Urban Heat Island Mitigation in Chicago

By: Vince Parisi, AmeriCorps State & National Member

Chicago set a record high temperature on Monday June 17th, with a temperature 16 degrees above normal as record breaking heat has become increasingly common in the city. As the effects of climate change continue to be felt by Chicagoans and humans across the globe, summers can only be expected to get hotter. 

Urban Heat Islands “occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This effect increases energy costs, air pollution levels, and heat-related illness and mortality” according to the United States Environmental Protection Agency. Chicago could see up to 30 more days per year with temperatures above 100 degrees with continued high greenhouse gas emissions. Heat Islands can disproportionately affect those living in historically low income neighborhoods, including most of the families that participate in our Chicago HOPES for kids after school programs. As part of our Chicago HOPES network it is important to understand the impacts of Urban Heat Islands and what can be done to mitigate their effect. 

Impacts of Urban Heat Islands
Increased Energy Consumption: Urban areas are typically densely covered in non-permeable surfaces like roads, sidewalks, and buildings, with few trees and little vegetation. As temperatures increase, heat is absorbed and reflected by the non-natural surfaces resulting in surface temperatures that can be 15 to 20 degrees warmer than surrounding areas. These higher surface temperatures cause air conditioning units to work harder to cool buildings, and while doing so consume more energy resulting in increased greenhouse gas emissions. During peak energy demand, typically on summer afternoons, heat island effects increase the strain on the electrical grid and may cause municipalities to implement rolling blackouts in order to prevent power outages. 

Elevated Emissions of Greenhouse Gasses: To supply the power for increased demand on air conditioning units during heat island effects, companies typically rely on fossil fuels to supply the power needed. Heat island effects, themselves exacerbated by climate change and increased temperatures, in turn cause greater greenhouse gas emissions and further the effects of climate change. Additionally, greater greenhouse gas emissions can increase urban air pollution. In 2023, Chicago experienced the second highest level of air pollution among major US cities. Urban heat islands will continue to exacerbate the impact air pollution has on the health, educational, and economic outcomes of city residents and children in particular.

Compromised Human Health: Heat-related mortalities are the leading cause of weather related deaths in the United States, despite being preventable. Urban heat island effects lead to an increase in heat-related deaths and heat-related illnesses like heat exhaustion, heat cramps, and non-fatal heat stroke. Heat islands disproportionally affect the health of older adults, young children, and low-income populations:

• Older Adults are more likely to be less mobile, isolated, already in poor health, and living on reduced incomes.
• Young children are more susceptible to extreme heat due to their small size, and rapid breathing rates increase their chances of respiratory conditions resulting from the increased pollution caused by heat island effects.
• Low-income populations are at a greater risk for heat-related illness and death due to poor housing conditions and access to air conditioning. ​

Heat Island Effect Mitigation
Local governments and municipalities can employ several strategies to reduce the increased surface temperatures that result from urban heat islands in tandem with efforts to raise awareness of residents that may be impacted. The implementation of early warnings of extreme heat to residents and availability of cooling centers are one way a municipality can protect the safety of residents while heat island effects are being experienced. The primary methods to mitigate heat island effects in the long term are as follows:
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Increasing tree and vegetative cover: On average, urban forests have temperatures three degrees lower than surrounding areas, as trees and vegetation lower surface temperatures by providing shade. Through a process called transpiration, they absorb water through their roots and release the vapor through their leaves, further cooling surrounding areas. In addition, trees and vegetation can decrease pollution by lowering air conditioning costs, and store carbon dioxide from the air. They improve water quality by storing and filtering runoff and treeshade can slow the deterioration of pavement. Trees cover just 19% of the area of Chicago, while by comparison Atlanta boasts 47.9% coverage. According to a 2010 editorial Chicago has lost 10,000 more trees than it has planted each year on average. 

Installing green roofs and cool roofs: Green roofs, or rooftop gardens, provide shade, remove heat and lower temperatures of the surrounding air through the same processes as trees and vegetation. Green roofs can be 30-40 degrees cooler than traditional roofs and reduce city-wide temperatures by 5 degrees. Cool roofs are made of highly reflective materials that remain cooler than other materials in peak temperatures. While both are effective strategies for reducing heat island effects, green roofs also absorb stormwater and carbon dioxide from the air while connecting residents and wildlife to the built environment through green space.

Installing cool and permeable pavements: While conventional pavements can absorb 80-95 percent of sunlight, cool pavements use additives to create a surface that reflects solar rays. Permeable pavements absorb less heat than traditional pavement by containing pores that allow water to drain and evaporate, retaining runoff and cooling the ground level surface temperature. 

In summary, as the effects of climate change continue to be witnessed, it is important for cities to acknowledge and prepare for increasing instances of extreme heat. Chicago lacks the tree canopy needed to sufficiently mitigate the strain on the electrical grid that will result from increased heat island effects. In addition, many of its most vulnerable residents are at risk of heat-related illness and death due to housing instability and a lack of access to air conditioning. If the city wants to ensure a sustainable and safe future for its residents, a concerted effort to plant more shade trees and surface vegetation should be made, with potential incentives for homeowners and businesses to install green roofs. 

References: 
U.S. Environmental Protection Agency. (n.d.). Reduce urban heat island effect. U.S. Environmental Protection Agency. Retrieved June 28, 2024, from https://www.epa.gov/green-infrastructure/reduce-urban-heat-island-effect

U.S. Environmental Protection Agency. (n.d.). Learn about heat islands. U.S. Environmental Protection Agency. Retrieved June 28, 2024, from https://www.epa.gov/heatislands/learn-about-heat-islands

U.S. Environmental Protection Agency. (n.d.). Heat island impacts. U.S. Environmental Protection Agency. Retrieved June 28, 2024, from https://www.epa.gov/heatislands/heat-island-impacts

Cool pavement. (2024, June 28). In Wikipedia. https://en.wikipedia.org/wiki/Cool_pavement
Green, M. (2023, June 12). Chicago prepares for rising summer temperatures. Axios. https://www.axios.com/local/chicago/2023/06/12/chicago-summer-temperature

Friends of the Chicago River. (n.d.). Restore Chicago’s tree canopy. Friends of the Chicago River. Retrieved June 28, 2024, from https://www.chicagoriver.org/blog/celebrate-margaret-frisbie-s-20th-year-at-friends/6/restore-chicago-s-tree-canopy
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Trees Atlanta. (n.d.). Urban tree canopy study. Trees Atlanta. Retrieved June 28, 2024, from https://www.treesatlanta.org/resources/urban-tree-canopy-study