Anyone lucky enough to have lived in both a city and in a rural area has experienced the weird phenomenon that cities trap heat in a way the farms and fields just can’t. For me, growing up next to a dairy farm meant more snow days than my city-dwelling friends, but come summer, those warmer city streets became unbearably hot.

Sunlight hitting fields and forests gets used by plants, reflected into the sky, and otherwise doesn’t really build up to be released later. Asphalt, however, will absorb that sunlight, get unbearably hot, and radiate the heat into the air long into the night. The same is true of black tar roofs, cement, and a lot of the other defining factors in cities. The way buildings, roads, and parking lots retain heat is called the urban heat island effect, and it is responsible not just for overheated summer days but also for the corresponding energy demands of cooling systems and the negative health effects experienced by folks who may not be able to afford the energy to cool their homes.

This is a human-made problem, and it demands a human-made solution. Recent efforts have explored the effects of green roofs and Sun-reflecting surfaces, and by Sun-reflecting surfaces, I mean they are painting things white.

In a newly published study in the journal Sustainable Cities and Societies, led by Kathryn McConnell, researchers use LandSat 5 Earth-science satellites to study how effective three large green roof projects were at reducing the urban heat island effect [in Chicago, Illinois]. Those three sites were Millennium Park, City Hall, and a Walmart shopping center. Data were collected from 1990 to 2011, providing significant pre- and post-construction data.

The massive Millenium Park, with its rich foliage, was able to significantly lower temps and fully mitigate climate warming in its region. The smaller but also well-planted City Hall had some effect, but its region also began to warm over time as the city grew. And that Walmart? Well, it was built where a field used to be, and despite its green roof, the Walmart region was warmer than the prior natural landscape.

This is just the latest data to show there is no replacement for Mother Nature. Green roofs can help, and the more the better, but they aren’t the same as the natural landscape. As stated by coauthor Christian Braneon: In a lot of places, you might be developing an area that didn’t have something there before; it just had overgrown vegetation. You might think that putting a green roof on your new building would make a significant impact. But what we see is that a lot of impervious material may also be added there – such as a parking lot around the building. As a result, you might reduce the impact of the parking lot, but you certainly haven’t created the cooling effect that the overgrown vegetation had.

More Information

NASA press release

“A quasi-experimental approach for evaluating the heat mitigation effects of green roofs in Chicago, Illinois,” Kathryn McConnell et al., 2021 September 25, Sustainable Cities and Society