10.3 Heat, water, and air

Heat

Creating a city involves a change in land use (Fig 1): areas of vegetation are replaced with roads, sidewalks, parking lots, and buildings. Impervious surfaces (those that can’t absorb water) such as roofs and pavement emit and reflect solar energy differently than natural surfaces, often absorbing heat during the day and then releasing it at night. Additionally, buildings can trap air between them and alter the movement of wind currents, and human activities like cooking and driving produce heat. All these factors combine to create the phenomenon of urban heat islands (UHIs; Fig 2), in which the air temperature in a city is higher than in the surrounding area. Usually, the difference is on the scale of 2-4ºC, but the air in a big city can be up to 5-10ºC hotter than in nearby rural areas.

Areas with more impervious surfaces and fewer plants have higher surface temperatures, and this pattern holds during both the day and night.^[1] Heat threatens human health by magnifying the effects of other health conditions, and heat waves can cause thousands of deaths, especially when high temperatures continue overnight. As climate change progresses, many regions are dealing with hotter temperatures, which exacerbate the heat-island effect.

Just like the locations of polluting industries, built-up areas of cities where the heat-island effect is worst are often home to disadvantaged communities, reflecting historic prejudice and perpetuating inequality in these communities’ physical environments.^[2] In addition to health impacts, heat leads to increased electricity use for air conditioning. Each degree of temperature increase leads to a 0.5-8.5% increase in electricity demand,^[3] raising costs for residents or causing blackouts. For buildings whose electricity comes from fossil fuels, this leads to increased GHG emissions as well. Heat-mitigating infrastructure and energy conservation will be discussed in later sections.

Water

When rain falls in a field or forest, most of it either infiltrates into the ground or collects on plants and then evaporates. On a road or sidewalk, however, most of it runs off (Fig 3). This can cause flooding in cities, and the extra water increases the runoff’s flow rate, causing erosion on the way to nearby streams and rivers. The water is often heated unnaturally by the city’s impervious surfaces, and warm water holds less oxygen than cold water. Many aquatic species are very sensitive to sediment, temperature, and oxygen levels, so this runoff can cause serious disruption to nearby stream and river ecosystems.

When just 10-20% of a watershed is covered by impervious surfaces, the amount of runoff into streams doubles.^[4] Densely built-up parts of cities tend to have a higher percentage of impervious surface cover than low-density areas and therefore lead to more stormwater runoff; however, a dense city that takes up less land area overall creates less total runoff than a low-density, sprawling city with the same population.^[5]

Impervious surfaces collect contaminants (gasoline, road salt, industrial waste products, litter and household waste, fertilizers and pesticides, and many more), which rainwater runoff or floodwater can then carry into waterways. Replacing just 10% of a watershed with impervious surfaces is enough to damage the water quality in that watershed.^[6]

Water pollution is one of the top threats to human health, connected to 80% of diseases and 50% of child deaths around the world.^[7] Impervious surfaces can also contribute to the spread of disease by holding pools of water where mosquitoes (whose natural predators may not live in urban areas) can lay their eggs.^[8]

Because impervious surfaces cause so many problems, an obvious solution is to replace them with permeable surfaces where possible. There are many alternatives to pavement, such as flagstones and gravel, as well as various types of permeable pavement. There are also ways to store rainwater so it doesn’t overwhelm watersheds, including gray infrastructure, such as rain barrels and large storage tanks, and green infrastructure, which makes use of plants’ ability to absorb and filter water. For an in-depth look at Copenhagen’s flood mitigation efforts, including both green and gray infrastructure, you can watch this 11-minute video .

Rivers and streams naturally meander, or follow a path that curves back and forth. This gives the water enough space that when it rains, the flow rate doesn’t increase drastically, and when a stream floods, there’s plenty of land along the banks to absorb it. In many cases when a city was built around a river, people constructed straight concrete banks for the river to flow through, a technique called channelization (Fig 4). This shortens the river’s path and prevents water from being absorbed into the ground. After a large rain, this type of channel can easily flood the surrounding areas of the city, and the fast-flowing water can have significant erosive force. You can watch a 6-minute video on the history of humans’ modifications to rivers and a few places in Europe that are experimenting with restoring rivers to their original meandering paths. Many rivers were not only channelized, but buried in pipes underground. You can watch a 7-minute video about a city in Canada that is working to daylight a buried river, bringing it back to the surface. Daylighting often involves restoring the riverbanks to a more natural form, improving the river’s ability to absorb rainwater and creating aquatic and riparian habitat.

Air

Most air pollution comes from burning things, including fuel for heat, cooking, transportation, and industrial processes. A lot of this happens in and around urban areas. In developing countries, cooking and industry produce the majority of air pollution, while in developed countries, most air pollution comes from vehicles (Fig 5).^[9]

Not all city residents are at the same risk for air pollution – disadvantaged communities deal with higher levels. For example, in the United States, Black people are 1.54 times as likely as white people to live near factories that emit PM_2.5.^[10]

As we saw in Chapter 2.3, the world death rate from air pollution dropped by nearly half from 1990 to 2021 (Fig 6). That change is due to a decrease in deaths from indoor air pollution, while deaths from outdoor air pollution have stayed fairly constant. Reducing outdoor air pollution depends on transportation and power generation solutions and on decreasing industrial pollution. Governments can raise air quality standards, carefully monitor industries, and set strong financial incentives to decrease pollution. Research and development of new, green technologies can also decrease the environmental impacts of industrial processes.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.