The forecast called for sunny skies and highs in the low 80s—mild weather by New York City summer standards. The shaded paths of Sakura Park in Manhattan’s Morningside Heights neighborhood were breezy and cool. But on the same block, the midday sun beat down on an exposed stretch of Riverside Drive, and heat radiated off the asphalt.
That exact phenomenon is what brought NASA remote sensing specialist Dr. Christian Braneon to the park on this midsummer day in late July, where he met with about two dozen New Yorkers who had volunteered to canvass neighborhoods in Manhattan and the Bronx by car, clocking temperatures as they went.
“We're really feeling the urban heat island right now,” Braneon said as he dragged the signup table into the shade.
This day of temperature mapping was part of a nationwide effort by the National Oceanic and Atmospheric Administration to better understand urban heat islands: areas where dense buildings, lots of concrete and little tree cover make life feel extra hot in the summer. Advocacy group South Bronx Unite as well as researchers from Columbia University’s Earth Institute coordinated the Bronx and Manhattan branches of the project.
“A temperature that we actually know is like 70-something or 80-something, or maybe even 90 degrees, we’ll feel it several degrees higher,” said Dr. Melissa Barber, South Bronx Unite founder and this mapping project’s organizer.
Urban heat islands will become even more dangerous as temperatures continue to climb worldwide, and new inhabitants flock to cities. A global study published this month involving 13,000 urban areas found that city dwellers’ exposure to life-threatening heat and humidity has increased threefold since the 1980s. This change was due in part to urban population growth putting more people in the crosshairs of rising temperatures. And the New York City Panel on Climate Change predicts that by 2050, the city will experience about twice as many days in excess of 90 degrees Fahrenheit as in the 2020s.
This year’s scorching summer sent close to 650 people to New York City emergency rooms, more than the average toll for the four summers prior. And extreme heat kills more New Yorkers than any other type of severe weather, according to a 2017 report compiled by Mayor Bill de Blasio’s office. But it affects some neighborhoods more than others.
Studies show that poorer neighborhoods and areas with more residents of color experience hotter temperatures in the summer. Barber says that’s because these places tend to be densely built--with lots of asphalt and few shade trees.
“We've been neglected in terms of many of the structures and infrastructures that need to be in place to kind of buffer heat,” she said.
Those differences can be dangerous for people vulnerable to extreme heat such as children and older adults and people who work outside. A city report found that heat stress has killed an average of about 10 New Yorkers each year between 2010 and 2019. Black New Yorkers died at twice the rate of their white neighbors.
“People in areas more vulnerable to heat risks are also less likely to have access to health care,” said Dr. Liv Yoon, a co-organizer and postdoctoral researcher at the Earth Institute. “That's not a coincidence.”
Working in pairs, the temperature mappers drove pre-planned routes through the city with white plastic temperature sensors hanging out of their cars’ passenger windows. The sensors fired off every second, capturing block-by-block differences in heat and humidity.
Some volunteers heard about the project through climate advocacy groups; others were roped in by family members. At least two were former cab drivers, putting their deep knowledge of the streets to use in a new way. All of them had firsthand experience of brutal New York City summers.
“I came from Africa, so I expected it,” Fatou Diop, one of the volunteers, said of the heat as she drove through midday traffic with the sensor peeking out from her car window. “But it was so hot, I was like, ‘What is this?’”
“Sometimes up here in Harlem, it’s like stagnant air,” added co-pilot Liz McMillan, who was directing Diop through the twists and turns of the pre-planned route. “You're getting all of this heat that's coming off the concrete. All the heat is coming off the ground. All the heat is bouncing off the buildings.”
Project organizers say they’ll analyze the findings and post the maps online. Scientists like Braneon will overlay them with other data to better understand how infrastructure affects our experience of heat.
South Bronx Unite and other groups can use these maps to advocate for heat-busting infrastructure projects like parks, accessible waterfronts and cooling centers—particularly in neglected areas mostly inhabited by residents of color.
“If we use some of this data, we can foster the idea that we, too, can enjoy things like a waterfront,” Barber said. “We, too, can enjoy things like greenspace. So if this can be one of the ways that we begin to move forward the idea of equity, let’s do that.”