On a recent August day in New York City's subway, Jack Klein, a 31-year-old construction site manager, was tracking soaring temperatures on his phone to understand how underground train stations can cool down in an era of climate crisis. Over the past two years, Klein has taken it upon himself to become a guerrilla subway heat operative, sticking teardrop-shaped thermometers to steel beams in seven of the hottest Manhattan stations and posting social media videos of the summer temperatures commuters endure.
Extreme heat readings in Manhattan stations
At the 4/5 line platform at 14th Street-Union Square, one of the city's most blistering stations, Klein dangled another monitor and saw the digits climb to 99F (37C), a good 10F (5.5C) hotter than aboveground. The heat index, which combines temperature with humidity, rose to 116F (46C), well above the level at which heatstroke can set in. "It's been worse – I've seen it get to 120F [48C]," said Klein. Shortly afterwards, the monitor's reading climbed to 101F (38C) with the "real feel" heat index hitting 132F (55C).
In an elevated section of the Brooklyn Bridge-City Hall and Chambers Street subway complex, the heat index reached 132F (56C). Passengers expressed discomfort, with one Italian tourist named Leo saying, "It's just too hot, it's terrible, too much," and a woman fanning herself likening the heat to an oven. The heat's biggest impact is on subway staff, such as maintenance workers and cleaners, who have to toil for hours in these temperatures, a situation their union describes as dangerous.
Klein's efforts and public health concerns
Most of Klein's magnetized thermometers have disappeared, either removed or fallen, but he has switched to helping relieve the heat by heaving down large dispensers of water to platforms and agitating for better seating and proper fans. He grumbled about a dust-clogged overhead fan at 14th Street-Union Square that blasted even hotter air at commuters. Older commuters have reminded Klein, originally from Ohio, that the subway has always been hot and the trains weren't fully air-conditioned until 1993. "A lot of people just say 'suck it up'," he said. "I get that. But I still see this as a public health issue."
Rising global temperatures – July was the hottest month ever recorded in the US – are ratcheting up pressure on cities to deal with stifling heat on public transport. Giorgia Chinazzo, an environmental engineer at Northwestern University, said, "The underground is overheating more than aboveground, which is creating these extreme temperatures. This summer is a clear sign that we are facing a climate crisis and our built environment isn't designed for it. In older systems like in London and New York, the metro systems just can't deal with this."
Proposed geothermal solution
Underground stations act as a hot box, soaking up heat in concrete and steel and slowly releasing it. Trains give off heat and air pollution as they brake, and dump out hot exhaust from air conditioning, adding to the stew. Each person gives off about 100 watts in heat, with temperatures sometimes 20F (11C) higher than aboveground. New York's subway was designed with a piston system, where incoming trains push out hot air via vents, but this crude ventilation isn't enough for extreme heat.
Janno Lieber, chief executive of the Metropolitan Transportation Authority (MTA), said, "The subway isn't a submarine, it's not an enclosed environment. Bringing down the temperature is a complex problem in a system where the stations are 100 years old. We know this is no fun for New Yorkers. It's brutal underground in the summertime and every year is getting hotter because of climate change."
A new city and state government study will explore if it's viable to capture station heat and push it deeper underground, then use this trapped heat in the winter to warm up surrounding buildings. This system, called a thermal energy network, would run chilled liquid through pipes behind walls and under flooring to pull heat away from platforms and funnel it into storage boreholes drilled up to 600ft underground. The study will focus on the Brooklyn Bridge-City Hall and Chambers Street station complex, which Lieber said is one of the "sweatiest" stations, with an average temperature of 96F (35C) in summer.
Zohran Mamdani, New York City's mayor, said at a media event, "What we could see is a first-of-its-kind approach that could lead the nation. We would be making these stations cool again, and we'd be making what is old new again. That means lower energy costs, a more resilient subway system and a cleaner energy future for New York City."
Feasibility and future challenges
Transferring heat in this way isn't a new idea – the ancient Romans and Persians mastered the flow of hot and cold air long before electricity. In New York, St Patrick's cathedral and its botanic garden both use geothermal heating, and the MTA is already starting several schemes elsewhere to draw away and bury heat. Alessandro Rotta Loria, an engineer who worked with Chinazzo on her research, said, "Extracting the heat and moving it away is a rational approach. You can deploy this universally, in New York City, London, Rome, wherever you are. The only issue is space restraints and the expertise and willingness to install it."
Even if New York opts to build this system, it will take billions of dollars and many years to refit the largest subway system in North America, which spans 472 stations, 283 of them underground. The 4 million people who ride the subway on a typical day will have to deal with rising heat – already the US's largest natural killer – with some interim measures, perhaps free water in stations. The MTA did not respond to questions as to whether it will introduce free water.
Mamdani acknowledged last week, "Our stations do not just get hot, they turn into underground saunas." Klein is planning to erect an actual wooden sauna on the platform of the 14th Street-Union Square station as a piece of art commentary. "It's almost done and it looks pretty solid. We will see how people react," Klein said. "I think not being from here perhaps it's like I recognize that [the heat] is actually not normal. I didn't grow up as a kid riding this so I have a different, outsider perspective. It really is like a sauna." More frequent trains, to whisk passengers away from the platform, will also help, but for now, stepping into the cool air of a subway car comes as a relieving summer joy.
