Globally, the annual average surface air temperature increased by approximately 1.8°F between 1901 to 2016 [1]. Within the United States, heatwaves have occurred more frequently and in the next few decades (2021-2050) the annual average temperature is anticipated to rise about 2.5°F. In the United States, the urban heat island effect results in daytime temperatures of 0.9° to 7.2°F higher and nighttime temperatures of 1.8° to 4.5°F higher when compared to rural areas.
Figure 1. Global land and ocean temperature anomalies (left) and surface temperature change (right) (Figure by Kennth Kunkel, 2017 Climate Science Special Report: Fourth National Climate Assessment by the U.S. Global Change Research Program)
Heat islands are areas where structures such as buildings, roads, and other infrastructure that absorb and re-emit solar heat are highly concentrated and natural landscapes are minimal [2]. There are two types of heat islands – surface heat islands and atmospheric heat islands. Surface heat islands occur because existing urban surfaces absorb and emit heat greater than natural surfaces. Atmospheric heat islands occur due to the temperature difference between the warmer urban areas and cooler outlying areas.
Heat islands are caused by certain urban material properties, urban geometry, human activities that generate heat, weather conditions, and the influence of the surrounding geography. The addition of structures and the removal of natural landscapes causes a reduction in natural shading and plant transpiration, which serve to cool the environment and provide moisture. Materials, such as pavements or roofing, tend to absorb and emit solar energy rather than reflect it. Therefore, these materials can contribute to the increase in local temperatures and foster heat island development. Urban geometry influences wind flow and the ability for trapped heat to be released within a built environment. In areas with dense structures and narrow streets, natural wind flow is blocked, which results in a reduction of cooling.
Human activities, including vehicular travel, air-conditioning, industrial facilities, and other activities create waste heat that goes into the environment and contributes to the heat island effect. Local weather and geography can influence the formation and the severity of heat islands. For example, Clear weather conditions allow greater amounts of solar energy to reach urban surfaces, while cloudy and windy weather conditions suppress heat and provide a cooling effect. Geographical features, such as mountains, impact wind flow and the severity of the heat island effect. Heat islands are typically measured by the difference in temperature between cities and their surrounding areas.
Figure 2. Showing surface and air temperature difference during the day and nighttime (Figure by the U.S. EPA)
Urban heat islands have different impacts on local communities including increased energy consumption, elevated emissions of air pollutants and greenhouse gases, compromised human health, and impaired water quality [3]. The heat island effect increases temperatures and as a result increases the demand for air conditioning in order to maintain cooler temperatures in buildings.
Case studies reveal that electricity demand for air conditioning increased between approximately 1% to 9% for each 2°F increase in temperature. Companies that supply electricity rely on fossil fuels, which results in an increase in air pollution and greenhouse gas emissions. The release of air pollutants is detrimental to air quality and contributes to issues such as the formation of smog, fine particulate matter, and acid rain.
Heat islands contribute to health issues including but not limited to general discomfort, respiratory difficulties, heat cramps, and heat exhaustion. According to the Center for Disease Control and Prevention, between 2004 to 2018 there were 10,527 deaths in the United States where heat was the underlying or contributing cause [4].
During excessive heat events, heat islands can increase health risks by increasing the maximum temperature locals are exposed to and the length of time they are exposed to such temperatures [5]. These events can warm pavements and rooftop surfaces and as a result stormwater runoff, which then drains into storm sewers and releases warmer water into streams, rivers, ponds, and lakes. The resulting temperature change in natural bodies of water can be stressful and fatal to aquatic life.
To address and respond to heat related hazards within local communities, Ladd Keith and Sara Meerow share strategies that can achieve equitable heat resilience in Beat Extreme Heat with These 8 Tactics. Such strategies fall into two categories – heat mitigation and heat management [6].
Heat mitigation strategies are focused on cooling cities and their respective communities. Such mitigation strategies can be accomplished through land-use planning, urban design, urban greening, and waste-heat reduction strategies.
Land-use planning is encouraged to facilitate broader efforts such as the conservation of natural areas, the development of ventilation corridors within urban areas, the dimensions and spacing between buildings, and the reduction of heat-trapping surfaces. The use of small-scale design opportunities is encouraged through the orientation of buildings and streets for shade, the addition of shade structures, and the use of cool materials. Shade structures, including bus shelters, provide cooler spaces for residents to occupy during extreme heat events.
Figure 3. Permeable asphalt that allows water to drain on the surface, unlike conventional asphalt (Figure by U.S. Department of Agriculture)[7]
To maintain cooler temperatures around buildings, cool roofs can be implemented and replace conventional roofs. Cool roofs tend to have high solar reflectance that lower the temperature outside of a building and thus reduces the heat island effect [8]. Co-benefits of cool roofs include the reduction in energy use in buildings and the associated reduction in air pollution and greenhouse gas emissions. Cool roofs can reduce heat health hazards and energy cost burdens faced by those most impacted by the heat island effect.
Greening strategies can mitigate the heat island effect and can be implemented through green roofs, greenways, green stormwater infrastructure, and urban forestry. For example, street and parking lot trees can be planted to provide shade and reduce direct solar energy from reaching surfaces and structures. The reduction of waste heat can be accomplished through the reduction of vehicle use, the use of cool surfaces, and the increase of building energy efficiency.
Figure 4. A green roof located at the EPA Region 8 – Denver, CO (Figure by U.S. EPA)[9]
The implementation of “tiny forests” is another greening strategy that can reduce the effects of extreme heat events. Tiny forests are dense forests that are approximately the size of a tennis court [10]. Heat management strategies focus on lowering temperatures and preventing future extreme-heat events. Such mitigation involves the preparation for and response to extreme heat events, which will require coordination of different government levels and other sectors.
Heat management strategies include indoor cooling, reduction of heat exposure, building public awareness, and the creation of a heat action plan. Regulations and assistance programs are implementable strategies to make cooling accessible and affordable to all. To reduce heat exposure, facilities and public infrastructure operations may require alterations and regulations for indoor and outdoor worker safety. In preparation for unprecedented extreme-heat events, communities can implement early-warning systems, plan coordinated responses, and set up designated cooling centers for shelter and assistance. Such preparation and coordination should be incorporated into local planning documents (e.g., Land Use Element, Resiliency Element, and Safety Element).
The most vulnerable populations to the heat island effect are those that reside within cities [11]. Within cities, there are “intra-urban” heat islands that form due to the uneven distribution of natural landscapes in comparison to heat-absorbing structures and pavements. Studies have shown that the uneven distribution of natural land cover is related to income [12]. As a result, low-income communities tend to have less natural land cover and face greater health risks during excessive heat events. Additionally, these communities face a reduction in air quality and the burden of higher cost air conditioning bills.
Local jurisdictions can develop policies and actions to tackle such inequities in their adopted Environmental Justice Elements. Policies and actions addressed in a jurisdiction’s Environmental Justice Element can provide direction for change and foster the implementation of necessary programs.
By Laylonni Laster, Assistant Planner
Laylonni Laster is an Assistant Planner, focusing on environmental planning and a commitment to addressing climate change impacts.
[1] USGCRP, 2017: Climate Science Special Report: Fourth National Assessment, Volume 1, ‘Executive Summary’, https://science2017.globalchange.gov/chapter/executive-summary/
[2] U.S. EPA, ‘Learn About Heat Islands’, https://www.epa.gov/heatislands/learn-about-heat-islands#heat-islands
[3] U.S. EPA, ‘Heat Island Impacts’, https://www.epa.gov/heatislands/heat-island-impacts
[4] Vaidyanathan A., Malilay J., Schramm P., Saha S. Heat-Related Deaths — United States, 2004–2018, Morbidity and Mortality Weekly Report (MMWR), https://www.cdc.gov/mmwr/volumes/69/wr/mm6924a1.htm#suggestedcitation
[5] U.S. EPA, ‘Extreme Heat Guidebook’, https://www.epa.gov/sites/default/files/2016-10/documents/extreme-heat-guidebook.pdf
[6] Ladd Keith, Sara Meerow, APA- ‘Beat Extreme Heat with these 8 Tactics’, https://www.planning.org/planning/2021/fall/beat-extreme-heat-with-these-8-tactics/
[7] U.S. EPA, ‘Reducing Urban Heat Islands: Compendium of Strategies Cool Pavements’ https://www.epa.gov/sites/default/files/2017-05/documents/reducing_urban_heat_islands_ch_5.pdf
[8] U.S. EPA, ‘Using Cool Roofs to Reduce Heat Islands’, https://www.epa.gov/heatislands/using-cool-roofs-reduce-heat-islands
[9] U.S. EPA, ‘Using Green Roofs to Reduce Heat Islands’, https://www.epa.gov/heatislands/using-green-roofs-reduce-heat-islands
[10] Bruns, Maarten, et al., ‘Handbook: Tiny Forest Planting Method’, https://www.greenflagaward.org/media/2136/tf_handbook_2019_english_0.pdf
[11] U.S. EPA, ‘Heat Islands and Equity’ https://www.epa.gov/heatislands/heat-islands-and-equity
[12] Ed G. & Shannon W., ‘The Relationship Between Urban Forests and Income: A Meta-Analysis’ https://www.sciencedirect.com/science/article/abs/pii/S0169204617302062?via%3Dihub