As urbanization rates top themselves year after year, the United Nations predicts that by 2050, 68% of Earth's population will live in big cities. That is up from the 55% estimated in 2018. The migration from rural areas into metropolises brings with them an array of obstacles. Infrastructure, city services, and job vacancies dominate the problems needing to be addressed when a population influx occurs. This is disregarding the actual quality of life that needs to be maintained in order to drive more people for jobs and infrastructure construction. Researchers from the United States Geological Survey (USGS) agency, however, shine a light on a different approach to issues regarding city planning.
In a study published in the AGU (Advancing Earth and Space Science) Advances journal, Tom Parsons, a geophysicist at the USGS, found that cities such as San Francisco may have sunk up to 80 millimeters throughout their history. Parsons states in his paper that cities have built up so much that they could weigh on average 1.6·1012 kg or simply, 1.6 trillion kilograms. Taking into account the elasticity of the earth's crust, otherwise known as the lithosphere, this weight is sufficient to literally bend the ground downwards.
The figures, are a rough estimate and may very well be higher than what Tom estimated. It is “virtually impossible to calculate the exact weight of a city," said Parsons. “I approximate the weight of urbanization by assuming that buildings and their contents make up the majority of it." Adding that The Millennium Tower in San Francisco has sunk by more than 400 millimeters over the last decade.
Nearing the end of the paper, the geophysicist adds an alarming finding that renders his paper in a new light of relevancy. "As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea-level rise may exacerbate risk associated with inundation." Effectively highlighting the issue with rising sea levels due to climate change and how coastal urbanization impacts it for the worse. According to a UN paper, 2.4 billion people live within a 100km radius of the sea as of 2017.
Scientists will now factor in city weight into their studies and reports when researching areas at risk of rising sea levels. Concluding his paper, Parsons argues that “satellite or air photos” could be used to “make more detailed analyses in likely flood zones” as sea levels continue rising, with the elevation of cities taken into account.