Models and estimates of Earth’s human carrying capacity vary widely and assume, rather than solve for, binding environmental constraints (the process or resource in shortest supply relative to human biological needs). Unlike “cultural” carrying capacity, which varies with human values and choices, carrying capacity as a property of the Earth is determined by fundamental biological and physical constraints--but the binding constraint, and therefore the true upper bound on the number of humans that Earth could sustain indefinitely, remains unknown. We seek to resolve this uncertainty. In our spatially-explicit model, humans depend on a population of primary producers, with which humans must share Earth’s stock of water and nutrient resources to meet biological requirements. This presents a potential stoichiometric constraint not previously explored in the literature. We also explore the dependence of human carrying capacity on our ultimate technological capabilities, sketching out a population frontier between "current", demonstrated technology and "perfect" technology constrained only by the laws of physics. Given technological uncertainties, e.g., around the feasibility of large-scale use of fusion, our model accounts for potential trade-offs between the use of solar radiation for primary production and for the electricity production necessary for agricultural activities.
Results/Conclusions
We find that under "perfect" technology, the photosynthetic production of calories constrains human population before nutrients become limiting. Under particular pathways of (limited) technological advancement, phosphorus availability becomes the limiting resource. More generally, total available photosynthetically active radiation constrains photosynthesis, though water availability binds in certain locations when storage and transport of water are limited. Our estimates of Earth's maximum human carrying capacity vary between the hundreds of billions under "current" technology to the hundreds of trillions under "perfect" technology. In no case does the estimate correspond to a desirable population. Rather, our work identifies hard upper limits on the sustainable human population and provides a modeling framework within which it would be possible to explore trade-offs between population and affluence. The model could also be used to explore the dependence of human carrying capacity on changes in climate.