For comparison, it was about 6.1 billion in the year 2000.
Professor Vollset told IFL Science: “If our forecast is correct, it will be the first time population decline is driven by fertility decline, as opposed to events such as a pandemic or famine.”
According to the study’s forecasts, the population of up to 23 countries could shrink by more than half.
These include Japan, Spain, South Korea, Thailand, Portugal and Italy.
Tow key factors, Professor Vollset argued, are improved access to contraception and education of women and girls.