Demographers marked the day the global population was estimated to hit five billion people, a doubling from roughly 2.5 billion in 1950. The United Nations Development Programme subsequently established World Population Day on this date to raise awareness of population issues, from family planning to resource sustainability.
The five billion milestone reflected a staggering acceleration in human numbers driven by declining mortality, especially child mortality, alongside continued high fertility in much of the developing world. It had taken all of human history until roughly 1800 to reach one billion people; by 1987 humanity was adding a billion more roughly every dozen years.
The UN Population Division designated this specific day somewhat symbolically—there was no way to pinpoint the exact date a real birth pushed the total past five billion—but the milestone captured global attention and anxiety about resource scarcity, urbanization, and the strain of rapid growth on developing economies. Population Day was subsequently established by the UNDP's Governing Council in 1989 to focus attention on these issues.
Subsequent milestones came even faster: six billion by 1999, seven billion by 2011, eight billion by 2022. The trajectory has since begun to bend as fertility rates fall worldwide, but July 11 remains an annual occasion for confronting questions the 1987 milestone first thrust into headlines: how many people can the planet sustain, and at what standard of living.