As nations develop their economies, younger generations are drawn from rural areas into cities. Urban growth rates in recent years have been particularly pronounced in lower and middle-income countries. Africa depicts this global trend in high relief.

For the past few decades, Africans have been moving from rural areas into cities, seeking work, so the continent’s urban population has skyrocketed. Nairobi is a prime example of that migration, with its population pushing upward and its boundaries pushing outward.

“Nairobi, like most cities in Africa, is experiencing dramatic growth,” said Assaf Anyamba, a USRA/GESTAR scientist at NASA who was born in Kenya. Anyamba pointed to areas that have seen a lot of change over the years, like the Mombasa Road and Karura Forest.

While such migration often mirrors economic advancement, it also presents socioeconomic and environmental challenges. Rapid urban growth strains existing infrastructure. Conservationists also note that it discourages the preservation of natural habitat in favor of building more developments. In Nairobi, urbanization has caused the city to encroach on former green spaces like the nearby national parks and forests.

At the time of the last official government census in 2009, the population of Kenya numbered more than 38 million people. The city of Nairobi houses 3.5 million of them, which is more than double the 1986 population. The images above, acquired by Landsat 5 in 1986 and Landsat 8 in 2016, show the expansion of Kenya’s capital in the past three decades.

The booming population in Kenya has sparked concern among some analysts, who say that the influx of people will bring rising rates of youth unemployment and poverty. “If Kenya’s rapid urbanization is left unmitigated, the country could see even higher rates of unemployment, specifically within its youth population,” wrote Raphael Obonyo, a representative for the World Bank Group.

Population density in Nairobi varies greatly, peaking in the city’s slums, which house roughly 2.5 million people in about 200 settlements. Roughly 60 percent of Nairobi’s population occupies just 6 percent of the land, according to Kibera UK. The city’s largest slum, Kibera (the light-colored patch north of Nairobi National Park) is made up of small, tightly packed structures.

Kenya’s government has recognized these challenges, according to its National Bureau of Statistics, and is working toward creating a national urban policy. The government set forward its plan in Kenya’s Vision 2030, which aims to transition to a “newly-industrialising, middle-income country providing a high quality of life to all its citizens in a clean and secure environment.”

Editor’s Note: Although both of the images above were taken at a similar time of the year, the more recent one depicts greener vegetation, the result of a rainy El Niño cycle, said Anyamba. “During an El Niño year, the tendency for East Africa is to get wet.” The January-February dry season follows a short wet season from September to November.

NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey. Caption by Pola Lem.