In addition to worrying about rising sea levels, we should also be afraid that cities will sink from being so heavy, according to new research.
By 2050, 70% of the Earth’s population will live in cities, according to scientific forecasts. Geophysicist Tom Parsons of the United States Geological Survey (USGS) has calculated the weight of the San Francisco metropolitan region and studied the changes in the solid ground below it, using numerical modeling techniques.
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It turns out that sinking under this weight is not insignificant, in addition to other causes of urban subsidence, such as groundwater pumping. Thus, the American city of San Francisco built on the ocean coast may have sunk as much as 80 millimeters as it grew over time, according to the geophysicist’s calculations.
“As the world’s population disproportionately shifts towards the coasts, this additional subsidence in combination with the anticipated rise in sea levels may exacerbate the risk associated with flooding,” Parsons writes in his article.
According to Parsons, all the buildings in the city and their contents weigh about 1.6 trillion kilograms, that is, about 8.7 million Boeing 747s. Not counting transportation infrastructure, vehicles and people.
The same type of subsidence is likely to occur in other parts of the world, although it depends on the local geology. It could be enough to bend the lithosphere on which the urban center sits and even to change the relative levels of fault blocks, the chunks of rock that make up the Earth’s surface.
“The specific results found for the San Francisco Bay Area are probably applicable to any major urban center, albeit with varying importance,” explains Parsons.
Many details remain to deepen the investigation, especially for cities that are already threatened by subsidence.
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“It should be possible to improve the methods presented here by using satellite or aerial photos to do more detailed analysis in probable flood zones,” Parsons summarizes.
The research has been published in AGU Advances.