Impact Climate

A Deadly Heatwave Turned Chicago Into the Country’s Green Roof Capital

The Windy City used over five million acres of roof space to go green.

by Meg Charlton
May 23 2017, 6:45pm

Photo via Wikimedia Commons

The day after a heatwave began in Chicago on July 12, 1995, temperatures hit 106 degrees fahrenheit. Two days after it began, with the humidity, it felt like 125. A week later, the heat had broken, and 739 people had died.

Kim Wasserman is a lifelong resident of the West Side neighborhood Little Village and a lauded local organizer with the Little Village Environmental Justice Organization. She had just graduated high school in the summer of '95 and remembers the heatwave vividly.

"It was so damn hot!" she says. "But more importantly we all had to go to the park to get ice and water and food."

She tells me that the whole neighborhood was out on the streets, because it was too sweltering to stay indoors, and that her mom kept their milk in coolers, because city-wide brownouts left them with no refrigeration.

Little Village was lucky. They had a death rate of 4 people per 100,000. The neighboring area of North Lawndale had a death rate 10 times higher. The rest of the city -- the Southside in particular -- was devastated. Over 50% of the victims were black and the vast majority were over 60; the path of destruction mirrored that of the most vulnerable.

The city -- excoriated for their response -- tried to course-correct. City Hall opened cooling centers, set up call centers to check on the elderly, distributed electric fans, and more. But there was another, more radical solution that Mayor Daley's administration began investigating: What if besides just cooling off the people, you could actually cool down the city?


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Heatwaves tend to receive far less public panic than say, Ebola or other natural disasters. Yet in the United States, excessive heat exposure kills more people annually than hurricanes, tornadoes, and floods combined. As the climate changes, extreme heat events are only predicted to get worse. And some of the areas most at risk from this change will be cities.

The Chicago heat wave was exacerbated by the Urban Heat Island effect or UHI, a phenomenon in which urban areas are hotter than the surrounding countryside. This occurs for several reasons: The "urban geometry" of large canyons of buildings trap heat and don't allow it to dissipate; the dark surfaces of roads and roofs that absorb heat rather than reflect it; the blasts of exhaust and heat from human activities (often ironically from air conditioners); and those same surfaces are usually impermeable. All of this conspires to keep urban areas much warmer overall, but the difference is most pronounced at night; once the sun goes down, the gap between an urban area's temperature and the surrounding suburban or rural areas can reach up to 22 degrees Fahrenheit.

But the most pressing issue is a lack of greenery. Plants cool the air around them through a process known as evapotranspiration. They absorb water from the rain and air and then let off an imperceptible mist as it evaporates, lowering the surrounding air temperature. With green roofs, urban areas can reduce the temperature (and attendant energy costs) of the buildings that house them, as well as cool the air overall.

Excessive heat exposure kills more people annually than hurricanes, tornadoes, and floods combined.

Michael Berkshire is the director of Green Projects for the city of Chicago. He says that the green roofs program was borne out of the EPA study ordered in the wake of the heatwave for lowering urban temperatures. But he also says that Mayor Daley traveled in Europe, particularly Germany, where green roofs are widely used.

"He came back and said, 'let's see if this can work here,'" Berkshire says, "In our political climate as well as our natural climate."

There are now 359 green roofs in Chicago, covering over 5.5 million acres, promoted through a carrot and stick approach of incentives and requirements. Beginning in 2004, all new developments in certain areas, those receiving certain city incentives or those exceeding a certain size, were required to put in green roofing.

The crown jewel and unofficial pilot roof is on city hall itself, which transformed the over 20,000 square foot area into a haven of local prairie plants and grasses. (It is, Berkshire says with some sheepish pride, "a pretty fancy green roof.") Over the course of the following 16 years, it has not only saved the city $5,000 annually on utility bills and provided a model for other developers, it's also proven a source of invaluable data.

The enormous, block-sized City Hall is divided into two halves. One of which is for city offices and the other for county, only the former is covered in vegetation. The county side was, up until a few years ago, standard black asphalt.

'We were monitoring the temperature differences on each side," Berkshire says. "On a 90 degree day, the roof on the vegetated side always hovered around 90 degrees. Whereas the county side was as high as 170 degrees fahrenheit."

Usually, however, green roofs do not provide such a neat demonstration of their efficacy like City Hall's 80-degree gulf.

Haley Gilbert researches heat islands for the Berkeley National Laboratory in California and leads the Heat Island Group's Cool Communities efforts. Even with all the tools at their disposal, Gilbert says it's "very difficult to measure" the changes from a green roofing program to a city's overall temperature. Installing sensors is expensive, so labs like hers do a lot of modeling work. "Meteorological models like WRF [The Weather Research & Forecasting Model] can be used to investigate effects of hypothetical city-wide programs, for example green roof adoption, on urban temperatures."

For the elderly, small kids and the isolated, even a few degrees, can make a huge difference.

In one recent study by the University of Notre Dame, researchers used WRF modeling to examine the effects of the city's green roof program. Ashish Sharma, the lead author, described the study as a kind of test-bed to evaluate the efficacy of green roofs.

"Assuming the city has 100 percent vegetated green roofs, or it has 25 percent or 50 percent, what is the impact going to be? What we found was the roof surface temperatures in downtown Chicago would decrease by seven degrees celsius," Sharma said. "That's a huge decrease."

For the elderly, small kids and the isolated, even a few degrees, he says, can make a huge difference. "Green roofs are aesthetically beautiful and help improve biodiversity and the whole ecosystem. People start walking and enjoying nature. But they are very costly to put in in the first place. So there are a lot of socio-economic and environmental justice factors that go into choosing heat mitigation strategies."

Back in Little Village, Wasserman doesn't see green roofing as the best solution to her community's concerns.

She says one of the big realizations the heatwave triggered in her community was, "How do we have no power when we have two coal powered plants in our neighborhood?" (She successfully campaigned to shut those plants down in 2012, to wide acclaim.) It jump-started conversations around climate change, fossil fuels and what Wasserman calls "culturally relevant" cooling-down practices in her majority Mexican-American neighborhood. She wants to focus on environmental projects that incorporate the health and needs of her neighbors, ones that make Little Village sustainable not just environmentally but economically.

"How do you balance the ability to tap into these programs [from the city] that require some capital and do require some financing," she says, "when folks are just barely trying to make their monthly payments on their spaces?"

Green roofs can indeed be expensive to maintain, better suited to developers than individuals. City Hall has competitive grants where residents can apply to get money for green roofs on their buildings, but the program has not been as fruitful as they had hoped; according to city data, the ratio of required to voluntary green roofs in the city exceeds five to one. This is not to say that residents don't want a lush garden or small meadow above their home or business, but rather that they can be beyond the means of, say, a renter, living paycheck to paycheck.

"You know, green roofs are great. But that's not where our neighborhood is at."

Learn more about how your city can use green roofs and other strategies to cool off from the C40 Cool Cities Network.

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