Changing global patterns of urban exposure to flood and drought hazards

The studies that quantify the human and economic costs of increasing exposure of cities to various natural hazards consider climate change together with increasing levels of population and economic activity, but assume constant urban extent. Accurate estimates of the potential losses due to changing exposure of cities, however, require that we know where they will grow in the future. Here, we present the first-ever estimates of the changing exposure of urban infrastructure to floods and droughts due to urban land expansion from 2000 to 2030. The percentage of the global urban land that lies within the low elevation coastal zone (LECZ) increases only slightly to 13% by 2030; nonetheless, this corresponds to a 230% increase in the amount of urban land within the LECZ (from 71,000 km2 to 234,000 km2). In 2000, about 30% of the global urban land (i.e., nearly 200,000 km2) was located in the high-frequency flood zones; by 2030, this will reach 40% (i.e., over 700,000 km2). The emerging coastal metropolitan regions in Africa and Asia will be larger than those in the developed countries and will have larger areas exposed to flooding. The urban extent in drylands will increase by nearly 300,000 km2, reaching almost 500,000 km2. Overall, without factoring in the potential impacts from climate change, the extent of urban areas exposed to flood and drought hazards will increase, respectively, 2.7 and almost 2 times by 2030. Globally, urban land exposed to both floods and droughts is expected to increase over 250%. There are significant geographical variations in the rates and magnitudes of urban expansion exposed to floods or droughts or both. Several policy options exist to safeguard urban infrastructure from flood and drought hazards. These range from directing development away from flood- or drought-prone zones to large-scale adoption of “green infrastructure” (or “eco-efficient infrastructure”). Decisions, taken today on managing urban growth in locations exposed to these hazards, can make a big difference in mitigating likely losses due to floods and droughts in the near future.