This decade began just after a historic inflection point, with 51% of the world's population living in urban areas.

By the numbers: That proportion has continued to rise steadily, reaching 55% as of 2018. It's climbed faster in China, up from 48% to 59% — meaning an additional 180 million people are living in Chinese cities.

China now has 130 cities of at least 1 million people, more than the U.S. (45), European Union (36) and South America (46) combined.

India, which won't become majority-urban until the 2040s, has 61 such cities. There are 63 in Africa.

Nigeria just became majority-urban in 2018, but urbanization in the West African giant will grow even more dramatically over the next decade.

Nigeria's 10 largest cities are home to 32 million people as of 2018, with 13 million of those in Lagos.

The UN projects their combined populations will rise to 50 million by 2030 — just over a decade away — by which time Lagos will have over 20 million residents.

By 2050, the UN projects Nigeria's total population will double to 400 million, with 70% living in cities. Compare that to the U.S. (82% urban), where the population will rise from 330 million to 375 million.

Zoom out: The world's biggest urban areas (UN, 2018):

Tokyo, Japan (37.5 million) Delhi, India (28.5 million) Shanghai, China (25.6 million) São Paulo, Brazil (21.7 million) Mexico City, Mexico (21.6 million) Cairo, Egypt (20.1 million) Mumbai, India (20.0 million) Beijing, China (19.6 million) Dhaka, Bangladesh (19.6 million) Osaka, Japan (19.3 million)

What's next: By 2030, Kinshasa in the Democratic Republic of the Congo will join the list, replacing Osaka.

Flashback: New York City held the No. 10 spot as of 2016 but was bumped off the list by Dhaka. European cities that would have been in the top 10 in decades past — Moscow (24th), Paris (28th), London (37th) — keep sliding.

Go deeper: The decade of the very poor and super rich