We estimate population trends throughout the 21st century for ~20,000 urban agglomerations in 151 countries, working within the Shared Socioeconomic Pathways (SSPs) and using a simple urban-growth model. Our results suggest that urban growth in this century will produce increasingly concentrated cities, some growing to enormous sizes. We also demonstrate that, although detailed urbanization trajectories differ for different SSP scenarios, in all cases, the largest projected agglomerations of the future are more populous than the largest agglomerations today. Our projection strategy advances urban-population research by producing urban-size projections—for agglomerations around the world—that correctly obey empirically observed distribution laws. Although our method is very simple and omits various aspects of urbanization, it nonetheless yields valuable insight into long-term SSP-specific urbanization trends to inform discussion of sustainable urban policies.

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