# What is the maximum sustainable population of the earth?

This question prompted by discussion regarding this answer to a question about vegetarianism.

I recently came across a U.N. review of studies seeking to determine the earth's maximum sustainable human population: "One Planet, How Many People? A Review of Earth’s Carrying Capacity".

The review itself is from 2012, when the world's population was 7.1 billion. The population today is around 7.5 billion. The range of estimates reviewed goes from 2 billion to 1 trillion(?!):

Conveniently, most studies put the limit at 8 billion, a number just below the present population.

In the six years since this review, has a more robust, consensus-driven method been developed to determine the limit? What limit has been determined?

I'm interested both in the estimate itself, and a discussion of how the methodology used to make the estimate has improved over time.

## 1 Answer

The largest issue with answering this question is that it depends heavily on other factors, some of which are quite hard to predict. Consider https://en.wikipedia.org/wiki/Energy_intensity , which measures how efficient nations are at powering their needs. Consider all environmental impacts (not just energy & climate change), this is a popular formula:

This formula describes the impact of human activity on the environment as the product of three factors: Population, Affluence, and Technology.

I = P × A × T https://en.wikipedia.org/wiki/I_%3D_PAT

Also notice that even when people or countries become more efficient, that doesn't necessarily reduce damage, since use may then rise:

https://en.wikipedia.org/wiki/Jevons_paradox

If you made assumptions for all of these factors and their dependencies and interactions, as well as unspecified behaviors (e.g., diet) then one could make a rough estimate of the carrying capacity of the Earth.