Cities are not only concerned about rising sea levels – they are also slowly sinking under the weight of their own development, according to new research, which emphasizes the importance of including subsidence in climate change risk models.
Geophysicist Tom Parsons of the United States Geological Survey (USGS) looked at San Francisco as a case study of how large urban developments could affect and depress the Earth’s real surface.
According to his calculations, San Francisco could have sunk to 80 millimeters (3.1 inches) as the city grew over time. Given that the Gulf area is threatened by a sea level rise of up to 300 mm (11.8 inches) by 2050, the additional variation added by slow yielding is significant enough to be worrisome.
“As global populations move disproportionately to the coast, this additional decline, combined with the expected rise in sea level, may exacerbate the risk associated with floods,” Parsons writes in his paper.
Taking into account an inventory of all the buildings in the city and their contents, the study calculated the weight of San Francisco (population: 7.75 million) to be around 1.6 trillion kilograms – about 3.5 trillion pounds or about 8.7 million Boeing 747s.
This could be enough to bend the real lithosphere on which the urban center sits and, perhaps, more significantly, to change the relative levels of the defect blocks – the floating pieces of rock that make up the Earth’s surface.
In fact, the 80 mm slip is probably a conservative estimate, as the weight calculations did not include things outside the buildings – including transport infrastructure, vehicles or people. The same type of diving is probably in other parts of the world, although it depends in part on local geology.
“The specific results found for the San Francisco Bay Area are likely to apply to any major urban center, albeit of varying importance,” Parsons writes.
“The effects of anthropogenic loading at tectonically active continental margins are probably greater than more stable continental interiors, where the lithosphere tends to be thicker and more rigid.”
There are plenty of other causes of sinking to consider, including changing tectonic plates and pumping groundwater needed to support a growing population – something we’ve seen cause significant city sinking in other parts of the world.
While this current study only looked at San Francisco and made some broad assumptions about modeling, the findings are notable enough to make the weight of the city another consideration when scientists figure out how it might be. change geography over time and what areas are threatened as sea levels rise.
There are still a lot of details to dig, especially in cities already threatened with sinking. The compaction of sediment and aquifer systems at the San Francisco International Airport on the coast – the heaviest building in the city – has already been calculated to cause 4 mm (0.16 inch) sinking each year.
“It should be possible to improve the methods presented here by using satellite or aerial photography to make more detailed analyzes of likely flood areas,” Parsons writes.
“Such detailed analyzes could also provide better information about changes in changes in subterranean porosity and the resulting liquid flow.”
The research was published in AGU Advances.