Temperatures on the London Tube and other underground train systems are continuing to increase, leading to escalating complaints from passengers. 

Northwestern University scientists have conducted a broad-ranging study that scraped social posts and online reviews published between 2008 and 2024. They then analysed more than 85,000 crowdsourced comments from across Boston, London and New York.

The findings show a clear pattern that as temperatures rise because of climate change, below-ground thermal complaints also increase. 

“No one wants to feel uncomfortable,” said Giorgia Chinazzo, who led the study. “But while discomfort might seem like a minor inconvenience, extreme heat also represents a serious threat to public health. Extreme heat causes more deaths per year than all other natural hazards combined. 

“Having information about when and where people feel uncomfortable could help transit agencies and policymakers make targeted interventions, such as increasing the number of operating fans during specific times of day.”

Urban heat islands are a common phenomenon in large metropolitan areas and significantly warmer than surrounding rural areas. This heat also seeps into soil, tunnels and underground transit systems where it lingers and can be difficult to expel even with air conditioning systems. 

Subsurface temperatures in the London Underground have reached 47°C, surpassing London’s highest recorded air temperature.

The researchers wanted to explore how the extreme temperatures impact commuters and focused on crowdsourcing data from social media posts by analysing more than 85,000 posts from X and Google reviews.

“We focused on these three cities because their transit systems are the oldest and biggest,” said researcher Rotta Loria. “London has the oldest metro system in the world. In the US, Boston has the oldest system, and New York has the largest one. In principle, this technique could be applied to any place on Earth, but this was our starting point.”

After collecting posts, the team used natural language processing to extract comments with keywords like “hot,” “warm,” “uncomfortable,” “roasting,” “scorching” and “boiling,” while omitting those keywords when unrelated to thermal discomfort. 

When comparing the complaints to recorded temperatures, Chinazzo and Loria found that for each increase in 1°C above 10°C, complaints increased by 10% in Boston, 12% in New York and 27% in London. Complaints peaked during summer months – especially July – and spiked during major heatwave years such as 2018 and 2019.

Although complaints spiked in summer months, commuters still expressed some discomfort in colder weather. As a possible justification, commuters may often be dressed warmly, prepared for freezing outdoor temperatures. But, after venturing underground, they may feel overdressed and uncomfortable in the warmer subterranean conditions.