How many people can Earth handle?

According to Alex Ezeh, a professor of Global Health at Drexel University, Pennsylvania, the absolute number of people in a country is not the most important factor. Instead it’s the rate of its population’s growth or decline that is key to a country’s future prospects – this determines how quickly things are changing.

Take Africa, where Ezeh explains that there are radically different rates of population growth currently occurring, depending on where you look.

“In a number of countries, particularly in Southern Africa [one of five regions defined by the United Nations], fertility rates have really dropped and contraceptive use is up – the rate of growth of the population slowing down, which is in some ways good news,” says Ezeh.

At the same time, some Central African countries still have high rates of population growth, as a result of high fertility and longer lifespans. In some places, it’s well above 2.5% per annum, “which is massive,” says Ezeh. The population will double every 20-plus years in a number of countries.”

Even within a single region, different countries can be on surprisingly different paths – Ezeh gives the example of the East African neighbours Burundi and Rwanda. While the former still has high levels of growth – at 5.3 births per woman – in the latter growth is slowing down, with 3.9 births per woman in 2020 compared to 4.5 in 2010.

“I think the conversation about size and numbers is a misplaced conversation”, says Ezeh. “Think of a city that is doubling every 10 years – and that’s a number of cities in Africa – which government really has the resources to improve every infrastructure that currently exists every 10 years, in order to maintain the correct level of coverage of those services?”

Ezeh explains that in particular, it’s difficult to support the development of human capital under conditions of extreme growth – which research has found plays an important role in the happiness of people in cities, even more than the amount of money they’re earning. It’s also thought to be an important predictor of economic growth, in addition to the sheer number of people in a country.

“When economists think about it, a large population is great for many different outcomes, but do you achieve that large population in 10 years or 100 years or 1,000 years? The longer it takes to get there, you can put in place the right structures in the system that will support that population,” says Ezeh.

One factor with a well-documented role in slowing down this rate of growth is the education of women, which has the side-effect of increasing the average age at which they give birth. “Over time, women get access to education, they have positions outside the family, jobs, all of those that compete with childbearing,” says Ezeh.

However, Ezeh is keen to emphasise the merits of education independent from their impact on population size – it’s one of the UN’s 17 sustainable development goals. This gets to the heart of one modern view on population engineering – policies should be implemented for the benefit of society, and if they happen to lead to beneficial demographic changes, that’s just a bonus.

“I think one of the things we don’t want to do is to instrumentalise female education and make it that we want them to go to school because we want women to have fewer children… there are a lot of positives that we cannot minimise by thinking about it in terms of fertility reduction,” says Ezeh.

In fact, the cascading side-effects of policies implemented for other reasons highlight a striking reality of population science – just how imprecise its predictions often are. Across the globe, the decisions made by governments over the coming decades will be hugely influential in determining how many people there are on the planet – with the power to bump us from a future in which there are 10 billion people, to one in which there are 15 billion, and vice versa.