The Weight of Cities: Urbanization Effects on Earth's Subsurface

Abstract

Across the world, people increasingly choose to live in cities. By 2050, 70% of Earth's population will live in large urban areas. Upon considering a large city, questions arise such as, how much does that weigh? What are its effects on the landscape? Does it cause measurable subsidence? Here I calculate the weight of San Francisco Bay region urbanization, where 7.75 million people live at, or near the coast. It is difficult to account for everything that is in a city. I assume that most of the weight is buildings and their contents, which allows the use of base outline and height data to approximate their mass, which is cumulatively 1.6·10 12  kg. I build a series of finite element models to study effects of pressure exerted by the weight distribution. Within the elastic realm, I look at compression, flexure, isostatic compensation, stress change, dilatation, and fluid flow changes. Within the nonlinear realm I show example calculations of primary and secondary settlement of soils under load. The combined modeled subsidence from building loads is at least 5–80 mm, with the largest contributions coming from nonlinear settlement and creep in soils. A general result is closing of pore space and redirection of pore fluids. While the calculated subsidence of the Bay Area is relatively small compared with other sources of elevation change such as pumping and recharge of aquifers, all sources of subsidence are concerning given an expected 200–300 mm sea level rise at San Francisco by the year 2050. By the year 2050, 70% of Earth's population will live in cities. The belongings and needs of these growing populations concentrate mass over relatively small areas. In this paper, I calculate the weight of a metropolitan region and study the changes to the solid earth beneath it using numerical modeling techniques. I find that the subsidence under this weight is not insignificant and that it adds to other causes of urban subsidence, such as ground water pumping. As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea level rise may exacerbate risk associated with inundation. The relative rate of global population growth is higher in cities than in rural areas Urbanization causes significant material imports and a net increase in weight centered over small areas Models show that applied weight caused at least 5–80 mm of subsidence in the San Francisco Bay region in addition to other causes