65 Owing to galloping urbanization, the soils in large cities are subsiding. This phenomenon is all the more striking in large coastal cities threatened with flooding due to rising sea levels. Without waiting for the necessary global response to this problem, many municipalities are already successfully implementing tried-andtested technical solutions. When soils are putting up RESISTANCE The findings are unanimous: hundreds of big cities around the world are sinking. And the phenomenon can only worsen given urban development trends: by 2050, almost 70% of the world’s population is set to live in mega‑cities. This prospect could almost be insignificant if, at the same time, rising sea levels were not causing coastline erosion problems, endangering constructions. According to the World Economic Forum, several cities, including New York (United States), Dhaka (Bangladesh), London (United Kingdom), and Bordeaux (France), could be partially or totally submerged by 2050-2100. In the first part of their sixth report, the IPCC’s(1) experts drove the point home: no fewer than 570 cities are facing this threat. THE EXCESSIVEWEIGHT OF BUILDINGS To what can this phenomenon be attributed? Two words provide the answer: urban growth. The older the city, the more it gains weight. The pressure has reached critical levels due to the ever-growing size of buildings and the increase in built-up areas in the middle of the 20th century. All the more so as groundwater pumping to supply the inhabitants and economic activities with water contributes to the weakening of the bedrock of large cities. TOWARD ZERO LAND TAKE? While rampant urbanization contributes to these kinds of geological disturbance, it has an impact on soils weakened both by storm water runoff and repeated floods with their string of infiltrations. Aware of the need to act, scientists, industrialists, and local authorities have already come up with solutions. The most obvious consists in combating land take in cities and urban sprawl, responsible for the disappearance of natural areas. However, we should be wary of the obvious, because building on the outskirts of cities helps regulate their weight! SPoTLIGHT (1) Intergovernmental Panel on Climate Change.
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