The sweltering heat radiating off a sidewalk or parking lot on a hot summer day is a sensation familiar to many around the world, especially in cities where paved spaces seem to outnumber the natural. A growing phenomenon in these metropolitan areas is known as the “urban heat island effect," a consequence of concrete, buildings, and other infrastructural elements in which heat from the sun is absorbed and re-radiated to create abnormally high temperatures in localized urban areas [1]. Urban heat islands not only endanger human health with their links to heat strokes and heat-related mortality in cities, but they also create a suite of socioeconomic issues pertaining to pollution and energy costs [2]. Although several methods of addressing urban heat islands have already been identified, the lack of a truly equitable and economical solution is fanning the flames of environmental justice concerns across urban landscapes.
Urban heat islands are prevalent in Charlottesville, Virginia, namely within the 10th & Page, North Downtown, Belmont, and Ridge Street areas of the city. According to the City of Charlottesville’s 2023 Climate Action Plan, the urban heat island effect has increased the temperature of these areas by up to an additional 5.5°F, or 3.1°C, on summer afternoons (as compared to areas not experiencing the effect). One of the primary characteristics of areas prone to urban heat islands is a general lack of tree canopy cover. When trees undergo transpiration, the process by which plants release water vapor out of the leaves due to photosynthesis, they reduce the amount of heat available to warm the air around them [3]. This means that the air within the shade of a tree can be much cooler than the air within the shade of a building [3]. While many urban areas experience shade throughout large parts of the day due to tall buildings, this alone is not enough to prevent the urban heat island effect from occurring.
A considerable amount of research has been conducted to examine the efficacy of trees and vegetation in buffering the urban heat island effect. A 2016 study from Leicester, UK found that trees and shrubs around the city (not including those planted in domestic green spaces like gardens) reduced mean maximum daily soil surface temperatures by 5.7°C in the summer compared to areas with solely herbaceous vegetation [4]. Research has also been done to show how different tree and vegetation characteristics are more effective than others in providing relief to the urban heat island effect. A 2021 study from Korea showed that urban areas containing trees with a high leaf area density (LAD) lowered the daily maximum temperature by 5.23°C more than urban areas containing trees with a low LAD [5]. The scientific community has come to the general consensus that with increased tree canopy cover comes decreased intensity of the urban heat island effect.
Therefore, it is logical that many of the areas most heavily impacted by the urban heat island effect are lacking in the tree department, and the residents of these treeless areas suffer the consequences. An increase in average summer temperatures and a lack of shade from trees can lead to significant increases in annual energy costs for those living within the affected neighborhoods. Generally, a 1°C increase in a city’s air temperature yields an electricity demand increase of 2-4% [6]. Unfortunately, many cities exhibit a noticeable correlation between low-income neighborhoods, areas with sparse tree canopy cover, and areas susceptible to urban heat islands. This is especially true in the 10th and Page neighborhood of Charlottesville. Insufficient city planning and greenscaping that leads to heightened energy costs in some neighborhoods, specifically those with lower household incomes, is a prime example of urban environmental inequity. The solution, however, is not as simple as planting trees and letting energy costs go down by themselves.
A fully equitable and inclusive solution must consider the full scope of socioeconomic and environmental impacts of inner-city tree planting. While the presence of trees in cities have been shown to positively impact the issues of air quality, stormwater runoff, crime rates, and public health, they also are known contributors to gentrification when introduced to pre-existing neighborhoods [7]. When trees are planted in established residential areas, rental rates increase due to the subsequent rise in curb appeal and property values [8]. A 2011 analytical study from Portland, Oregon found that the addition of a singular tree to a lot raised monthly rent by $5.62, and the addition of a tree to the public right of way caused rent to increase by $21 [8]. Although these figures differ by city, it becomes difficult for residents of low-income neighborhoods to pay rent as more and more trees are added, oftentimes forcing them to relocate as more affluent groups move in and change the demographic makeup of the neighborhood.
The environmental justice issues tied up in urban heat islands and the nature-based solutions presented to resolve them are an ongoing challenge for urban planners. While there is no one-size-fits-all fix for these problems, it is clear that the needs of underserved communities should be prioritized, as they are the recipients of the short end of the stick whether they end up living in heat islands or dealing with rising rent costs. Regardless, the preservation of human health and the limiting of heat-related deaths should be paramount in the search for an appropriate solution, and it is safe to assume that this is why many cities are starting to adopt tree-planting schemes for lowering summer temperatures. As temperatures continue to rise in the coming decades due to climate change, this challenge will call on the world’s most resourceful urban planners to integrate heat-absorbing greenspaces into cities without encroaching on the economically disadvantaged.
1. Rizwan, A. M., Leung, D. Y., & Liu, C. (2008). A review on the generation, determination and mitigation of Urban Heat Island. Journal of Environmental Sciences, 20(1), 120–128. https://doi.org/10.1016/s1001-0742(08)60019-4.
2. Amani-Beni, M., Zhang, B., Xie, G., & Xu, J. (2018). Impact of urban park’s tree, grass and waterbody on microclimate in hot summer days: A case study of Olympic Park in Beijing, China. Urban Forestry & Urban Greening, 32, 1–6. https://doi.org/10.1016/j.ufug.2018.03.016.
3. Rahman, M. A., Stratópoulos, L. M. F., Moser-Reischl, A., Zölch, T., Häberle, K., Rötzer, T., Pretzsch, H., & Pauleit, S. (2020). Traits of trees for cooling urban heat islands: A meta-analysis. Building and Environment, 170, 106606. https://doi.org/10.1016/j.buildenv.2019.106606.
4. Edmondson, J. L., Stott, I., Davies, Z. G., Gaston, K. J., & Leake, J. R. (2016). Soil surface temperatures reveal moderation of the urban heat island effect by trees and shrubs. Scientific Reports, 6(1). https://doi.org/10.1038/srep33708.
5. Esfehankalateh, A. T., Ngarambe, J., & Yun, G. Y. (2021). Influence of tree canopy coverage and leaf area density on urban heat island mitigation. Sustainability, 13(13), 7496. https://doi.org/10.3390/su13137496.
6. Akbari, H., Pomerantz, M., & Taha, H. (2001). Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy, 70(3), 295–310. https://doi.org/10.1016/s0038-092x(00)00089-x.
7. Donovan, G. H., Prestemon, J. P., Butry, D. T., Kaminski, A. R., & Monleón, V. J. (2021). The politics of urban trees: Tree planting is associated with gentrification in Portland, Oregon. Forest Policy and Economics, 124, 102387. https://doi.org/10.1016/j.forpol.2020.102387.
8. Donovan, G. H., & Butry, D. T. (2011). The effect of urban trees on the rental price of single-family homes in Portland, Oregon. Urban Forestry & Urban Greening, 10(3), 163–168. https://doi.org/10.1016/j.ufug.2011.05.007.