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For those who often find themselves grumbling about the stifling heat in subway systems during the warmer months, especially in cities like New York, Boston, and London, there’s now scientific backing for those complaints. A recent study indicates that these discomforts are a widespread issue.
Published in the journal Nature Cities, the study reveals that as surface temperatures climb, more subway passengers report feeling overheated underground. This trend is concerning as climate change, largely fueled by fossil fuel emissions, continues to contribute to global warming.
Researchers from Northwestern University conducted an extensive analysis of over 85,000 social media posts and Google Maps reviews, spanning from 2008 to 2024, to assess the situation in these three bustling cities. They specifically targeted terms indicative of heat discomfort, such as “hot” and “warm,” while excluding unrelated terms like “hot dog.” These cities are among the oldest and most heavily trafficked subway systems in the world.
The study highlights that many subway users expect cooler conditions below ground. However, it found that a mere 1-degree Fahrenheit (0.56-degree Celsius) increase in outside temperature correlates with a significant rise in heat-related grievances: 10% more in Boston, 12% in New York, and a striking 27% in London. Notably, the National Oceanic and Atmospheric Administration reports that from 2008 to 2024, Earth’s average temperature has risen by 1 degree Fahrenheit (0.56 degrees Celsius).
The researchers meticulously examined posts across various seasons, times of day, and days of the week to ensure a comprehensive understanding of the issue.
“Interestingly, over the weekend, people complained less,” said Giorgia Chinazzo, assistant professor in Northwestern’s Department of Civil and Environmental Engineering, who co-authored the study with associate professor Alessandro Rotta Loria. Chinazzo speculated that one reason may be that people were dressing differently than on workdays.
Some limitations
Flavio Lehner, an assistant professor of Earth and atmospheric sciences at Cornell University who was not involved in the work, said the research “follows the template of previous studies that link environmental conditions to human behavior using social media data.” He has also studied how warm conditions trigger a stronger online reaction.
Lehner said limitations of the research include only monitoring three city transit systems, and it being difficult to control for other factors influencing social media behavior.
University of Washington public health and climate professor Kris Ebi, who was also not involved in the study, said the actual impact of subway heat is likely to be greater than researchers found because vulnerable groups are underrepresented on social media.
Ebi said the size of the study “provides compelling evidence that cities should be planning for measures to keep people safe during hot weather.”
The potential for energy savings
Work such as this could certainly play a role in influencing how policymakers and subway operators adjust to heat extremes.
“We’re all experiencing rising temperatures. So those above will be reflected underground, and this will be reflected in people complaining more and more,” Chinazzo said. “Mitigation and adaptation strategies are things that will be much more implemented in the future.”
This could mean installing fans or operating cooling at more specific times of day. It could also mean offering drinking water at certain times.
“We need new technologies and tools, new methodologies that people can use to face these changes in temperatures that everyone is aware of and experiencing nowadays,” she added. “And it will be worse in the future.”
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