Cities not only have sea level rise to worry about, but it is also slowly sinking under the weight of their own development, according to new research, highlighting the importance of sinking into models of the risk of climate change.
Geophysicist Tom Parsons, of the United States Geological Survey (USGS), considered San Francisco to be a case study of how large urban developments can affect and suppress the actual surface of the earth.
According to his calculations, San Francisco may have sunk as much as 80 millimeters (3.1 inches) as the city has grown over time. As the Bay is threatened by as much as 300 mm (11.8 inches) of sea level rise by 2050, the extra variation added by the slow subsidence is significant enough.
“As world populations move disproportionately to the shores, this additional sinking in combination with the expected sea level rise could exacerbate the risk of flooding,” Parsons wrote 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) at about 1.6 billion kilograms – about 3.5 billion pounds, or about 8.7 million Boeing 747s.
It can be enough to bend both the actual lithosphere on which the urban center sits, and perhaps even more, to change the relative levels of fault blocks – the floating pieces of rock that form the earth’s surface.
The 80 mm slip is probably a conservative estimate, as the weight calculation does not include things outside buildings – including transport infrastructure, vehicles or people. The same type of zinc is likely to be found 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, although they are of varying importance,” Parsons writes.
“Anthropogenic loading effects against tectonically active continental margins are likely to be greater than more stable continental interiors where the lithosphere is thicker and rigid.”
There are also many other causes of subsidence that you can think of, including the displacement of the tectonic plate and the pumping of the groundwater needed to support a growing population – something we have seen causing the city in other parts of the world calmed down.
While this current study looked only at San Francisco, and made some assumptions about modeling, the findings are striking enough to make the weight of the city another consideration when scientists find out how geography can change over time, and which areas are threatened the sea level becomes higher.
There are still many details to dig into, especially in cities that are already under threat of subsidence. The compaction of sediment and aquifer systems under the San Francisco International Coastal Airport – the heaviest building in the city – has already been calculated to sink 4 mm (0.16 inches) each year.
“It would be possible to improve the methods presented here by using satellite or aerial photographs to conduct more detailed analyzes in probable floodplains,” Parsons writes.
“Such detailed analyzes can also provide better insights into changes in porosity changes underground and the resulting fluid flow.”
The research was published in AGU advances.