Cities have become so big and crowded that they are gradually collapsing under the weight of their own development, according to a new study.

The researchers specifically looked at San Francisco as a case study and found that the city might have already sunk by 8 centimeters (or 3.1 inches) – something that they argue is likely happening in other cities too.

The finding is especially concerning as cities (especially coastal cities) are already exposed to sea level rise because of climate change. Sea level has already risen between 21 and 24 centimeters (or 8-9 inches) since 1880 and the rate is accelerating, increasing the risk of floods, extreme weather events, and coastal erosion.

“As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea-level rise may exacerbate risk associated with inundation,” Geophysicist Tom Parsons, from the United States Geological Survey (USGS) agency, wrote in his recent paper.

At the same time, a steady migration of Earth’s population from rural to urban centers is occurring in virtually every part of the world. Currently, about 50% of Earth’s population lives in urban settings, and by 2050 it is projected that number will grow to 70%, according to the UN. Along with people, urbanization has caused a redistribution of mass into concentrated areas. In other words, cities are constantly becoming heavier, and it’s not just because of the people moving in.

Nearly everything necessary to sustain a city’s population must be imported. A global network of suppliers ships food, fuel, water, cars, mass transit, pavement, pipes, concrete, and steel from great distances. Every conceivable object that people want or need is brought to and stored within relatively small areas. Researchers wondered whether all this concentrated weight wouldn’t have an effect. Lo and behold, it does.

For his research, Parsons used San Francisco as a case study to understand why and how many cities are sinking. The San Francisco Bay region has over 7.7 million inhabitants and is the cultural, commercial, and financial center of Northern California. He discovered that the city sunk by 80 millimeters (3.1 inches), this being the city’s level of subsidence (sudden or gradual sinking of the ground’s surface).

Parsons calculated the weight of the bay area at 1.6 trillion kilograms or roughly 8.7 million Boeing 747s, taking into account all the buildings in the city and their contents. This would be enough to bend the lithosphere, which is the rigid outer part of the Earth consisting of the crust and upper mantle, causing it to sink, according to Parsons.

The study didn’t consider the weight of things outside buildings, such as transport infrastructure, vehicles, or people. This means the city could have actually sunk more than the estimated 80 millimeters. For the researcher, it’s a clear sign that the same type of sinking is likely to be happening in other parts of the world, depending on the geography of each city.

“The specific results found for the San Francisco Bay Area are likely to apply to any major urban centre, though with varying importance,” Parsons wrote in the study. 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.”

While other causes of sinking also have to be taken into account such as tectonic plate shifting and groundwater pumping, these findings are significant. And they could even be improved further, using satellite photos to better analyze the Earth’s surface and predict where likely flood zones might occur, Parsons argued.

The study was published in the journal AGU Advances.