Next Generation Science Standards:
Key vocabulary: Temperature, conductor, dispersed, urbanization, greenhouse gases, localized, moisture, urban heat index (UHI), vegetation, ecosystem, pollutant, smog
Over the last 100 years, people have drastically changed how and where they live. In 1900, 14% of the world’s population lived in cities. Today roughly half of all humans on Earth live in cities, a figure researchers expect to increase over the 21st century. While urbanization has given more economic opportunity to regular citizens than ever before, it has also caused a lot of ecological problems including habitat destruction and pollution. While water and air pollution have been discussed and well regulated, thermal pollution has become a cause for concern. Without natural vegetation, cities have become areas of higher temperatures called heat islands, causing increased energy demands, higher levels of air pollutants and greenhouse gases, and stresses on both humans and local ecosystems.
What are heat islands?
Heat islands are areas of increased temperatures, usually centered around places of high urbanization. The term island was coined because the higher temperatures tend to be localized in cities compared to rural areas. The difference in temperature becomes particularly stark during the summer, where cities can be as much as 22°F (12°C) warmer than the air in neighboring, less developed regions.
So what makes urban areas so much hotter? Kevin Czajkowski from the University of Toledo studies the heat island effect. He says that in cities, the natural environment of trees and vegetation has been replaced by the concrete, metal, and pavement that make up the roads and buildings. These materials are strong and durable, but they are also good thermal conductors. “Urban areas are made up of buildings, streets, sidewalks, parking areas, etc,” says Dr. Czajkowski. “These surfaces warm up in the sunshine and retain heat at night because they are dense and bulky.” As a result, they absorb the sun’s energy instead of dispersing it like plants. Without trees and other vegetation to shade the land and retain moisture, more energy from the sun is converted to heat, raising the air temperature.
Additionally, cities burn fossil fuels for energy. “Heating and cooling buildings, electricity generation, transportation, and industry all give off heat,” explains Dr. Czajkowski. The electricity used to power the economy is usually sourced from burning of coal or natural gas. Cities also have a higher concentration of automobiles, the vast majority of which burn gasoline. Since highways and roads have nearly no tree cover, that heat is absorbed by the air, further raising the temperature. Even the shape of cities plays a factor. Tall buildings block the wind, which keeps the heat from getting dispersed to the surrounding environment. All this has caused not only an increase in temperatures but also an increase in days with dangerously high temperatures.
A problem to sweat over
Besides making summers uncomfortable for city residents, there are a lot of issues that heat islands cause. The most obvious of which is increased temperatures means more energy to be consumed to keeping buildings cool and safe for people. This not only is expensive but requires more fossil fuels to be burned to keep up with energy demand, further exacerbating the problem.
And while hot days in the summer may certainly be uncomfortable for most people, it can be deadly to the others. Increased temperatures and increased days of high temperatures can cause respiratory difficulties, heat cramps and exhaustion, and non-fatal heat stroke. The elderly, who may not have home care or air conditioning, are, particularly at risk. The Centers for Disease Control estimates that 600 premature heat-related deaths occur each year.
Air pollution also increases with higher temperatures. Ozone the oxygen molecule found in the upper atmosphere is another pollutant. While critical to protecting the Earth’s surface from solar radiation, the gas is toxic to living things and forms the haze in cities. “Cars and factories add nitrogen oxides (NOx) and hydrocarbons into the air that will produce ozone (smog) on warm sunny days,” says Dr. Czajkowski. The compound forms faster at higher temperatures and some studies have shown an increase in cases of children’s asthma in children could be linked to air pollution.
The high pressure systems that give us the warm, sunny days of summer also contribute to the problem. “These high pressure systems produce a temperature inversion a couple of kilometers above the surface,” explains Dr. Czajkowski. “This inversion keeps the pollution from mixing up into the atmosphere and allows pollutants to build up.”
Increasing temperatures also cause big problems for ecosystems as well. While many species have adapted to city living and higher temperatures may help plants photosynthesize in the short term, species are only so tolerant. If it gets hot enough, the plants may start to die. Heat islands also cause storm waters to be warmer. That warmer water will runoff into local waterways, causing stress for aquatic animals and plants.
The heat island problem will likely get worse as cities across the globe grow both in population and in area. As cities get bigger, they tend to spread out, taking up more land, destroying more habitat, and cutting down the vegetation that would otherwise absorb a city’s heat. Increased sprawl and a growing population will also mean city residents will need to continue to use cars to get around, adding more heat and smog to the atmosphere. Additionally, rising global temperatures due to increased atmospheric carbon dioxide will make the problem worse.
So what can the average person do to reduce a city’s heat island? One of the best ways is to make your home as energy efficient as possible with your appliances. Cut down the number of cars on the road by walking, biking, or taking public transit to get around town. If you have property, planting trees or other vegetation can not only provide shade but also help absorb the summer heat. If you really want to step up your game, you can install a green roof on your home or building. Green roofs not only help keep buildings and cities cool, but the plants also remove pollutants from the air and reduce stormwater runoff.
But to really address the heat island problem, larger steps need to be taken by local and regional governments. “There are several cities around the world that are adding many trees to reduce the UHI,” Dr. Czajkowski says. Increase tree cover in cities is one of the most effective ways to beat the heat. Setting aside more parkland in urban centers is a great way to do this. Cities have also found creative solutions to make more green space by building green roads. These roads not only have trees lining them but are made with a more porous surface, allowing for water seep in and grass to grow, cutting down on the amount of heat absorbed by the road’s surface.
Investing in public transit would also cool things down. Better transit options would cut down on the number of cars on the road, meaning less heat and exhaust. It would also encourage cities to become denser and use less area, allowing for more habitat to be protected on the outskirts. Buildings made with less dense materials and lighter colors can also help.
However, the most impactful policy is managing city growth to grow with the local ecosystem and not in spite of it. With a human population that could reach 10 billion by the end of the 21st century, a delicate line must be towed to provide enough housing and economic opportunity for people while at the same time protecting the habitat of plants and animals. Lacking forward planning could land many cities in hot water.