They say that the world is getting smaller, by which it is meant that modern communications have made of it a sort of village — but the world may be getting smaller in other ways, too.
Earth will be home to 8.8bn people in 2100 — 2bn fewer than current UN projections, according to a new study that foresees declining fertility rates and ageing populations.
By the end of this century, 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.
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 1.4bn people today to 730m in 80 years.
Fewer younger workers and more retired elderly means less tax to take in and more welfare to hand out. At a national level, that is the reasoning behind the recommendations made by the Irish Fiscal Advisory Council (IFAC) that the pension age should rise to 69.
It notes that older people will soon represent a much higher proportion of the population, with those over 65 almost doubling from 14% in 2020 to 27% by 2050.
Life expectancy at 65 is expected to rise from 85 now to 89 by 2050.
The IFAC's recommendations do not make comfortable reading for anyone planning to retire at 65 or 66, but the logic of the argument is inescapable.