Commuters, residents, and tourists who have long complained about the sweltering conditions within London, New York, and Boston’s underground transport systems now have scientific backing for their grievances.
New research confirms that rising surface temperatures directly correlate with an increase in reports of uncomfortable heat below ground, a problem set to intensify with climate change.
A study published in the journal Nature Cities on Tuesday reveals that as temperatures climb aboveground, the number of subway riders reporting thermal discomfort significantly increases.
Researchers from Northwestern University analysed over 85,000 crowdsourced social media posts on X and Google Maps reviews from 2008 to 2024 across these three major cities, which boast some of the world’s oldest and busiest subway networks. They specifically searched for keywords such as "hot" and "warm," filtering out irrelevant mentions.
The findings indicate a clear link: a 1-degree Fahrenheit (0.56-degree Celsius) increase in outdoor temperature led to a 10 per cent rise in complaints in Boston, 12 per cent in New York, and a substantial 27 per cent in London.
This period saw Earth’s average temperature warm by 1 degree F (0.56 degrees C), according to the National Oceanic and Atmospheric Administration.
Giorgia Chinazzo, an assistant professor at Northwestern’s Department of Civil and Environmental Engineering and co-author of the study, noted that subway riders often expect cooler temperatures underground.
She also observed an interesting trend: "Interestingly, over the weekend, people complained less." Ms Chinazzo speculated this could be due to people dressing differently than on workdays.
While the research provides compelling evidence, experts not involved in the study offered some caveats. Flavio Lehner, an assistant professor of Earth and atmospheric sciences at Cornell University, acknowledged the study’s alignment with previous work linking environmental conditions to human behaviour via social media.
However, he pointed out limitations, including the focus on only three transit systems and the difficulty in controlling for other factors influencing online behaviour. Kris Ebi, a public health and climate professor at the University of Washington, suggested the actual impact of subway heat might be greater than found, as vulnerable groups are often underrepresented on social media. Ms Ebi stressed that the study’s scale "provides compelling evidence that cities should be planning for measures to keep people safe during hot weather."
The implications of this research are significant for policymakers and subway operators as they adapt to extreme heat. Ms Chinazzo warned, "We’re all experiencing rising temperatures. So those above will be reflected underground, and this will be reflected in people complaining more and more."
She added that "mitigation and adaptation strategies are things that will be much more implemented in the future," suggesting solutions like installing fans, operating cooling systems at specific times, or offering drinking water. "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 concluded, predicting that conditions "will be worse in the future."
