干旱期，城市涨潮

Policy Research Working Paper11099 Dry Spells, Urban Swells Analyzing the Drought-Induced Expansion of Citiesin Sub-Saharan AfricaPublic Disclosure Authorized Rafael Van der BorghtOscar A. IshizawaJean ThuretJoaquin Muñoz Urban, Disaster Risk Management,Resilience and Land Global DepartmentApril 2025 A verified reproducibility package for this paper isavailable athttp://reproducibility.worldbank.org,clickherefor direct access. Policy Research Working Paper11099 Abstract Droughts are increasingly cited as a driver of urbanizationacross Sub-Saharan Africa, yet little is known about the rolethey play in shaping the spatial expansion of cities. Com-bining satellite imagery on built- up areas with climatic datafor 1984–2015, this study empirically examines whetherand to what extent droughts influence the spatial expansionof African cities. It further investigates the heterogeneityof these effects across cities and countries. The findingsindicate that extreme droughts significantly accelerate thebuilt-up growth rate of cities, while more frequent butless severe droughts have negligible impacts. Importantly,these effects are strongly differentiated across cities. The1 percent most extreme droughts boost the average speedat which new settlements emerge in the surroundings ofa country’s largest city by 75 percent, yet they do not alter the expansion of smaller cities and towns. These drought-in-duced effects intensify the sprawl of the largest cities andbear important policy implications. Extreme droughts putadditional pressure on the largest and often overcrowdedcities,potentiallydeepeningcongestioneffects.Theyalso contribute to exacerbating the speed at which citiesexpand in flood-prone areas, thereby magnifying urbanflood risk. As the climate changes, the frequency of bothextreme droughts and extreme rainfall events is projectedto increase across the region, aggravating the likelihoodof future drought-induced expansions of the largest citiesand worsening urban flood risk prospects. These findingscall for urgent and tailored risk reduction measures in bothcities and rural areas. The Policy Research Working Paper Series disseminates the findings of work in progress to encourage the exchange of ideas about developmentissues. An objective of the series is to get the findings out quickly, even if the presentations are less than fully polished. The papers carry thenames of the authors and should be cited accordingly. The findings, interpretations, and conclusions expressed in this paper are entirely thoseof the authors. They do not necessarily represent the views of the International Bank for Reconstruction and Development/World Bank andits affiliated organizations, or those of the Executive Directors of the World Bank or the governments they represent. Dry Spells, Urban Swells: Analyzingthe Drought-Induced Expansion ofCities in Sub-Saharan Africa Rafael Van der Borght, Oscar A. Ishizawa, Jean Thuret, and Joaquin Muñoz JEL classification: Q54, 018, 013, R11 Keywords: Drought, City extent, Climate risk, Earth Observations, Africa 1.Context Urbanization is one of the most profound transformations that the African continent will experiencein the 21stcentury.With 59% of the population living in rural areas in 2020, urbanization in Sub-Saharan Africa (SSA) is still incipient. However, since 2000, the population living in urban areas hasmore than doubled and it is expected to continue growing at a rate of 40% per decade over the nextthree decades.1SSA is now the fastest urbanizing region globally, with projections indicating that anadditional 750 million individuals could be living in urban areas by 2050. The projected increase inurban population surpasses the current urban population of India (approx. 500 million) and exceedsthe entire population of the Latin America and the Caribbean region (approx. 660 million). This massiveinflux of new urban dwellers will reconfigure urban systems, creating a unique window of opportunityto shape the future of African cities.2 The future urbanization of SSA will unfold in a changing climate, yet little is known about the effectsof extreme weather events on cities’ dynamics.3During the past decades, urbanization in SSA has notbeen associated with the economic development observed elsewhere (seeFigure 1). This strikingdifference suggests that the drivers of urbanization in SSA might differ from those observed elsewhere.Long-standing literature has focused on the disconnect between growth and urbanization, highlightinghow in poor countries urbanization could be increasingly driven by push factors that make people leaverural areas, rather than by pull factors that attract them to cities (Lipton, 1977; Bairoch, 1988; Fay andOpal, 2000; Bloom et al., 2008; Glaeser, 2014; Jedwab and Vollrath, 2015; Castells-Quintana andWenban-Smith, 2020). In rural areas of SSA, which concentrate 82% of the people living in extreme poverty in the region and where livelihood means are heavily r