Not only do cities have sea-level rise to worry about, they are also slowly sinking under the weight of their own development, according to new research, which highlights the importance of weighing land subsidence in models of climate change risk.
Geophysicist Tom Parsons, of the Office of the United States Geological Survey (USGS), viewed San Francisco as a case study of how major urban developments could affect and depress the actual Earth’s surface.
According to his calculations, San Francisco could have sunk as much as 80 millimeters (3.1 in) as the city has grown over time. As the Bay Area is threatened by a whopping 300 mm (11.8 in) sea level rise by 2050, the extra variation added by the slow subsidence is significant enough to be of concern.
“As the world’s population moves disproportionately to the coast, this additional subsidence, combined with the expected sea level rise, may increase the risk of flooding,” Parsons writes in his paper.
Taking into account an inventory of all buildings in the city and their contents, the study calculated the weight of San Francisco (population: 7.75 million) as about 1.6 trillion kilograms – about 3.5 trillion pounds, or about 8. 7 million Boeing 747s.
That could be enough both to bend the actual lithosphere on which the city center resides, and perhaps more importantly, to change the relative levels of fracture blocks – the floating chunks of rock that make up Earth’s surface.
In fact, the 80mm slip is probably a conservative estimate, as the weight calculations did not take into account things outside of buildings – including transportation infrastructure, vehicles, or people. The same kind of sinking is likely in other parts of the world, although it depends in part on the 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.
“Anthropogenic loading effects on tectonically active continental margins are likely to be greater than more stable continental interiors where the lithosphere tends to be thicker and stiffer.”
There are also a host of other causes of subsidence to consider, including tectonic plate shifting and the pumping of groundwater needed to support a growing population – something we’ve seen significant urban subsidence in other parts. of the world.
While this current study only looked at San Francisco and made some broad assumptions in terms of modeling, the findings are remarkable enough to make the weight of the city another consideration when scientists are figuring out how geography might change over time and which areas are threatened if the sea level gets higher.
There are still plenty of details to dig into, too, especially in cities that are already threatened by land subsidence. It has already been calculated that the compaction of sediment and water-bearing systems beneath the coastal San Francisco International Airport – the heaviest building in the city – causes 4 mm (0.16 in) of sinking every year.
“It should be possible to improve the methods presented here by using satellite or aerial photos to make more detailed analyzes of likely flooding areas,” Parsons writes.
“Such detailed analyzes can also provide better insights into changes in subsurface porosity changes and the resulting fluid flow.”
The research is published in AGU advances