Cities not only have concerns about rising sea levels, but are also slowly sinking under the weight of their own development, according to new research, which emphasizes the importance of considering subsidence in models of risk of climate change.
Geophysicist Tom Parsons of the U.S. Geological Survey (USGS) examined San Francisco as a case study of how large urban developments could affect and depress the Earth’s actual surface.
According to his calculations, San Francisco could have dropped to 80 millimeters (3.1 inches) as the city has grown over time. Given that the bay area is threatened by up to 300 mm (11.8 inches) of sea level rise in 2050, the additional variation added by a slow sinking is significant enough to worry about.
“As global populations move disproportionately toward shores, this additional subsidence in combination with projected sea level rise may exacerbate the risk associated with flooding,” Parsons writes in his paper.
Taking into account an inventory of all the city’s buildings and their contents, the study estimated that San Francisco’s weight (7.75 million people) was about 1.6 trillion kilograms, approximately 3 , 5 trillion pounds, or about 8.7 million Boeing 747s.
This could be enough to bend the actual lithosphere on which the urban center sits and, perhaps more significantly, to change the relative levels of fault blocks: the floating pieces of rock that make up the Earth’s surface.
In fact, the 80mm slip is probably a conservative estimate, as the weight calculations did not include things outside the buildings, including transportation infrastructure, vehicles, or people. The same type of subsidence is likely in other parts of the world, although it depends in part on local geology.
“The specific results found in the San Francisco Bay Area are likely to apply to any major urban center, albeit of varying importance,” Parsons writes.
“The effects of anthropogenic load on tectonically active continental margins are probably greater than more stable continental interiors where the lithosphere tends to be thicker and more rigid.”
There are many other causes of subsidence that need to be considered, including the displacement of tectonic plates and the pumping of groundwater needed to support a growing population, which we have seen cause the city to collapse significantly in other parts of the world. world.
Although this current study only analyzed San Francisco and made some broad hypotheses in terms of modeling, the findings are remarkable enough to make the city have another consideration when scientists find out how geography could change over time. and which areas are threatened. sea level rises.
There are still plenty of details to delve into, especially in cities that are already threatened by collapse. Compaction of sediment and aquifer systems at San Francisco International Airport on the coast, the heaviest building in the city, has already been calculated as the cause of 4 mm (0.16 inch) sinking each year.
“It should be possible to improve the methods presented here by using satellite or aerial photos to make more detailed analyzes in likely flood zones,” Parsons writes.
“These detailed analyzes could also provide better insights into changes in subsoil porosity changes and the resulting fluid flow.”
The research has been published in AGU Advances.