Washington, Jul 15 (IANS): World population is likely to peak in 2064 at around 9.7 billion people and fall to 8.8 billion by century's end. Twenty three countries will see populations shrink by more than 50 per cent, including Japan, Thailand, Italy, and Spain, says a new analysis published in the journal The Lancet.

With 1.09 billion, India will be the most populous country by 2100, followed by Nigeria (791 million), China (732 million), the US (336 million) and Pakistan (248 million), according to the forecast.

Dramatic declines in working age-populations are predicted in countries such as India and China, which will hamper economic growth and lead to shifts in global powers.

But India will still be one of the four major powers along with Nigeria, China, and the US by the end of the century when the world will be multipolar.

The study forecasts global, regional, and national populations, mortality, fertility, and migration for 195 countries worldwide.

The new study also predicts huge shifts in the global age structure, with an estimated 2.37 billion individuals over 65 years globally in 2100, compared to 1.7 billion under 20 years, underscoring the need for liberal immigration policies in countries with significantly declining working age populations.

The modelling research uses data from the Global Burden of Disease Study 2017 to project future global, regional, and national population.

Using novel methods for forecasting mortality, fertility, and migration, the researchers from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington's School of Medicine estimate that by 2100, 183 of 195 countries will have total fertility rates (TFR) below replacement level of 2.1 births per woman.

TFR represents the average number of children a woman delivers over her lifetime.

"Continued global population growth through the century is no longer the most likely trajectory for the world's population," said IHME Director Christopher Murray, who led the research.

"This 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."

In 2100, the US is forecasted to have the fourth largest working-age population in the world (around 181 million), after India, Nigeria, and China -- with immigration likely sustaining the US workforce, with the largest net immigration in absolute numbers -- more than half a million more people are estimated to immigrate to the US in 2100 than will emigrate out.

While the US had the largest economy in 2017, China is set to replace it in 2035, but the US is forecasted to once again become the largest economy in 2098 -- bolstered by immigration, showed the analysis.