Graphic by Annabel Driussi for ODP

by Ashira Morris, ODP Staff Writer

How much does a city weigh? You can’t put San Francisco on a scale, but new research from the US Geological Service estimates that the number is 1.6 trillion kilograms, about the same as 250,000,000 elephants. 

This isn’t just a clever math problem, though: all that weight is causing San Francisco and other coastal cities like it to slowly sink. The sinking, called “urban subsidence,” is caused by a range of factors, including a concentration of weight from buildings, people, and cars on the earth and groundwater pumped to support the city. The research estimates that San Francisco has sunk 80 mm over time as its population and size have grown. 

Why this Matters: By 2050, 70% of the world population is expected to live in an urban center. With many of those cities on the coast and sea level rise happening even faster than anticipated, there isn’t any extra land to spare. With rapid new building construction, smart planning and water management are necessary to keep sinking to a minimum and keep residents safe. A better understanding of urban subsidence could improve our understanding of flood plains and future areas at risk of inundation. 

“As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea level rise may exacerbate risk associated with inundation,” wrote Tom Parsons, the USGS geophysicist who did the study. 

Sinking cities worldwide: Nearly 10% of the global population lives at 10 meters above sea level or lower, putting cities worldwide at risk of coinciding rising seas and sinking ground.

• In Jakarta, building weight coupled with lots of groundwater being pumped to the surface has sunk the city by 15 cm. 
• In Shanghai, groundwater has been better managed, but lots of underwater construction, from aquifers to building foundations, could lead to anywhere from 3 cm to 2 meters of sinking. The city is expected to see sea levels rise by 43 centimeters.
• In Bangkok, extracting groundwater was the primary culprit until a building boom in the ‘90s added to the weight of the city, which is now sinking by 2 centimetres annually. 
• In Mexico City, aquifers readily tapped for drinking water are causing buildings around the city to noticeably sink and tilt. 

Tides already cause flooding: For some coastal U.S. cities, flooding is a present problem, not just a future risk. Miami and Boston both have recently had water inundate the city because of extreme high tides, called king tides. The annual high tides, which begin in the fall, are part of the lunar cycle and Gulf Stream current. But because climate change has pushed sea levels higher, these tides now flood streets and turn yards into swamps.