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Urban Heat Island: An Environmental and Public Health Crisis by ClimateYou blogger Paul Rivers

With the coming impacts of climate change it is important that government agencies begin to address the forms of city infrastructure that prolong hot weather conditions. Climate change will result in increasing air temperatures in city areas, exposing residents to harmful high temperatures. It is well documented that cities around the world experience what is known as an Urban Heat Island or UHI effect. This phenomenon takes advantage of the energy absorption in building materials. Impermeable surfaces like concrete, tall buildings, black roofs, and the lack of dominant vegetation and parks contribute to an enhanced absorption and release of heat derived from sunlight. The combination of debilitating UHI heat stress and underlying climate change warming will put real strains on urban resident health and well-being. Researchers on UHI are Stuart R. Gaffin, Cynthia Rosenzweig with the Center for Climate Systems Research at Columbia University, and Reza Khanbilvardi, who is affiliated with and City College’s Department of Civil Engineering & Earth and Environmental Sciences. According to their 2009 study, “heat island intensity in many urban areas is already comparable to the amount of warming expected regionally over the next century”. This highlights urban areas as the next battleground for confronting climate change.

urban heat island graph

Several academic and governmental institutions have published studies that form a reliable consensus on the dangers of UHI. A study by researchers at Arizona State University stated that “when coupled with these [global surface temperature] increases local warming from UHI intensifies the climate discomfort of urban residents and increases their vulnerability to heat stress”. The urban heat island effect impacts the city in a number of ways including air pollution management and heat stress. A study by K. Vikayaraghavan, a researcher at the Indian Institute of Technology Madras, cited that “urban air often contains elevated levels of pollutants that are harmful to human health and environment”. Air pollutants can include nitrous oxides, ozone and particulates. Clark, Adriaens & Talbot, three researchers at the University of Michigan, found that “Nitrogen oxides (NO­x) alone or in combination with other air pollutants such as ozone, sulfur oxides, and particulate materials (PM) can cause respiratory diseases and increase the risk of heart attacks”. How does this problem impact New York City?

Gaffin, Khanbilvardi & Rosenzweig found that New York City has a predominantly nocturnal heat island, during which temperatures between the city and rural surroundings can average 2.5°C. This is consistent with the way urban building materials absorb energy. A ‘nocturnal heat island’ responds to heat stored in building materials during the day, which is then released at night.  Hanson & Schmidt, authors affiliated with the Brooklyn Botanical Garden, argue that conventional building materials absorb and retain heat from sunlight during the day and then release that stored heat at night. The absorption of sunlight and heat during the daytime can increase the discomfort of pedestrians. It also prolongs high temperatures throughout the night, when conditions are normally cooler. The nocturnal heat island forces city residents to endure higher temperatures without a nighttime cooling period. This also necessitates an increased reliance on air conditioning and electric cooling systems. Urban heat island challenges the ability of cooling systems and electricity providers to perform in a sustainable and efficient manner. Hanson & Schmidt outline that black rooftops in New York City can reach intense temperatures, up to 170°F on hot summer days. Air conditioning or HVAC systems which are often located on roofs use massive amounts of energy to cool this superheated air. The increased operation of non-renewable electric utilities to provide this power further contributes to air pollution, climate change, and urban heat island. This feedback cycle is dangerous to the future of our cities. Clark, Adriaens & Talbot argue that the combination of urban heat island and emissions from the electric utility industry impacts not only local but regional air quality. The researchers also stress that city-derived warming and pollution will strengthen the urban heat island resulting in increased heat stressed mortality and illness.

The lack of vegetation, a contributor to UHI, is a symptom of human behavior and can be attributed to the conversion of vegetative suburbs to strictly urban cities. Land use decisions which promote the replacement of natural soils and vegetation, which would allow for cooling, with impervious surfaces like those in cities, lead to a reduced evapotranspiration. Evapotranspiration is a process that relies on the presence of moisture released by plants to cool the surrounding area. Another variable, albedo, also plays a big impact. Albedo represents the reflectivity of a surface, and is also a principal factor. Vegetative surfaces have more moisture than rooftops, and a higher albedo than many darker urban materials like tar roofs. Therefore, they reflect more solar radiation and help shade people and public spaces.

Many of the factors that strengthen UHI emanate from the form and material of urban sites themselves, which makes it difficult to address. Cities suffer from concerns of spatial fixity, which represents the immovability of a built-up landscape . We cannot reasonably remove already existing tall buildings, or dredge up impermeable concrete roads because they inhibit cooling processes or absorb heat. We can however, transition and amend these features to work more harmoniously with natural cooling cycles. It is here that greenroof technology can address the sources of urban heat island, provide direct solutions to air pollution, and effect greater cooling on heat stressed areas. Check for the next upcoming article in this series with more information on how greenroofs and other technologies can help mitigate UHI!

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