Mike Davis’s Planet of Slums has cast a long shadow over the world’s cities since it was published nearly 20 years ago. Expanding on his influential 2004 New Left Review essay of the same title, Davis argued that the global urban future would look more like the favelas of Brazil than the gleaming megacities of China. For Davis, the Third World debt crisis of the late 1970s and subsequent IMF-led structural adjustment had set off a process of “urbanization without growth” across sub-Saharan Africa, Latin America, the Middle East, and South Asia. Millions fled the impoverished countryside — yet there was no “pull” factor to answer this “push,” as cities in the Global South were either deindustrializing or creating jobs too slowly to absorb the rural exodus. In metropolises like Lagos and Jakarta, but especially in neglected second-tier cities, these refugees were forced into shantytowns and informal labor.

Davis predicted that this “perverse urban boom” would only worsen, confounding development experts who thought they could replicate the successes of early industrial cities like Manchester and Chicago in the late-industrializing world. He was right to stress the central role of urbanization patterns in shaping 21st-century capitalism: for the first time in human history, most people live in cities. So far, slums haven’t exploded in the way that Davis feared, but the global effort to eliminate them is facing headwinds that could prove overwhelming.

In 2000, the United Nations pledged to achieve a “significant improvement” in the lives of 100 million slum dwell-ers; 15 years later, it adopted the more ambitious goal of ensuring “access to safe and affordable housing” for all while upgrading slums worldwide. The optimism made sense at the time. Between the turn of the millennium and 2015, slums’ share of the global urban population was in pronounced decline, even if the absolute number of slum residents was gradually increasing. Yet the trend soon reversed: slums’ absolute growth accelerated around 2015, and by 2020 their share of the global urban population was expanding too. This concerning data reflects backsliding in sub-Saharan Africa and Southeast Asia, whose rapid slumification more than cancels out continued modest progress in all other regions. As Davis predicted, slums have spread fastest in smaller cities and recently urbanized towns.