Cities are sinking under their own weight, increasing danger from rising sea levels

MOUNTAIN VIEW, Calif. — Have you ever looked at a major city like New York and wondered how all those giant skyscrapers packed together don’t sink into the ocean? Well, a new study finds they are sinking and it could be a major problem as sea levels keep rising. A geophysicist from the United States Geological Survey says major cities are so heavy and so many people are moving into them that they may actually be bending the outer shell of planet Earth.

“By the year 2050, 70% of Earth’s population will live in cities,” says Tom Parsons in the journal AGU Advances. “The belongings and needs of these growing populations concentrate mass over relatively small areas.”

Looking specifically at San Francisco, Parsons calculates that the California metropolis has sunk by over three inches as the city has developed. On top of this, scientists believe the city’s sea levels will rise by about a foot by 2050. When factoring those two together, the growing weight of cities becomes a real issue for future floods.

“As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea level rise may exacerbate risk associated with inundation,” Parsons continues.

Just how heavy is a city?

When looking at all the buildings and the people and items inside of them, the study estimates San Francisco (home to 7.75 million people) weighs about 3.5 trillion pounds. This figure doesn’t even account for things outside of the buildings like vehicles, transportation infrastructure, or additional people coming to town. Therefore, the three-inch estimate of San Francisco’s sinking is likely a conservative number.

Parsons believes such weight in urban areas around the globe is enough to actually bend the planet’s lithosphere. Cities may even be dense enough to alter the fault blocks which make up the Earth’s surface.

“The specific results found for the San Francisco Bay Area are likely to apply to any major urban center, though with varying importance,” the researcher explains.

In fact, previous studies have discovered that the major Middle Eastern city of Tehran is sinking by nearly 10 inches a year in some parts. The Iranian capital is home to around 15 million people. In California, the study finds massive urban centers like San Francisco Bay, Monterey Bay, Los Angeles, and San Diego are all sinking to some degree. The problem tends to show itself more in areas where the Earth’s shell isn’t thick enough to withstand the growing weight.

“Anthropogenic loading effects at tectonically active continental margins are likely greater than more stable continental interiors where the lithosphere tends to be thicker and more rigid,” Parsons adds.

What’s causing cities to sink?

While growing populations play a role in cities packing on the pounds, it’s not the only cause. The study notes that as cities grow, they start pumping more groundwater, which can significantly increase sinking.

Looking at San Francisco International Airport, the city’s heaviest building sitting right on the coast, the study finds sediment and aquifer system compaction is making the transportation hub sink by 0.16 inches each year. The study author believes scientists can use these calculations to figure out how the surface and sea levels will change over time.

“It should be possible to improve on the methods presented here by using satellite or air photos to make more detailed analyses in likely flood zones,” Parsons concludes. “Such detailed analyses might also yield better insights about changes to subsurface porosity changes and resultant fluid flow.”

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