Photo: Courtesy City of Chicago
September 2007 | From the Commish
Addressing Urban Heat Islands
By Sadhu A. Johnston
In last month’s column, I described the work the City of Chicago is doing to develop a climate change action plan. An important component of that work is understanding how the city can better adapt to changing climactic conditions. Addressing urban heat islands is a major component of our adaptation strategy.
Urban heat islands are areas that are warmer than their surroundings. As natural and green spaces are replaced with asphalt, buildings and other infrastructure, the temperature rises because these surfaces and buildings absorb heat from the sun during the daytime, hold on to that heat and release it at night. The difference in temperature can vary by as much as ten degrees between urban and rural areas.
The City is undertaking a number of initiatives to understand how to balance the need for new development with the urban heat islands that exist in the city and how to lessen their impact. One major initiative that helps reduce the effects of urban heat islands is various green roof programs the City operates. Chicago has more than 3.5 million square feet of green roofs either constructed or underway — more than any other city in North America — that help to cool the city. The Mayor’s commitment to leading by example means that green roofs have been installed on City Hall, fire stations, schools and libraries, to name of a few of the structures that implement this green technology. We know that on a hot summer day that the ambient air temperature above a green roof compared to a black tar roof can be anywhere from 60 to 70 degrees lower.
There are numerous other ways to cool the city. Adding a reflective coating to a black roof can bring the temperature down significantly as well. We have a reflective roof ordinance which also can reduce energy costs for building owners. The City, often with nonprofit and private partners, has planted more than 500,000 street trees — those trees not only help cool the city but help remove millions of pounds of carbon dioxide from the atmosphere. The Chicago Department of Transportation’s Green Alleys program uses high albedo pavement, a lighter-colored reflective surface that absorbs less heat and helps reduce the urban heat island effect.
The Department of Environment is currently working with Arizona State University to model the health and emergency response effects of urban heat islands as well as determine specific building contributions to urban heat islands. We have taken infrared video via helicopter over various parts of the city that will enable us to pinpoint those areas that are indeed hotter and what the correlation is to heat emergency calls and responses. We may be able to adjust the strategies available to us in order to help make the city cooler and have a positive effect on the health impacts that occur as a result of a hotter environment.
If it’s summer in Chicago, we know it will be hot, but the City remains committed to doing all we can to adapt our environment to reduce the urban heat island effect and provide resources to residents to help them deal with the heat, such as well-being checks on seniors and cooling centers where residents can take advantage of an air conditioned environment. New tools, such as the modeling work Arizona State University is doing, will help us in our efforts to make Chicago a cooler place to live, work and play.
Sadhu A. Johnston is commissioner of Chicago’s Department of Environment.
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