TEMPE, Ariz. — Asphalt, concrete, and steel absorb and re-emit much more heat than vegetation. It’s why urban cities often reach temperatures much higher than their rural surroundings.
A crowded downtown area with lots of high rises, parking lots, and pavement can be much hotter than a lush, green park. It’s a phenomenon known at an urban heat island.
“The typical definition is of local air temperatures that are warmer because of the built environment,” said Paul Coseo, associate professor of landscape architecture at Arizona State University.
“It's a worldwide phenomenon. It happens differently in these different regions because of their local climate. And their local environment,” he added.
Since 2015, Coseo has been studying the benefits of green rooftops and the impact they have on potentially reducing the urban heat island effect and capturing storm runoff.
“They're reducing the amount of water that goes into sewers and things like that. So, they reduce flooding,” said Coseo. “That water will then evaporate and cool the air. It cools that surface and then the plants, also through evapotranspiration.”
Last year, the city of Phoenix established the first heat response and mitigation department in the country.
They recently planted the city’s first "cool corridor." The hope is that the 259 trees will eventually provide shaded relief from hot weather.
“It's basically a street with a sidewalk and how can we actually make places that people have to walk to take the bus or to walk to school? How can we make those cool?” said Coseo.
But heat islands exist in cool-weather cities like Baltimore, Cleveland, and Chicago as well and deadly spikes in temperatures are happening.
Extreme heat causes an estimated 1,300 deaths in the U.S. each year. Most notably, in 1995, a deadly heatwave led to the deaths of more than 700 residents in Chicago – with additional fatalities in Milwaukee and St. Louis.
“Heat's a rising issue. Heat kills more people in this country than all other natural weather events combined,” said Coseo.
It’s why cities have been experimenting with ways to bring surface temperatures down. One strategy has been to use more green rooftops.
Looking at satellite data, NASA climate scientists recently studied three rooftop gardens in Chicago, including the 24-acre Millennium Park Project, which is one of the largest in the world.
“I think what's really stood out to me is that potentially the impacts of soil volume on surface temperature,” said Coseo.
Across the city, they found that of the three, which included a Wal-Mart and another one atop City Hall, only the Millennium Park location was able to significantly lower average temperatures over the course of the study.
Larger roofs with more diverse plant species, they found, had greater cooling benefits.
Scientists say using strategies that harness the cooling power of plants could provide a roadmap for policymakers and urban designers as temperatures continue to rise.
“Particularly here in the desert, we see ourselves on the forefront of all of this because we are super-hot from March until October,” said Coseo. “And that heat season is actually increasing every year.”