The colossal weight of cities sinks them, even as sea levels rise

Cities don’t just have rising sea levels to worry about – they’re also slowly sinking under the weight of their own development, new research has shown, stressing the importance of including subsidence in climate change risk models.

Geophysicist Tom Parsons of the U.S. Geological Survey (USGS) viewed San Francisco as a case study of how major urban events can affect and depress the Earth’s actual surface.

According to his calculations, San Francisco could have sunk as much as 80 millimeters (3.1 inches) as the city grew over time. Given that the bay area is under threat of as much as 300 mm (11.8 inches) of sea level rise by 2050, the additional variations added by the slow subsidence are significant enough to worry.

“As the global population moves disproportionately towards the coast, this additional descent combined with the expected rise in sea levels may exacerbate the risk associated with flooding,” Parsons writes in his paper.

Taking into account the list of all buildings in the city and their contents, the study calculated that the weight of San Francisco (population: 7.75 million) was about 1.6 trillion pounds – about 3.5 trillion pounds, or about 8.7 million Boeings 747

This could be enough to bend the actual lithosphere on which the urban center is located, and perhaps more significantly, to change the relative level of fault blocks – the floating pieces of rock that make up the Earth’s surface.

In fact, an 80mm slip is likely to be a conservative estimate, as the weight calculations did not include things outside the buildings – including transport infrastructure, vehicles or people. The same type of sinking is likely to occur in other parts of the world, although it depends in part on local geology.

“Specific results found for the San Francisco Bay Area are likely to apply to any major urban center, albeit with varying importance,” Parsons writes.

“Anthropogenic load effects on tectonic active continental margins are probably greater than more stable continental inland where the lithosphere tends to be denser and stiffer.”

There are many other causes of subsidence to consider, including the movement of tectonic plates and the pumping of groundwater necessary to support a growing population – something we have seen that a significant city has sunk in other parts of the world.

Although this current study dealt only with San Francisco, and made some broad assumptions regarding modeling, the findings are notable enough to give city weight another significance when scientists devise how geography could change over time and which areas are threatened as sea level becomes senior.

There is still plenty of detail to explore, especially in cities already threatened by landslides. The compaction of the sediment system and aquifers under San Francisco International Airport on the coast – the heaviest building in the city – has already been calculated to cause a 4 mm (0.16 inch) sinking each year.

“It should be possible to improve the methods presented here by using satellite or aerial photographs to produce more detailed analyzes in likely flood zones,” Parsons writes.

“Such detailed analyzes could also provide better insight into changes in surface porosity changes and the resulting fluid flow.”

The research was published in AGU Advances.

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