Terrawatch: cities that change the shape of the planet

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It’s well known that ice sheets are heavy enough to bend the underlying rocks, but what about cities? Are some cities capable of reshaping the bit of planet they sit on?

By 2050 around 70% of Earth’s population are projected to live in cities. This set Tom Parsons, a geophysicist with the United States Geological Survey, to wondering if the associated redistribution of mass into concentrated urban areas is capable of causing subsidence. Using the San Francisco Bay region (7.75 million people) as a case study, Parsons estimated the weight of all the buildings and their contents to be around 1.6 trillion kg – comparable to the weight of water behind a dam. Taking into account the underlying geology of San Francisco, Parsons modelled the pressure that the city exerts and showed that San Francisco’s buildings are responsible for between 5 and 80mm of subsidence. The findings are reported in the journal AGU Advances.

When it comes to flood risk those millimetres matter, particularly given that San Francisco is expected to see a rise in the sea level of 200 to 300mm by 2050. Parsons suggests that many coastal cities will be sinking under their own weight, while inland cities are less vulnerable because they tend to be situated on thicker lithosphere – the rocky, outer part of the Earth.

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