World population could rise by three billion more than expected

Life on earth is set to become even more cheek-by-jowl than previously thought, a report by a team of scientists and statisticians warned last night. The study, by researchers from the University of Washington and the United Nations, says there is an 80 per cent chance that by the end of this century the global population could have reached between 9.6 billion and 12.3 billion people.

The numbers, which will raise fresh alarm about the future sustainability of the human race, were arrived at using new projection models that purport to show a far narrower margin of error compared with earlier studies. The population numbers it predicts could significantly exacerbate global strains in areas such as climate change, poverty, disease control and food supply.

Published yesterday by the journal Science, the report is certain to reverberate at next week’s gathering of world leaders at the United Nations in New York, who are set to hold parallel summits on both climate change and population and development. Around 100,000 supporters of stronger steps to curb global warming are expected to march through Manhattan on Sunday. A smaller march is also planned for London.

Previous studies had led to the expectation that population, currently more than seven billion, would peak at around nine billion by the end of the 21st century before levelling off, or even declining. However, that scenario may have been premature.

Adrian Raftery, a University of Washington professor of statistics and of sociology, and a co-author of the report, said: “The consensus over the past 20 years or so was that world population [which is currently around seven billion] would go up to nine billion and level off or probably decline. We found there’s a 70 per cent probability the world population will not stabilise this century.”

The sudden uptick in projected world population is in part due to new methodology, where authors took existing UN census numbers and extrapolated future trends using additional data on variables such as fertility, mortality and sociological pressures.

Geographically, it is Africa that will contribute the most dramatically to the rise. The population in sub-Saharan Africa is expected to quadruple from around one billion people today to four billion by the end of the century. Researchers noted high levels of fertility on the continent and still-lingering traditions that give rise to large families. Other parts of the world will have less dramatic levels of growth, including North America and Europe. Asia is projected to peak at around five billion people in 2050 and then begin to decline.

While the researchers are still offering a range for the 2100 global population tally, they expressed confidence that their overall picture of higher-than-expected growth is correct. “This paper brings together the research from the past seven years, and also brings in recent data,” Professor Raftery said. “We can answer questions about future population growth using standard principles of statistical inference, which has not really been done before.”

These projections also rely less on expert opinion. “Earlier projections were strictly based on scenarios, so there was no uncertainty,” Patrick Gerland, a UN demographer who worked on the new study, said. “This work provides a more statistically driven assessment that allows us to quantify the predictions, and offer a confidence interval that could be useful in planning.”

12.3 billion Highest estimate of world population by 2100
5 billion Predicted population of Asia in 2050
7 Number of years it has taken to compile the report