If you live in a densely populated urban area, you may relate to the feeling of relief when entering a park or green space. The vibrant foliage, the call of a nearby songbird, and the temporary escape from the hustle of city life eases your senses for a brief moment. The shade provided by the trees cools you off from the sweltering summer heat, and as you take a deep breath, you allow yourself to enjoy the cooler air. Why do we experience this vast difference in general comfortability when we’re in a dense urban landscape versus a green space such as a park, forest or lake? The answer requires an understanding of the urban heat island (UHI) and its effects on the microclimates in urban spaces. 

Urban areas are, on average, 10°F warmer than surrounding natural land cover. According to the U.S. EPA, UHIs are urbanized areas that experience higher temperatures than outlying landscapes due to the high concentration of buildings, roads and other impervious surfaces that absorb and re-emit the sun’s heat while simultaneously impeding the percolation of water necessary to support vegetation. Green spaces, trees and water bodies within urban areas, on the other hand, are correlated with lower surface temperatures given their natural cooling effect on the environment through the provision of shade, evapotranspiration of plants, and the evaporation of water bodies (Chen et al., 2023, Cortesão et al., 2023, Ampatzidis et al., 2022).

The lack of vegetation and abundance of heat-absorbing infrastructure are not the only factors contributing to the UHI effect. UHIs can be exacerbated by tall buildings and narrow streets that restrict airflow and trap hot air, a phenomenon referred to as urban canyons. Furthermore, the waste and heat-producing emissions from vehicles, factories, and housing in urban areas can also exacerbate UHIs. While air conditioning may be the only way to cool down your home in a heat wave, the leakage of freon waste can compound the heating phenomenon of UHI.

Urban heat islands and environmental justice

UHIs disproportionately affect low-income residents and communities of color compared to their wealthier and predominantly white, counterparts. A recent study conducted by Hsu et al. 2023, used surface urban heat island (SUHI) data and census tract-level data to analyze disparities in heat subjugation among urban neighborhoods. They found stark inequities between race and income, with the average person of color living in a census tract with higher SUHI intensity than non-Hispanic whites in all but six of the 175 largest urbanized areas in the United States. With respect to race and ethnicity, the authors found that Black residents have the highest average SUHI exposure across all climatic zones (arid, snow, temperate, and equatorial). In terms of income specifically, the authors found that people living below the poverty line have the highest exposure to SUHI. 

In another study, Voelkel et al. 2018  investigated the impact of extreme heat on vulnerable populations at the neighborhood level during the 2014 heatwave in Portland, Oregon. The authors examined socio-demographic factors (e.g., income, race, education, age, and English-speaking ability) in relation to the disproportionate distribution of UHI with the aim to better understand vulnerability based on (1) heat exposure among socio-demographic groups and (2) access to refuge from heat (either public cooling facilities or residential central air conditioning). They found that neighborhoods with predominantly low-income, non-white residents with low education levels and limited English proficiency, including those living in affordable housing, are disproportionately exposed to extreme heat. They also have limited ability to adapt to the negative effects of climate-induced extreme heat due to the lack of access to resources such as air conditioning and cooling centers. 

While the study doesn’t explore potential factors that could contribute to these spatial and social patterns of inequitable distribution of extreme heat, the authors point out how local development practices have typically placed heat-ameliorating landscape features (e.g., large trees) in higher-income neighborhoods, exacerbating heat exposure in low-income and minoritized communities that have been historically excluded from these areas. 

Urban heat islands and climate justice

Over the past 60 years, the frequency, duration and intensity of heat waves across the U.S. have increased, a trend the Intergovernmental Panel on Climate Change (IPCC) expects to continue globally. Climate change-induced extreme heat events have caused more fatalities than hurricanes, tornadoes and floods combined. For example, the 2003 heatwave in France claimed 14,947 lives, underscoring extreme heat as the deadliest “natural” disaster. 

Scientists have shown that warming caused by global climate change is compounded by the effects of UHIs. Therefore, as the climate continues to warm the planet, people who live in urban areas, especially those who already live in a UHI, will face heat events with greater intensity and frequency, becoming particularly vulnerable to heat-related illnesses and morbidity. 

UHI can affect human health in a myriad of ways, including exhaustion, dehydration, circulatory disorders, and even death. Vulnerability to extreme temperatures and other environmental hazards is usually defined by the degree to which individuals are likely to experience harm from exposure (Turner et al. 2003). Chow et al. 2012 describe how exposure to harm depends on both (1) physical exposure to extreme heat and (2) one’s ability to mitigate risk, such as having access to AC by staying indoors or their ability to find help in case of an emergency.

Extreme heat events primarily pose a danger to vulnerable communities, which, according to Chow et al., are those “lacking in economic assets and access to public support systems, with diminished physical or cognitive capacities to respond to warnings and missing strong and enduring social support systems.” These are the groups most vulnerable to hazardous events and the least able to adapt. They go on to explain that the adaptive capacity of individuals to respond to heat-related hazards is linked to racial and ethnic groups, income level, gender, age, citizen status, and housing tenure. 

A variety of factors can exacerbate an individual's vulnerability to extreme heat. For instance, individuals with mental health issues who use Psychotropic medications are at greater risk of heat-related illness since the medication type can impede the body’s ability to thermoregulate. Similarly, for the very young and older populations, their body’s ability to thermoregulate is also compromised (Hsu et al., 2021). In addition, many older individuals have comorbidities that increase the likelihood of heat-related illness and death, with 39% of heat-related deaths in the U.S. occurring in ages 65 or older (Vaidyanathan et al., 2020). Finally, low-wage jobs that involve strenuous outdoor work and those lacking adequate cooling facilities can lead to increased heat exposure (Rany & Alamgir, 2023). 

Low-income communities face additional challenges. Approximately 70% live in rental apartments with small AC units that landlords are under no obligation to replace. When it comes to public housing, many buildings are decades old and have no AC, and there are no federal requirements regulating access to AC. More problematic, Rosenthal et al., 2006, found that many low-income seniors feel that they cannot afford to use their AC due to increased utility bills, which puts them at a greater risk. They also found that populations who were impacted by housing foreclosures and economic recession were disproportionately vulnerable to heat-related morbidity due to increased housing instability, causing greater stress in the physical and social environment. Lastly, low-income communities who do not have access to a car or means of transportation are also disproportionately vulnerable as their ability to seek shelter or relocate to cooler areas is inhibited.

Immigrant, migrant and limited-English proficiency populations are also vulnerable. Language barriers can hinder access to resources and emergency alerts. Furthermore, undocumented populations are particularly at risk, as they are ineligible for federal aid and may avoid seeking help due to fears of surveillance, detention, and deportation. In addition, undocumented populations are often the last to receive aid during emergencies. Moving forward, these populations will require special attention in planning and recovery efforts during extreme heat events.

With half of the global population currently living in urban areas and an expected rise to 70% by 2050, the issue of UHIs will continue to grow. As climate change progresses, addressing UHI effects is critical, with special consideration needed for disproportionately vulnerable communities.

Mitigating urban heat islands

Effective UHI mitigation requires careful planning and collaboration among urban planners and policymakers. Urban planning offers crucial tools for reducing UHI effects, particularly in the context of climate change and urban sprawl (Gartland, 2010). The strategic inclusion of both vegetation (green) and water (blue) landscape infrastructure can be an effective UHI mitigation solution. 

As mentioned at the beginning of the article, UHIs largely form from the lack of heat-ameliorating vegetative cover. Thus, the addition of trees, green roofs, and vegetation can significantly reduce UHI effects through evapotranspiration (Peng et al., 2012). Water bodies can also serve as a landscape feature by reducing thermal load, thereby cooling the surrounding environment (Lieu & Weng, 2008). However, these strategies require careful consideration of maintenance requirements, types of vegetation, and placement (Lanzana & Stone, 2016). For example, drought-prone areas should use drought-tolerant plants, and water-stressed areas should consider using recycled gray water for blue infrastructure (Coutts et al., 2014)

Implementing sustainable practices and materials in urban design can also mitigate UHI effects. Researchers have explored the potential of certified LEED Green Buildings, which use sustainable and resource-efficient materials. While the concept is still being researched and developed, many have already made significant findings. For example, Shin et. al 2017 found that the construction of a certified LEED building has a cooling effect of 0.35 celsius in a 30-meter boundary.

Reflective materials, such as reflective roofs and pavements, are another design strategy to reduce UHI due to their high albedo and ability to reflect solar radiation. Yang et al. 2015, show that reflective roofs can be effective in reducing daytime surface temperature, but the impact of reflective materials on nighttime surface temperatures are negligible. They also found that reflective roofs can reduce precipitation, causing severe hydroclimate impacts that can adversely affect already water-stressed cities and urban areas. Adding to this concern, Brener and Lindsey 2006, show how reflective pavements can actually reflect solar radiation onto the walls of nearby buildings, thereby increasing surface temperatures, which is particularly concerning in high-density urban areas. While some researchers have found reflective materials impactful in mitigating UHI, more research on the potential negative impacts on the surrounding environment must be done in more depth before recommendations can be made.   

In addition to the mitigation strategies discussed above, urban planners should also aim to reduce urban sprawl in order to minimize anthropogenic heat. They should also design buildings that provide shade to cool the microclimates and better airflows to avoid urban heat canyons (Filho et al., 2018). To further minimize urban heat canyons, planners should also consider higher ratios of street width to street height and randomizing the placement of taller buildings (Kleerepoer et al., 2012 & Gago et al., 2013). Most importantly, urban planners should prioritize historically underserved communities in UHI adaptation and mitigation efforts.

Adapting to urban heat islands

Local governments and policymakers can increase the adaptive capacity of vulnerable communities by enacting extreme heat action plans. These plans could include issuing advanced heatwave warnings, providing public cooling centers and air-conditioned spaces, and providing necessary support to vulnerable populations. Outreach programs to increase awareness of heat-related symptoms and guidance on staying cool and hydrated are essential (Vaidyanathan et al., 2020). Additionally, policymakers should consider HVAC and home improvement subsidies, as well as utility caps or reduced electricity rates for low-income populations (Rosenthal et al., 2006; Vaidyanathan et al., 2020). Training and education programs for UHI experts in local governments can aid communities in finding heat refuge. Emergency responders should also be trained on the impacts of UHI protocols for treating heat-related illnesses. 

Given the unequal risk of extreme heat among different populations, governments should take an intersectional approach that prioritizes disproportionately vulnerable populations (Chow et al., 2012). Community-based adaptation planning and programs that identify neighborhood hotspots while also addressing economic disparities can address the social justice dimensions of extreme weather impacts on vulnerable populations while significantly reducing health impacts (Rosenthal et al., 2006). 

In conclusion, UHIs pose a significant environmental and public health challenge in densely populated urban areas. As climate change progresses, communities living in UHIs will be disproportionately exposed to extreme heat events. Thus, its effects not only raise temperatures but also exacerbate social and environmental inequities. Therefore, addressing UHIs requires an integrative approach that combines urban planning, policy, environmental justice, and climate resilience to ensure that the adaptive capacity of vulnerable populations is prioritized.