A crowded market in Ibadan, Nigeria, a country in which the population may reach 532 million people by 2100, according to new population projections. | International Institute of Tropical Agriculture/ Flickr

The population of Earth is unlikely to stabilize this century, according to a new analysis published in the 19 September issue of the journal Science. The findings are contrary to past studies, which have predicted that the world population will peak around 2050 and then level off or decline.

The results — based on a statistical analysis of the most recent population projections from the United Nations — suggest the global population will continue to grow through and beyond 2100. Based on their analysis, the researchers estimate an 80% probability that the world population, now 7.2 billion, will increase to between 9.6 and 12.3 billion by 2100.

"This finding is not completely in line with the conventional wisdom of the past 15 years," said co-author Adrian Raftery, professor of statistics and sociology at the University of Washington, "and this made us check all our results even more carefully."

"Our work," he said, "showed different results for two main reasons: new data and new methods."

The main driver of global population growth in their study is an increase in the projected population of Africa, the researchers found. Demographers had projected that the decline in fertility seen in Asia and Latin America since 1950 would continue in Africa, too, but Raftery and colleagues show that this decline has actually stalled in Africa.

What's more, many African women are still having larger families (the median size is 4.6 children), in part due to a lack of contraceptives. Mortality from HIV has been reduced in Africa as well, and the results of the study show the clear impact of this improvement.

Based on their results, the researchers also indicate that the ratio of working-age people to older people is almost certain to decline substantially in all countries, meaning there will be far fewer workers to support the needs of an increasing number of elderly. This shift in demographics will present economic challenges in places where elderly people are supported greatly by public welfare programs.

Rapid population increase in high-fertility countries can create numerous other challenges. "The rising population could exacerbate world problems such as climate change, infectious disease and poverty," Raftery said.

The researchers estimate an 80% probability that the world population, now at 7.2 billion, will increase to between 9.6 and 12.3 billion by 2100.

Raftery explained that most recent projections of global population have been based on data at least five years old, and on expert-based assumptions about future mortality and fertility rates, rather than analyses from statistical models.

To perform their analysis, he and his colleagues began with the latest U.N. population report, published this July, which Raferty called "the most complete and up-to-date dataset on population in the world."

The U.N. population reports feature "high" and "low" projections that have been criticized for lacking a basis in historical data, however. The high projection, for example, is obtained by adding half a child to the total fertility rate for all countries and all future time periods, something critics say is unlikely to happen for all nations and in all future years. To overcome this limitation, the team applied advanced statistical techniques to the latest U.N. data to estimate future demographic-related trends, such as fertility levels, with the greatest possible accuracy.

In addition to the U.N. report, the team used other scientific data including historical information about the rate of change in the maximum age at death in a country. This helped them calibrate possible variations in life expectancies going forward — something past studies haven't been able to do well.

Though the results of their analysis point to more of an increase in population than many scholars had predicted, the researchers also provide a hopeful note. The projected population growth could be moderated, they say, by more substantial investments in girls' education and by family planning programs that provide contraceptives. Both factors influence fertility.

"[The data could be used by] other U.N. organizations such as the Food and Agriculture Organization, by governments — particularly those that don't have their own official statistical systems — and as inputs to other models," Raftery said. "Social and health researchers dealing with international issues could use these data, too."