It was only a century ago that there were 2 billion people. Since then, the global population has quadrupled to eight billion. Around 108 billion people have ever lived on our planet. This means that today's population size makes up 6.5% of the total number of people ever born.

The primary (and perhaps most obvious) cause of population growth is an imbalance between births and deaths. The infant mortality rate has decreased globally, with 4.1 million infant deaths in 2017 compared to 8.8 million in 1990, according to the World Health Organization (WHO).

Population change, defined generally, is the difference in the size of a population between the end and the beginning of a given time period (usually one year). Specifically, it is the difference in population size on 1 January of two consecutive years.