Earth will be home to 8.8 billion souls in 2100, two billion fewer than current UN projections, according to a major study published yesterday that foresees new global power alignments shaped by declining fertility rates and greying populations.

By century's end, 183 of 195 countries -- barring an influx of immigrants -- will have fallen below the replacement threshold needed to maintain population levels, an international team of researchers reported in The Lancet.

More than 20 countries -- including Japan, Spain, Italy, Thailand, Portugal, South Korea and Poland -- will see their numbers diminish by at least half.  China's will fall nearly that much, from 1.4 billion people today to 730 million in 80 years.

Sub-Saharan Africa, meanwhile, will triple in size to some three billion people, with Nigeria alone expanding to almost 800 million in 2100, second only to India's 1.1 billion.

"These forecasts suggest good news for the environment, with less stress on food production systems and lower carbon emissions," lead author Christopher Murray, director of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, told AFP.

Social services and healthcare systems will need to be overhauled to accommodate much older populations. 2.37 billion people -- more than a quarter of the global population -- will be over 65 years old by then.  Those over 80 will balloon from about 140 million today to 866 million.