It’s easy to see how environmental wellness changes as urban areas develop; buildings and roads replace open land and vegetation, and once permeable and moist land becomes hard and dry. When this happens, you get what’s known as an urban heat island, or an urban region that’s warmer than its rural surroundings, forming an “island” of higher temperatures in the landscape.
According to the United States Environmental Protection Agency (EPA), ‘Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining. In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with one million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.3 On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).’
But why should you care about urban heat islands? Does it have an impact on your wellbeing? The EPA notes, ‘Elevated temperature from urban heat islands, particularly during the summer, can affect a community’s environment and quality of life.’ Some of the impacts of heat islands seem positive, as they can lengthen the season of plant-growing, for example. However, most of the heat island impacts are, unfortunately, negative. These include:
- Increased energy consumption: The EPA explains, ‘Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand. One study estimates that the heat island effect is responsible for 5–10% of peak electricity demand for cooling buildings in cities.’
- Elevated air pollutant and greenhouse gas emissions: ‘Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants,’ the EPA says. ‘Higher air temperatures also promote the formation of ground-level ozone.’
- Poor human health and comfort: When the days and nights become warmer, and the air pollution level rises, this can contribute to general discomfort, as well as respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.
- Poor water quality: ‘Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes,’ details the EPA. ‘Rapid temperature changes can be stressful to aquatic ecosystems.’
Heat islands, then, are no picnic – but what can be done about it? According to the EPA, there are four main strategies that, as a community, you can take to reduce the heat island effect:
1. Increase cover from trees and vegetation.
2. Create green roofs, otherwise known as rooftop gardens or eco-roofs.
3. Install cool—mainly reflective—roofs.
4. Use cool pavements.
The EPA concludes, ‘Typically heat island mitigation is part of a community’s energy, air quality, water, or sustainability effort. Activities to reduce heat islands range from voluntary initiatives, such as cool pavement demonstration projects, to policy actions, such as requiring cool roofs via building codes. Most mitigation activities have multiple benefits, including cleaner air, improved human health and comfort, reduced energy costs, and lower greenhouse gas emissions.’
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