The Urban Heat Island Effect

Figure 1. The downtown area of the city of Sao Paulo (Hirama, 2011).

The Urban Heat Island (UHI) effect is a well-documented phenomenon in which urban areas experience elevated temperatures compared to their surrounding areas. Examining the Figure 2 above demonstrates that as you move outward from cities, vegetation increases and temperatures drop. For a more visual understanding of how the UHI effect works, see Figure 3 below. Essentially all urban areas around the world experience the UHI effect to varying degrees and Sao Paulo’s UHI effect is very pronounced (Peng et al. 2012). With more than 70% of the global population expected to live in cities by 2050, mitigating the UHI effect to avoid negative environmental, social, and economic effects is necessary (UNDESA 2018). Also essential is understanding the relationship between climate change and its amplification of the UHI effect. This case study of Sao Paulo attempts to analyze the UHI effect in a major global city and provide strategies for mitigation.

Figure 3. An explanation of the UHI effect by NPR (Harlan and Joyce, 2018).

While the UHI effect is not caused by or directly contributing to climate change, rising global temperatures will amplify the intensity and duration of the UHI effect. The IPCC emphasizes that “increased frequency of hot days and warm spells will exacerbate urban heat island effects” with “the current trend of increasingly frequent extreme events..expected to increase with climate change” (IPCC Chapter 8 2018, p. 554, 555). Cities are already hotter than their surroundings and as the climate warms, the UHI effect and its negative impacts will be more pronounced and damaging.

Many UHI mitigation strategies, like adding trees and green spaces around cities, can also help lessen climate change since vegetation removes carbon from the atmosphere. Thus, while UHI and climate change are not causally linked, they are related phenomena. 

Brazil has experienced rapid and dramatic urbanization. From 1960 to 2000, Brazil’s urban population grew from 46% to over 80%, skyrocketing from 3.97 million to 17 million in just four decades (Richtie and Roser 2018). Currently, about 86% of Brazil’s population reside in urban areas, with the largest city being Sao Paulo (UNDESA 2018). The Metropolitan Area of Sao Paulo is home to more than 21.57 million individuals, making it one of the most populous cities in the world (“Região Metropolitana de São Paulo” 2018).

Sao Paulo has the fourth largest daytime UHI effect compared to other global cities. Sao Paulo is more than 5°C hotter than its surrounding areas, a dramatic difference posing major threats to human well-being (Peng et al. 2012). Given its massive urban population and elevated temperatures, addressing UHI in Sao Paulo is critical. 

UHI impacts and risks are not distributed evenly amongst the population. Populations particularly vulnerable to extreme heat include low-income, young children, elderly adults, and individuals with chronic illnesses. Low-income communities tend to lack access to parks, air conditioning, and healthcare services and may be more impacted by severe storm damage (US EPA 2014). In cities, parks tend to be much cooler than built structures and in Sao Paulo, only wealthy neighborhoods have high amounts of vegetation. Lower income neighborhoods in Sao Paulo suffer from a stark lack of parks and green spaces, greatly increasing the intensity of the UHI effect locally (Lima and Magana Rueda 2018). 

UHI and climate change may also increase human mortality by increasing heat wave frequency, duration, and intensity. Across a 14-year period, Son et al. (2016) discovered that extreme heat led to increased mortality in Sao Paulo. Heat exposure can result in vascular changes and systemic inflammation leading to respiratory distress and death. (Son et al. 2016). Heat exposure can send the body into heat exhaustion and heat stroke, triggering heart attacks, strokes, and cardiovascular failure. In the United States, heat is the primary cause of weather-related deaths. For vulnerable populations, including those that work outside, those with cardiovascular or respiratory illnesses, young children, those older than 65, and low-income individuals, excessive heat can be life threatening (US EPA 2016).

UHI increases disease prevalence. Araujo et al. (2015) found that UHI increases dengue incidence, a mosquito-borne illness with health effects ranging from mild to life threatening illness. Referring to Figure 10 on the left, it is clear that cases of dengue (represented by the black dots) correspond to higher temperatures (represented by the red, yellow, and orange background colors). In São Paulo, the vast majority of dengue cases were found in areas with surface temperatures greater than 28°C and in areas with low vegetation, indicating that the urban environment is more conducive to disease spread. Further, high land surface temperatures (>32°C) showed a stronger correlation to dengue than any other factor, like socioeconomic status and population density (Araujo et al. 2015). As climate change will only serve to further elevate city temperatures, there is even greater disease spread risk as more areas of the city become hotter. Considering the serious health impacts of dengue as well as documented heat mortality, UHI and climate change pose a significant threat to public health.

City vehicles, power plants, refineries, and other industries spew carbon dioxide, nitrogen oxides, volatile organic compounds, and other pollutants into the atmosphere. The formation of ozone occurs through a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight. Elevated city temperatures, combined with a large number of NOx and VOC emissions sources, leads to more ozone pollution. Ozone pollution harms human health, leading to chronic respiratory and cardiovascular diseases (Weaver et al. 2009). Further, with global warming and climate change, this relationship between UHI and ozone formation will be amplified, posing risks for human wellbeing.

High temperatures diminish economic productivity, lower economic output, reduce average incomes, and widen income inequality (Burke et al. 2015). Urban areas account for most economic productivity and already experience hotter temperatures from UHI so as climate change continues, urban economies will be hit particularly hard. When calculating the economic costs of climate change for global cities, researchers found that accounting for the local UHI effect increases economic losses by 260% (Estrada et al. 2017). This indicates how cities will face major economic losses as a result of elevated temperatures.

Sao Paulo is Brazil’s most critical financial and political hub, accounting for more than 17% of Brazil’s GDP (“Região Metropolitana de São Paulo” 2018). Any loss of economic productivity in Sao Paulo could have devastating repercussions. Thus, mitigating the UHI effect and implementing climate change strategies is necessary to secure economic stability in the region.

There are many strategies cities can use to alleviate the UHI effect. Successful plans involve increasing vegetation and changing surface albedo (reflectivity) so that less heat is absorbed and trapped in cities. Vegetation is critical as it works to increase evapotranspiration, provide shade, and increase albedo if it replaces a dark surface, all leading to a cooling effect (Feyisa et al 2014). Combining various strategies to create effective policy plans is essential.

"green vegetation can improve both indoor and outdoor thermal comfort, while at the same time providing multiple environmental services such as carbon storage, reduced air pollution, and act as urban biodiversity hotspots" (Feyisa et al. 2014, p. 88).

Depending on the study area, park air temperatures may be between 1-7°C cooler (Feyisa et al. 2014). Despite variation, urban green spaces effectively lower air temperatures while providing other benefits. Feyisa et al. (2014) explains that “green vegetation can improve both indoor and outdoor thermal comfort, while at the same time providing multiple environmental services such as carbon storage, reduced air pollution, and act as urban biodiversity hotspots.” Looking at Sao Paulo, a study focusing on the downtown area of Luz found that a local park had ambient air temperatures between 2°C and 6°C lower than the open city square area. Simulations found that streets with high-density tree canopies lowered air temperatures only by about 1°C, but lowered street surface temperatures by up to 12°C. Dense shading trees dramatically improved thermal comfort and eliminated the hot afternoon peak in discomfort (Spangenberg et al. 2008). Beyond simply lowering temperatures, increasing the amount of green spaces can improve physical and mental well-being and increase available habitat and species diversity in cities.

Strategically arranging and selecting tree species can maximize temperature reductions. Planting trees and vegetation in wind corridors can lower air temperature almost twice as much as planting trees in leeward areas (Tan et al. 2016). This placement enhances cool air circulation while lowering local temperature as well. Tree species also influences temperature reduction as some species have more dense canopy cover and higher evaporation rates. For example, of selected tree species in Addis Ababa, Eucalyptus, Olea, and Acacia provided the greatest cooling effect (Feyisa et al. 2014). A study of tropical locations emphasizes that the “dimension, shape, and color of leaves” influences how much temperature decreases and found that out of twelve tree species, Caesalpinia pluviosa performed best. In terms of how hot it feels to humans, clusters of C. pluviosa lowered temperatures as much as 16°C (de Abreu-Harbich et al. 2015). These combined results indicate the importance of tree species selection and vegetation design in creating UHI mitigation plans.

Another strategy involves green and cool roof technologies. Rooftops represent roughly 25% of the city surface area and tend to be dark grey, but converting them to green and cool roofs could lower city temperatures significantly. Green roofs are unique in that they provide two cooling mechanisms, “active by evaporative cooling and passive by insulation” (Aflaki et al. 2016). While green roofs lower outdoor temperatures, they also lower indoor air temperature and reduce energy demand for air conditioning. Green roofs can also be used for urban agriculture and increase biodiversity and food production in cities. Similarly, changing the color of a rooftop from a dark to a “cool” color (white, pale colors) reduces heat storage, minimizing the UHI effect. A simulation of the Baltimore-Washington area found that if just 50% of roofs were converted to cool roofs, surface UHI could be lowered by nearly 2°C (Li et al. 2014). Along similar lines, changing the albedo of other surfaces like roads, sidewalks, and parking lots and adding vegetation can help offset the UHI effect. 

Climate change is amplifying the UHI effect, increasing the intensity, frequency, and duration of periods of hot temperatures. UHI has detrimental impacts on human health and well-being, economic stability, and environmental conditions. From severe weather to disease incidence to air pollution, the impacts of UHI in Sao Paulo are well documented and have serious implications. As a global megacity with more than 21 million residents, Sao Paulo would be wise to implement effective UHI mitigation strategies including increasing tree cover, vegetation, and green spaces and converting conventional rooftops to green and cool roofs. Along with mitigation, adaptation strategies are also needed to manage UHI and protect public health. Adaptation strategies may include creating accessible cooling centers, ensuring access to healthcare services, creating a neighborhood check-in program to identify and assist vulnerable residents, educating the public about UHI, and increasing awareness of how to avoid heat-related illnesses (“Mayor Announces Program” 2017). To ensure that Sao Paulo remains a thriving and livable city, a comprehensive UHI mitigation and adaptation plan should be created and implemented. 

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