There are currently 7.6 billion people on Earth. The United Nations (UN) projects that the planet’s population will increase by one billion within the next 15 years. By 2100, over 11 billion people could be inhabiting the planet.

The concept of overpopulation is not new. Thomas Malthus, an 18th century English economist famous for his theories regarding population growth and demographics, argued that the human population’s growth rate exceeded that of food production. To combat this disparity, Malthus urged for population control as an effort to conserve natural resources.

While logical in some ways, Malthus’s theory had numerous shortcomings. Firstly, he failed to account for technological advancements that would greatly increase food production. He also didn’t account for the many advances in public health, family planning, and contraception that would control human population. Furthermore, Malthus blamed the poor for their poverty, arguing that welfare would worsen the population problem by encouraging early marriage and having more children. Malthus supported regulation of the size of poorer families. This argument led the British government to proclaim that the Irish potato famine of the late 1840s was an effective method to reduce population, and bolstered the eugenics movements that selectively bred out what were considered socially undesirable—or non-white—traits.

Though flawed, Malthus’ ideas are important. Large, rapidly expanding populations lead to increased violence, poverty, and unemployment. Humanity currently depends on finite resources for energy and food production. In the natural world, carrying capacity refers to the maximum number of individuals of a species that can survive in a given environment. Using this theory, ecologists have been able to explain and observe the effects of overpopulation.

“When predators have been removed from areas, we see the effects of too many prey animals on vegetation, and their own precipitous decline in numbers,” Anthony Masi, professor of industrial relations and organizational behavior in the Desautels Faculty of Management, wrote in an email to The McGill Tribune.

Unlike other animals, humans can consciously change their behaviour patterns, such as by reducing fertility or deciding to migrate. They can also engage in trade, or expand the boundaries of their habitats. The carrying capacity of the earth regarding human population isn’t uniformly distributed between, or even within, countries. Coming up with an exact number is complex and dependent on many variables. According to Masi, the calculations are heavily debated and controversial, but various estimates put Earth’s carrying capacity for humankind at around 15 billion people.

Peter Brown, a professor in the Department of Natural Resource Sciences, said that Earth’s  carrying capacity depends on lifestyle.

“If you have a large number of people who want to eat red meat, the carrying capacity would be a lot lower than if you have a lot of people who want to eat lentils or rice,” Brown told the Tribune.

In Canada, discussions about human population growth often revolve around countries like India, since, in general, high-income countries have slower-growing populations than low-income ones. But most low-income countries are not growing at exponential rates. For Brown, the biggest problem is additional people in rich, consumerist countries.

Advanced economies are often guilty of being stuck on a “treadmill,” where they are constantly in pursuit of economic expansion via population growth, either through births or immigration. The consumerist attitude that accompanies continuous economic growth is a large part of the problem, according to Sarah Brauner-Otto, an associate professor in McGill’s Department of Sociology.

“If everybody lived the way […] that the people I study in Nepal do, we would not be talking about the population and the sustainability problem to the same degree,” Brauner-Otto said. “The resource use in wealthy countries is so astronomical that it far outweighs the high populations in any place.”

Humans have so heavily impacted the environment that many scientists are now calling the current geological era the Anthropocene—an epoch dominated by humans. We cover grass with tarmac, frack for oil and natural gas, and drain entire lakes for irrigation. But even if we’re living in  the age of humans, human lives aren’t the only ones that matter.

“All kinds of species […] are being driven to extinction, or at least to much lower numbers, in part by human population,” Brown said. “We’re one species among many, and we should learn to share the household.”

In Earth’s history, there have been five mass extinctions causing between 50 and 90 per cent of the planet’s species to die off. According to a study in Science, we’re entering a sixth mass extinction, due mainly to human activity. Brown emphasized that even though lifestyle can make an impact, a larger population will still lead to greater strain on the environment.

“Any organism, no matter how benign, degrades the environment in which it lives,” Brown said.

Pope Francis’ 2015 encyclical on climate change argued that the problem isn’t the number of human beings, but how they act.Pointing out that even if everyone were as virtuous and thoughtful as the Pope, Brown argued that the number of people is an unavoidable factor leading to environmental degradation.

Environmental impact is related to the size and growth rate of the population, how the Earth’s resources are used, and technological innovation. It can be modelled with the IPAT equation, I = P x A x T, in which “I” stands for impact, “P” for population, “A” for affluence, and “T” for technology.

Technology is a particularly important variable in the equation because it has the ability to increase Earth’s carrying capacity. People think that they can always invent something to get past limitations. However, this is public perception, though. Brauner-Otto pointed out that while innovation has the potential to expand carrying capacity, there may be limits to our ingenuity.

“We do not have evidence that our inventions can move at the pace that our current use of resources shows,” Brauner-Otto said.

On the global scale, human population depends on the world’s births and deaths. Demographic transition is the shift between low growth and high growth based on birth and death rates. Low birth and high death rates result in low growth. With improvements to public health, death rates decrease, resulting in higher overall growth. Over time, improved education, which increases job opportunities for women and makes contraceptives more available, lowers the birth rate, and low growth returns.

Factors that affect growth vary by location, even within countries. For example, mortality is extremely dependent on the quality of medical care. According to Brauner-Otto, there’s very high variation in mortality in places like the United States, where the quality of medical care is good, but access to it is inconsistent.

When Malthus started this controversial conversation in 1798, which has since pitted pessimists against optimists, he simplified the issue. To solve the population problem, its full complexity must be considered. Factors like culture, technology, social institutions, policy, and ethics cannot be ignored. High-income societies should consume less and help low-incomes ones by sharing technologies to empower women, improve living standards, and increase education—thus preventing humans from exceeding Earth’s carrying capacity.

In his 1998 book, How Many People Can the Earth Support?, mathematical biologist Joel E. Cohen presented the three schools of thought in balancing natural resources and the human population: “Bigger pie” through technological advancement, “fewer forks” through reductions in population growth rates, and “better manners” through improved governance and elimination of inequalities. Cohen envisioned bringing these philosophies together through universal primary and secondary education. Masi agreed that all three must be observed.

“They are not mutually exclusive, and we should move ahead on all three fronts to avoid ever reaching the Earth’s carrying capacity for human beings,” Masi said.