By Sydney O'Shaughnessy

A man dressed in gray slacks, a white-striped button down shirt and a blue tie blots his brow and takes a cursory look at the honking cars, concrete buildings, sidewalks and streets. He’s been walking for ten minutes, and already his back is soaked and his dark hair is slick with sweat. He ducks into the nearest Starbucks, orders an iced vanilla latte, and sits near an AC vent to cool off. He isn’t sure how much more of this heat he can take. He listens to the café radio and recognizes the local pop music station. The DJ gives the weather report.

“Hope your Friday is treating you well, folks ‘cuz it ’s going to be a hot one. We’ll see temperatures rise into the triple digits on Saturday. The April heat is settling in over the East Coast. And no relief in sight…”

The man throws his cup away, walks out of the café and is greeted once more by the relentless and heavy wall of heat. Above him, a clear blue sky stretches endlessly over the streets of Philadelphia, interrupted only by the skyscrapers of Center City. The white-hot sun reflects off windows and shimmering concrete. At street level, hundreds of cars speed from stoplight to stoplight. At the far end of the sidewalk, the uneven heating of air above the asphalt creates wavering mirages of people and uniforms. Pedestrians stop momentarily in front of open shop doors where the air conditioning spills out onto the sidewalk and briefly coalesces into children and panting dogs. Sweaty shirts cling to the backs and chests of street vendors.  It’s so warm there are no bugs.

“Cheers to the weekend,” the man says, waving down an Uber.  

Once on campus, the man hands the driver a handful of damp, sweaty bills and gets out, heading for shade. The towering sycamore, oak, and maple trees that line the Philadelphia streets shade the browning grass of the University of Pennsylvania’s campus. Dozens of people are stretched out beneath these drooping goliaths.  In the shade, the temperature difference is palpable. Where the heat was once unbearable, now it is just too hot. The man decides to join the others and rest beneath the trees. People are escaping to air-conditioned rooms and green spaces as often as they can to get some relief from the shoe-burning heat. City life in the summer is asphalt, concrete-slab hot and many degrees hotter than the suburbs and towns that surround metropolitan areas. But it’s not the sun alone that is making the cities hotter; it’s the literal makeup of the city itself that creates the scorching temperature difference. 

The increased amount of concrete, metal, emissions, and glass change the composition of the earth’s surface from green space to an absorptive and highly reflective urban space, which, in turn, holds the heat in and down. Because of the tall buildings, how they are arranged, and the amount of heat they absorb and hold, cities fail to cool down adequately at night. This “urban heat island effect” can be deadly.

Meet Richard Keller, a professor in the department of medical history and bioethics at the University of Wisconsin–Madison. He is a scholar specializing in the study of the social and health effects of urban heat islands (UHIs). According to Keller, UHIs are much more of a threat to human and environmental health than they are given credit for. He speaks passionately about this topic and races through story after story about his data – and the dangers of urban heat.

“About 1500 people per year die because of heat – compared to 200 deaths combined because of other weather events,” Keller says. “Extreme heat is increasingly widespread – which means it is hot in multiple places, at once, versus, say, a tornado which has extreme locality.”

All day in the city, the sun pounds down on the pavement and buildings, warming and illuminating them. Roadways are filled with idling vehicles and blocked by rush hour traffic jams. Due to the different thermal and radiative properties of the cars, pavement, and buildings, solar energy is absorbed and stored during the day and slowly released into the night air. This constant absorption and eventual release of the sun’s energy slowly changes the climate of the city, making hot microclimates that create real dangers for the vegetation, wildlife, and humans inside the city.

“There is a four to five degree Celsius, or about 10 degrees Fahrenheit, difference between cities and surrounding areas – all due to the urban heat island effect,” Keller says.

Typically, in an urban heat island, the more concentrated warmth is in the center, usually the most populated part of the city, with gradually lower temperatures near the edges of the city.  During summer heat, the center of the city, the older neighborhoods with their aging populations, the newer areas filled with modern brick and glass buildings and airless concrete apartment towers can turn deadly. With climate change and loss of urban green space, cities will turn even deadlier.

Brian Stone, a professor of urban planning at Georgia Institute of Technology, affirms that cities in the United States are warming very quickly, even when compared to the planet.

“What we find is that most large cities of the United States are warming at a rate that is double that of the planet as a whole,” Stone notes. “And double that of the surrounding countryside around these cities.”

That’s a lot of heat. Worse, as Stone adds, “Extensive urban infrastructure, such as street paving, rail lines, and airport runways can fail in response to extreme heat exposure.”  

Similarly, Keller says that extreme heat may not be getting the attention it deserves because of the populations who, so far, have been most severely affected.

“The kind of people heat kills tend to be on the margins of society,” Keller adds. “They are very old or homeless or mentally ill people that live at the social margins. Medicine dehydrates you so these people have a higher risk of death.”

According to Keller, this “harvesting effect” occurs because the human body is unable to cool down in high temperatures, but these mortalities should not be overlooked.

“It’s the wrong argument to say, ‘Is it a big deal that they died earlier? They were already on the way out anyway.’ Who are we to say that a couple more weeks of life didn’t matter?” notes Keller.

There are not many heat sinks, or places that get rid of heat, present within the city. But parks and trees help. In New York City’s Central Park, there is a dramatic difference in temperature because trees and other vegetation work to lower the temperature through evapotranspiration and shade. Even individuals can help lower the heat in their homes by keeping houseplants and creating green areas.

“During the Paris heat wave, a woman lived next door to someone who died,” Keller says. “The surviving woman had house plants but her neighbor who died did not. There was a measurable difference in temperature between the two apartments. It was only a few degrees but that was the difference between life and death.”

Since the catastrophic Paris heat waves, the city has taken many steps to combat extreme heat, including using vegetation to manage building temperatures and passing legislation that requires all new buildings to have ‘green’ roofs.

“We live in an urbanizing world,” Keller says. “More than half of the population is living in cities, mostly in the developing world. We need to keep extreme heat on our horizon.  But things are changing – there is more awareness about urban heat islands and these devastating heat waves. We should be thinking how to design cities that can cope with extreme heat by creating sustainable urban environments.”


At 2:30 p.m. in Philadelphia, the heat is reaching its crescendo. Beneath one of the tall and swaying speckled sycamore trees sits the man, trying to enjoy his afternoon off from work. He undoes his blue tie and snakes it from around his neck. Carefully, he folds it and places it in his bag. He sits for a few minutes and rolls up his sleeves. Even though the trees block most of the sun, the heat is still oppressive – a weight sitting on his chest. But being in the grass feels better to him than walking on the sidewalk so he stays put.


Brian Stone says that any individual can help combat the effects of urban heat islands by volunteering to plant trees or by using public transportation.

“We advise cities to adopt a zoning tool known as a ‘green area ratio’,” Stone notes. “The adoption of such a policy ensures that green cover will increase, or decrease more slowly, over time as city develops and redevelops.” 


The campus is beginning to empty out. Finally, man stands up, brushes off his slacks, stretches and decides to start the final leg of his walk home. Luckily, there is a fruit stand selling fresh peaches on the way.  He buys one for $0.35, tosses it into the air, catches it, and bites into its cooling, juicy flesh. It’s only a few more blocks home to his air-conditioned apartment. He is excited to get out of his sweaty clothes and relax, seemingly far away from the urban heat.

“Better take a cold shower tonight,” he thinks. The rest of the walk home, he tries to sink into the heat, calm his heartbeat, remember the feel of snow on his face. “It could be a long weekend.”

Sydney is a senior environmental science and journalism double major at Ithaca College. She grew up in a rural town in northern New York where she learned to value the environment. She hopes to use her passion about science to inspire and educate others about the intricacies of the natural world. In her free time, she likes to snowboard, cross-stitch and read.

Heat Islands in Ithaca

According to the EPA, “trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings or to shade pavement in parking lots and on streets.” Tree canopies can decrease temperatures by 20 degrees compared to an area of pavement. Everything from community gardens to planting and protecting trees can have an impact on urban microclimates.

In Ithaca, NY, the city foresters understand the importance of planting trees and increasing green spaces in city environments. Jeanne Grace, a seven-year city forester for the City of Ithaca, understands the value of her work. Ithaca has been named Tree City USA for 25 years.  However, Grace says that managing all of the trees is challenging. She stresses the importance of urban forestry precisely because of the positive impact it can have on mitigating the urban heat island effect.

“I think it’s the most tangible reason for protecting trees -- heat affects people day to day,” Grace notes. “It’s the difference between walking down a highway that’s hot, dry and dusty and walking down a residential sidewalk with mature trees and shade.”

Without green spaces, cities would be facing other environmental problems, like increased pollution, as well as extreme heat.