(Bloomberg) — For London commuters squished into cramped carriages, faces against sweaty armpits, every day is a heat wave, and every heat wave is an inferno.
Unusually hot weather is making it almost unbearable to travel on the UK capital’s underground train network. As pedestrians bake in 30C heat at street level, seeking shade among the wilting roadside trees, temperatures inside some tube carriages can be nearly 5C hotter.
It’s a situation that can sap a city’s economy and threaten the health of passengers, and it’s getting more unbearable with each passing summer due to climate change.
It’s not easy to find data on how hot it is below ground. Transport for London, or TfL, “periodically” measures platform temperatures, but doesn’t monitor them in real-time or record the level of heat in train carriages.
Bloomberg News is creating a London Tube Heat Index, gathering data on the Central Line at the same time every day using an Aranet4 sensor, about the size of a drinks coaster, that logs temperature and humidity at one-minute intervals.
Data recorded during a heat wave earlier this month showed carriages on the central line — one of the oldest and deepest in the network at 20 meters (65.617 feet) below surface level — were consistently 3C higher than street level during a commute from Bank station, in the city’s financial district, to Bond Street station, next to luxurious Mayfair, according to measurements taken by Bloomberg. Even on more typical summer days, temperatures were at least 1.5C hotter.
Put another way, using the official definition of a heat wave for London of three consecutive days of 28C maximum temperatures, London’s commuters are enduring worse-than-heat-wave conditions constantly.
In publicly available reports, TfL acknowledges extreme heat as a major and growing risk to the health and safety of passengers, staff, and the physical network itself. Britain was at least 1C cooler when the first below-ground trains went into service in 1863, and temperatures have risen even faster underground than at the surface.
London isn’t the only city in the world with infrastructure that’s unsuited to a warming climate. But the Tube is particularly vulnerable, with deep, narrow tunnels that trap heat from rail-car brakes, friction and motors, and its packed carriages have limited ventilation.
Research published on June 20 by Imperial College London shows that global warming has increased the chances of an early-season heat wave in the UK from once every 50 years in a pre-industrial climate to every five years today. This summer has already seen two periods of extreme heat this month, including the one forecast to peak on Monday and Tuesday. Max temperatures could reach 34C this week, which has only happened three times in June since the 1960s, according to the UK Met Office.
The biggest demonstration so far of the tube’s lack of heat resilience was in July 2022, when temperatures in the UK pushed past 40C for the first time. Train tracks buckled, overhead cables sagged and transportation equipment failed, according to TfL.
For TfL, short-term heat wave planning involves placards in stations and announcements on trains encouraging passengers to carry water with them. In the long term, the age of the network’s tunnels, along with its physical limitations and geographic location underneath dense city streets, severely limit the options for dealing with extreme heat, according to Sean Colfer, a spokesman for TfL.
“There’s not a huge amount more we can do,” he said. “The temperature on the underground is not immediately changeable.”
Just 40% of carriages on the subway network are air-conditioned, according to TfL. The city’s newest line, which came into operation in 2022 and is named after Queen Elizabeth, is the only service with cars that are entirely cooled.
Londoners have long grumbled and joked about stifling heat underground, but that misery takes a measurable toll on a city’s economy and culture, said Andrew Renninger, a researcher at University College London.
Working with Carmen Cabrera-Arnau, a scientist at the University of Liverpool, he used databases of fare payments and anonymized mobile phone data to research how temperatures affect travel patterns within cities. While the pair haven’t studied London, they found that people responded to heat waves in Spain and other European countries by staying home.
“Heat acts as a tax on activity, and different activities have different elasticities,” Renninger said. The first trips to disappear are the most discretionary — leisure, entertainment and dining — while essential travel to work and school take less of a hit, the research shows.
Data from TfL back this up, with 5 million fewer passenger journeys recorded during the three-day heat wave in 2022, resulting in an £8 million ($11 million) loss in fare revenue.
It’s not just a matter of turning up to work sweaty, heat can have serious health impacts, particularly among those of age 65 and over, according to the government.
Those risks are poised to grow, said Friederike Otto, a climate scientist at Imperial College London, who herself commutes on the underground. When temperatures rise as high as 35C, even very healthy and fit people accustomed to warm climates start to suffer adverse effects, she said. Bloomberg recorded temperatures as high as 33C on the Central and Victoria Lines this month.
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