In the bustling street of Calcutta, India, the specter of excessive population growth casts a looming shadow, exacerbating resource scarcities that are already quite limited.

The current world population stands at approximately 7.2 billion, a figure that has more than tripled since the end of World War II, earning the past half-century the moniker of "Great Acceleration". This dramatic increase in human activity has been accompanied by a number of significant demographic shifts.

One of the most notable is the global decline in fertility, which has seen the median number of children per woman drop from about 5 to about 2.5 since 1970. This trend is particularly apparent in sub-Saharan Africa, where the median remains at 4.6, well above both the global mean and the replacement level of 2.1. Reducing fertility is essential if future population growth is to be reined in.

The United Nations previously projected that the world population would reach about 9 billion by 2045 and begin to level off soon after. However, ongoing studies suggest that the global population may peak at a higher figure later this century. A study in the journal Global Environmental Change projects a peak of 9.4 billion, while another UN and University of Washington study predicts a population of 9.6 billion by 2050 and up to 11 billion or more by 2100. The new UN study estimates that there's an 80% chance the actual number of people in 2100 will be between 9.6 and 12.3 billion.

The ongoing high fertility in sub-Saharan Africa is a major factor in these revised projections. Elsewhere, countries like the United States now have fertility rates slightly below replacement level. Youthful nations in the Middle East and Africa, where there are more young people than old, struggle to provide sufficient resources such as land, food, water, housing, education, and employment for their growing populations.

Improved education, especially for girls, is cited as a key driver of declining family size. Having access to reliable electricity, a luxury still denied to about one-fifth of the world's population, can be a gateway to better education for millions of young people. It can open their eyes to opportunities and choices beyond bearing many children.

Concerns about the sustainability of these growing populations are not new. The Reverend Thomas Malthus proposed in a 1798 essay that human population would grow more rapidly than our ability to grow food, leading to mass starvation. His theories have been debated ever since, with Jeffrey Sachs and Paul Ehrlich among those who continue to worry that Malthus might eventually be right.

The current rate of resource consumption is also a cause for concern. An ongoing Global Footprint Network study says we now use the equivalent of 1.5 planets to provide the resources we use, and to absorb our waste. This overconsumption has already exceeded the institute's boundaries for biodiversity loss, nitrogen pollution, and climate change, according to a study by the Stockholm Resilience Institute.

Lifting people from the darkness of energy poverty could help improve lives and potentially slow population growth. Migration, driven by the need to escape political disruption or declining environmental conditions such as chronic drought and food shortages, is another complex factor in global population trends.

Cynthia Gorney wrote about the dramatic story of declining Brazilian fertility as part of our website's 7 Billion series. Average family size dropped from 6.3 children to 1.9 children per woman over two generations in Brazil, a stark illustration of the potential for change.

As the world continues to grapple with these challenges, it's clear that finding solutions will require a concerted effort from governments, communities, and individuals alike.

In the bustling street of Calcutta, India, the specter of excessive population growth casts a looming shadow, exacerbating resource scarcities that are already quite limited.