The United Nations estimate of future world population is 9.3 million by 2050, rising to 10.1 billion by 2100. However, the U.N. figures are given with a very wide range of uncertainty – between 6.2 billion and 15.8 billion in 2100.

A new model developed by a team from the Autonomous University of Madrid (UAM) and the CEU-San Pablo University, both in Spain, indicates that the U.N.'s low estimate for 2100 (a population of 6.2 billion) is more likely – with the population by 2050 being similar to, or even slightly lower than, the present 7.1 billion.

Félix F. Muñoz, a co-author of the project, points out that "the mortality rate fell sharply in the second half of the 20th century as a result of advances in healthcare and increased life expectancy and it seemed that the population would grow a lot. However, the past three decades have also seen a steep drop-off in the number of children being born worldwide."

In fact, the world fertility rate has fallen by more than 40% since 1950. As recently as 1992, it was predicted that there would be 7.17 billion people on Earth by 2010 but the actual number was only 6.8 billion.