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Dry Spells, Urban Swells: Drought-Driven Urbanization in Sub-Saharan Africa

Research Background

The study examines how droughts influence urban spatial expansion in Sub-Saharan Africa (SSA), a region experiencing rapid urbanization, but with a unique pattern not strongly tied to economic development. Urbanization in SSA is often driven by “push” factors, such as climate shocks affecting rural areas, rather than pull factors from cities.

Drought as a Driver of Urbanization

• Extreme Droughts: Have a statistically significant positive effect on the spatial expansion of the largest cities. A 1% increase in the severity of extreme droughts is linked to a 75% acceleration in the growth rate of built-up areas. For example, the most extreme droughts boost the average expansion rate of the largest cities by 1.2–1.8 percentage points.
• Less Severe Droughts: Do not significantly accelerate urban growth.
• Spatial Concentration: Effects are concentrated mainly on the largest cities (primary cities), which account for the vast majority of urbanization dynamics. Smaller cities and towns show negligible impacts.

Moderating Factors

• Rainfall Levels: Drought-induced expansion is more pronounced in countries with low average rainfall (e.g., Niger, Mauritania). High-rainfall areas like Liberia show insignificant effects.
• Initial Urbanization: No significant differentiation based on initial urbanization rates, suggesting pull factors for migration are not strongly tied to city size or economic structure.

Policy Implications

• Congestion and Infrastructure: Accelerated expansion of large cities intensifies overcrowding and strain on public services, transportation, and the environment in SSA's primary cities.
• Flood Risk: Faster urban growth in flood-prone areas increases exposure to extreme floods. The study shows that a 10% increase in built-up areas leads to a 1.3% rise in flood-exposed built-up, with primary cities affected more severely.
• Climate Change: As climate change increases the frequency and severity of droughts and floods, the cycle of drought-driven expansion and flood risk could worsen, threatening sustainable urban development.
• Action Needed: Urgent measures are required to strengthen risk reduction, improve land-use planning, and promote climate-resilient practices in both rural and urban areas.

Key Findings

Drought-induced urban expansion is a major driver in SSA, particularly for the largest cities. However, only extreme droughts, not milder events, significantly accelerate urban growth. This non-linear relationship has profound consequences for both urbanization patterns and disaster risk management, requiring tailored policy interventions to mitigate negative externalities.