Doubling times that keep shrinking
The global population reached 1 billion in roughly 1800, after 200,000 years of slow growth constrained by high mortality and agrarian living standards. The second billion was added by 1927 (127 years). The third billion by 1960 (33 years). The fourth by 1974 (14 years). The fifth by 1987 (13 years). The sixth by 1999 (12 years). The seventh by 2011 (12 years). And the eighth by 2022 (11 years). Each billion is arriving faster than the last, reflecting fertility in developing nations where death rates have fallen (via modern medicine) but fertility remains high.
This acceleration is now slowing: the United Nations projects the ninth billion will take 15+ years and the tenth may never arrive. Fertility is falling in most countries as women gain education and economic agency.
Growth dynamics driven by mortality declines
Population growth is not a single phenomenon but a consequence of the gap between birth and death rates. When death rates fall (via vaccines and sanitation) but birth rates stay high (still cultural in agrarian societies), populations boom. The fastest growth occurred 1950-1970 when death rates collapsed in the developing world but fertility remained at 5-8 children per woman.
Today, birth rates have fallen sharply in developed nations (sub-replacement, roughly 1.5 children per woman) and more slowly in low-income nations (still 4-5 in parts of Africa and the Middle East). This creates a demographic divide: developed nations are aging and shrinking; developing nations are young and still growing, shifting global economic and political gravity.