A correspondent brought this new paper to my attention:
Binder, Seth, Ethan Holdahl, Ly Trinh, and John H. Smith. 2020. “Humanity’s Fundamental Environmental Limits.” Human Ecology, April.
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). 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 by considering a full range of technological possibilities and incorporating a potential stoichiometric constraint not previously explored. We find that limits to photosynthesis constrain population before micronutrients become limiting unless technological capabilities for utilizing nutrient resources lag far behind other technologies. With ideal technology, human carrying capacity runs into the tens of trillions, while with currently demonstrated technology Earth could support more than 200 billion humans. These numbers reflect neither a desirable nor a natural equilibrium population level, but represent a rough estimate of the maximum number of humans Earth could sustain.
This is in the same order of magnitude as my ~100 billion estimate.
Our results also approximate those of models tied to current technologies. De Wit (1967) and Franck et al. (2011) estimate carrying capacity subject to the current technological frontier while allowing harvest on all land surface. De Wit assumes away water flux constraints, producing a carrying capacity estimate of just over 1 trillion. With similar land use and technology assumptions, our model suggests a carrying capacity of 2.11 trillion. Unlike De Wit, Francket al.incorporate waterflux constraints and effectively assume run-of-river irrigation without water storage, resulting in an estimate of 282 billion in their “Genghis Khan” scenario. Under similar assumptions, our model estimates a carrying capacity of 771 billion individuals. Indeed, we estimate that current farmland alone could support 245 billion.
In my AoMI series, I speculated that the most likely Malthusian constraint in a much higher population and higher fertility world would be increasingly frequent and lethal epidemics.
This would make sense from two perspectives:
(1) Bigger population = more experiments that viruses can undertake, in the cases where they are not contained.
(2) Decline in nutritional quality makes people like physiologically robust, along with continued accumulation of genetic load and perhaps an idiocracy-driven deterioration of epidemiological capacity allowing for the return of endemic diseases once suppressed.