NEW YORK • United Nations demographers have anticipated since last year that the world's population may stop growing by 2100 as fertility rates decline, projecting a peak of 10.9 billion people by the century's end, compared with roughly 7.8 billion now.
But a study published on Tuesday in medical journal The Lancet has challenged that forecast, with major economic and political implications.
The study asserted that the global population could peak at 9.7 billion by 2064 - nearly four decades earlier - and decline to 8.8 billion by 2100.
Moreover, the study concluded that the elderly will make up a bigger chunk of the total than projected in the UN forecast, and the populations of at least 23 countries, including Japan, Thailand, Italy and Spain, could shrink by more than 50 per cent.
The study also projected significant declines in the working-age populations of China and India, the two most populous countries, portending a weakening in their global economic power.
The study's projections, if borne out, also carry significant consequences for the United States, whose economy is expected to trail China's in size by 2035.
As China's working-age population declines in the second half of the century, the study said, the US could reclaim the top spot economically by 2098 - if immigration continues to replenish the US workforce.
India's gross domestic product will rise to take the No. 3 spot, while Japan, Germany, France and Britain will stay among the world's 10 largest economies.
Brazil is projected to fall in the rankings from the eighth largest economy today to 13th place, and Russia from the 10th spot to 14th.
Historical powers Italy and Spain, meanwhile, will decline from the top 15 to 25th and 28th, respectively.
Indonesia could become the 12th largest economy globally, while Nigeria - currently 28th - is projected to crack the top 10.
"By the end of the century, the world will be multipolar, with India, Nigeria, China and the United States the dominant powers," said Dr Richard Horton, editor-in-chief of The Lancet, describing the study as outlining "radical shifts in geopolitical power".
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S'pore fertility rate: Small rise expected
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According to the Lancet report, several governments have pursued explicit policies to increase fertility rates.
Some, such as Singapore, Sweden and Taiwan, have tried to create positive environments, including paid maternity and paternity leave, as well as financial incentives for more children, to encourage women to have more children, said the report.
Sweden has seen an increase in its total fertility rate (TFR) from 1.5 in the late 1990s to 1.9 last year.
"By contrast, positive incentives have had little effect in Singapore and Taiwan" where TFR levels were 1.26 and 1.04 respectively, said the report, which, however, expects Singapore's TFR to rise slightly to 1.27 in 2100.
Dr Christopher Murray, director of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington's School of Medicine and who led the study, said: "Continued global population growth through the century is no longer the most likely trajectory for the world's population."
Founded in 2007 and supported by the Bill and Melinda Gates Foundation, the IHME has become a global reference for health statistics, especially its annual Global Burden of Disease reports.
Dr Murray said the population study "provides governments of all countries an opportunity to start rethinking their policies on migration, workforces and economic development to address the challenges presented by demographic change".
An important underlying reason behind the conclusions is the improvement in access to modern contraception and the education of girls and women, which the study said would "hasten declines in fertility and slow population growth".
While the most recent UN population forecast, made in June last year, also noted declining fertility, the new study said the consequences would be felt much sooner and with greater impact.
Mr John Wilmoth, director of the population division in the UN Department of Economic and Social Affairs, which produces the organisation's projections every two years, said on Tuesday that he had not yet fully read the study.
But he said that it had made some assumptions about fertility, mortality and migration that helped shape the conclusions.
One of the biggest assumptions, he said in a phone interview, was that countries with low fertility rates would do nothing to raise them between now and 2100.
"It's kind of an extreme assumption to think that countries aren't going to think their way out of the problem for the next 80 years," he added.
Mr Wilmoth also said the UN had been tracking population trends for 70 years and that its projections "represent a consensus view" among demographers.
Still, he said: "I welcome this sort of creative inquiry about other ways of seeing these things."
The forecasting methodology used in the study found that by 2100, 183 of 195 countries would have total fertility rates - the average number of children a woman has over her lifetime - below the replacement level of 2.1 births. That is the level needed to prevent population decline.
The study also suggested that the decline could be offset by immigration, with countries that promote liberal immigration policies better able to maintain their population and support economic growth.
Some countries with fertility rates lower than replacement level, such as the US, Australia and Canada, would probably replenish their working-age populations through net immigration, the study said, although it noted uncertainty about such a forecast.
The backlash in the US against immigration, the study said, could threaten "the country's potential to sustain population and economic growth".
Sub-Saharan Africa may become more powerful as its population rises, according to the analysis.
NYTIMES, AGENCE FRANCE-PRESSE
