Ranking the Efficacy of Urban Heat Island Mitigation Strategies
Description & Overview
For this project, I want to do a study of the effectiveness of the various strategies for urban heat island mitigation. Urban areas are generally a few degrees hotter than their surrounding environments for a number of reasons:
– the darker materials typically used in urban construction absorb more solar radiation and warm the surrounding air.
– the canyon geometry of large urban environments help to trap more of the sun’s radiation near the surface level.
– urban environments generally have a higher concentration of pollutants (cars, factories etc.) which trap heat in that environment, and also lower air quality.
– there is a lower concentration of plants in urban environments than in the surrounding areas – plants use CO2, a common urban pollutant, for photosynthesis.
A difference of a few degrees may not seem significant, however there can be significant consequences. Heat waves may be more severe, and last longer as a result of the urban heat island effect. An example of this was the 1995 heat wave in Chicago, during over 700 people died from heat-related causes in just over 5 days. Additionally, higher temperatures cause people to use more water and energy in order to stay comfortable. In addition to humans, warmer urban environments can have adverse effects on other life forms. The higher temperature can disrupt animals both on land and in water near cities.
The UHI effect is not a new revelation – awareness of elevated temperatures in cities has been around for decades – but it is more recently that cities have seen the consequences of the effect and have, as a result, tried to mitigate it. There are a few common ways to do this:
– to counteract, in part, the use of darker materials for construction, highly reflective roofs (painting roofs a lighter color) help to cool not only the inside of a building, but the surrounding area as well.
– Increasing urban vegeatation, whether with parks or just more plants in general, helps to not only replace highly-absorbant materials, but also decreases the amount of CO2 in the area. An example of this is replacing the asphalt that usually covers parking lots with plants.
– As a kind of combination of these two strategies, green roofs are also used. Notably, Chicago City Hall has had a green roof since 2000.
What I’d like to do with this project is to use past studies of these strategies to create some kind of ranking of their effectiveness, while keeping in mind the cost and upkeep of each one. Additionally, most of the studies are case studies of particular cities – each city is unique, and so what works well in one place might not be as effective in another. I’d like to be able to explain why this happens in these cases.
Introduction to urban heat islands
– what it is about cities that causes elevated temperatures: darker materials, urban structure, pollutants, less plants, urban sprawl vs. high-density areas
– harmful and costly effects of heat islands: increased severity of heat waves, increased energy and water use, effects on other life forms
– mitigation strategies: cool roofs, cool pavements, parks, trees, green roofs, reduction of car usage
Studies of individual strategies: effectiveness, cost of implementation and maintenance, possible secondary consequences, and cities where these strategies have been implemented
– cool surfaces:
– roofs: Washington, DC; Guangzhou, China
– increased vegetation
– parks, trees etc.
– green roofs; Washington, DC; Barcelona; Palermo; Cairo
– green parking lots: Nagoya, Japan
– Rankings based on the above information
Other ways that UHI effect can be reduced
– electric cars
– reduction in car usage
– cleaner energy sources
Debbage, N., & Shepherd, J. M. (2015). The urban heat island effect and city contiguity. Computers Environment and Urban Systems, 54, 181-194. doi:10.1016/j.compenvurbsys.2015.08.002
This study looks at whether sprawling cities or the more dense variety create more intense UHIs. When thinking about mitigation policies, this is an important variable to consider.
Takebayashi, H., & Moriyama, M. (2012). Study on surface heat budget of various pavements for urban heat island mitigation.Advances in Materials Science and Engineering, 2012 doi:10.1155/2012/523051.
This looks at how various pavement surfaces either increase or mitigation heat island intensity. Because sidewalks and roads are so ubiquitous in urban environments, this is another important factor to consider.
Onishi, A., Cao, X., Ito, T., Shi, F., & Imura, H. (2010). Evaluating the potential for urban heat-island mitigation by greening parking lots. Urban Forestry & Urban Greening, 9(4), 323-332. doi:10.1016/j.ufug.2010.06.002
Parking lots are other surfaces which are also typically paved and common in urban areas. This article looks at how using green parking lots can help to mitigate the UHI effect.
Zinzi, M., & Agnoli, S. (2012). Cool and green roofs. an energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the mediterranean region. Energy and Buildings, 55, 66-76. doi:10.1016/j.enbuild.2011.09.024
Cool roofs and green roofs are similar mitigation strategies, and this study compares the two in three mediterranean cities.
Fallmann, J., Forkel, R., & Emeis, S. (2016). Secondary effects of urban heat island mitigation measures on air quality. Atmospheric Environment, 125, 199-211. doi:10.1016/j.atmosenv.2015.10.094
This an interesting study that looks at how some mitigation strategies, such as highly-reflective surfaces, can have adverse on greenhouse gas levels. This is something that is not often considered in UHI mitigation strategy.
Lee, J. S., Kim, J. T., & Lee, M. G. (2014). Mitigation of urban heat island effect and greenroofs. Indoor and Built Environment,23(1), 62-69. doi:10.1177/1420326X12474483
Investigation specifically of the effectiveness of green roofs as a mitigation strategy.
Li, D., Bou-Zeid, E., & Oppenheimer, M. (2014). The effectiveness of cool and green roofs as urban heat island mitigation strategies. Environmental Research Letters, 9(5) doi:10.1088/1748-9326/9/5/055002
Another study comparing cool and green roofs.
Magliocco, A., & Perini, K. (2014). Urban environment and vegetation: Comfort and urban heat island mitigation. Techne : Journal of Technology for Architecture and Environment, (8), 155-162. doi:10.13128/Techne-15070
How increased urban vegetation can help to cool cities.
Guhathakurta, S., & Gober, P. (2007). The impact of the phoenix urban heat island on residential water use. Journal of the American Planning Association, 73(3), 317-329. doi:10.1080/01944360708977980
How UHI intensity is related to the amount water that people use in Phoenix, Arizona. An example of one of the consequences of cities being hotter than their surroundings.