of what other species “produced” (1 Earth). The total of other species with populations of 1980's size consumed nearly 1.9 Earths, which was all turned into biomass that could be consumed by others. That and the extra population consumed and recycled a total of 3.1 Earths, a measure of the total ecological resources of the planet.

As human consumption grew, other species had to do with less. The pressures of habitat loss, introduction of destructive species, pollution, and direct killing by humans caused their populations to decline, and in an increasing number of cases, go extinct. The survivors still needed the same amount of consumption for basic survival and, by extension, fulfilling their roles in maintaining the planet's habitability; but they needed to consume more resources to do so, since food was harder to find, and there was greater competition for what was left. Thanks to humans, the amount of remaining resources was shrinking a little slower than their populations, and they used all of it.

That's the picture that emerges from my latest research, based on projections of the Living Planet Index using my population-consumption model. We humans are currently consuming nearly 1.6 Earths, leaving 1.5 Earths for other species, of which they need a minimum of 1.2 Earths for survival (and are using the remaining 0.3 Earth to attain it). Because their minimum consumption is converted into both mass and function, at least 1.0 Earth is needed to keep our planet habitable (in economic terms, the “production” equivalent of their “capital”); this corresponds to human consumption of about 1.8 Earths.

If this analysis is correct, then we need to do everything we can to avoid reaching the maximum by lowering our consumption, preferably without casualties to our own population. My worst-case curve fit to our consumption over time shows that our consumption would peak when we're only within 6% of the maximum, which is close enough to add credibility that it is a real limit.