5.10 Impacts of Urbanization

Urbanization is the growth of cities and the movement of people from rural areas into them. More than half of the world's people now live in urban areas, and the share is still rising, especially in Africa and Asia. People move to cities for jobs, education and services.

Cities aren't all bad for the environment. Dense cities can use land efficiently and make it easier to use public transit, walk and share services. But how cities grow makes a big difference.

Impervious surfaces are surfaces water can't soak through, like roads, parking lots, sidewalks and roofs. When rain falls on them, it can't infiltrate the soil. Instead, it runs off quickly into storm drains and streams.

That has several effects. Streams rise higher and faster during storms, so flooding increases. Runoff carries oil, metals, fertilizer, pet waste and trash straight into waterways. Water heated on hot pavement warms streams. And with less infiltration, less water recharges groundwater, so streams can run lower between storms.

Cities also deplete local water resources by pumping groundwater for drinking water and industry. Along coasts, pumping fresh groundwater faster than it's replaced lowers the water table, and salt water from the ocean moves in to fill the space. This saltwater intrusion makes wells too salty to use.

Urban sprawl is low-density development spreading outward from a city into the surrounding countryside: subdivisions, shopping centers and highways. It's driven by things like cheaper land, car ownership, new highways and people wanting more space.

Sprawl converts farmland, forests and wetlands into development, fragmenting habitat. Because homes, jobs and stores are far apart, people drive more, burning more fuel and producing more air pollution and greenhouse gases. Sprawl also adds a lot of new impervious surface in the form of roads and parking.

Cities affect the carbon cycle by burning large amounts of fossil fuels for transportation, electricity and heating, which adds CO₂ to the atmosphere. Landfills, where cities' trash ends up, release methane as organic waste decomposes without oxygen. Paving over vegetation also removes plants that would absorb CO₂.

Cities are often warmer than the surrounding countryside, an effect called the urban heat island. Dark pavement and roofs absorb sunlight, there are fewer trees for shade and cooling, and cars and buildings release heat.