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The fierce heat waves engulfing much of the Northern Hemisphere, including the United States and Europe, are renewing concerns among experts who say extreme heat poses an increasing threat to some of the world’s largest public transit systems. With mass transit in many big cities already facing serious budget shortfalls and transportation accounting for a significant portion of global greenhouse gas emissions, the situation adds another challenge to the international effort to curb climate change.

Research has shown that public transportation is a highly effective way to reduce emissions from the transportation sector, which is now the leading single-sector source of U.S. carbon emissions. One analysis found that public buses produce about one-quarter fewer carbon emissions per trip-mile than personal vehicles, with trains producing about 80 percent fewer emissions.

But as temperatures soared above 100 degrees Fahrenheit this week in the U.S. and across Europe, civil engineers and transit authorities reiterated warnings that railway tracks could potentially bend or buckle under the extreme heat. Some agencies, including in London and in different regions of the U.S., issued speed restrictions on rail lines and even suspended entire routes on some of the hottest days to mitigate the risks.

When exposed long enough to triple-digit temperatures, steel railway lines can soften and deform as heavy trains pass over them. As climate change makes extreme heat more frequent and intense, researchers say that could lead to more disruptions to transit operations as agencies are forced to shut down lines to make repairs. And a growing number of experts say not enough is being done to prepare for those added costs, which could make it harder for cities to improve and expand their transit systems to cut emissions.

“The U.S. is not prepared,” Paul Chinowsky, a professor of civil engineering at the University of Colorado Boulder, told Bloomberg. “While the rail system is incrementally being improved, there is significant work to do and what is being done is not being done fast enough.”

In fact, extreme heat was cited for the first time by California transit authorities as the reason a subway train derailed near San Francisco last month, causing some minor injuries and prompting the evacuation of about 50 passengers. Local officials said a subway track had developed a bend after a heat wave brought about temperatures as high as 102 degrees, with the tracks themselves reaching 140 degrees—25 degrees hotter than what the metro area transit authority considers safe.

Extreme heat can lead to other transit-related problems as well. A summer heat wave in the Pacific Northwest last year melted the cables that power the light rail system for Portland, Oregon. That forced city officials to suspend all train services for two days.

Research shows that those types of problems are only expected to get worse as the planet continues to warm. Operational delays due to extreme heat could cost U.S. transit authorities and other train operators anywhere between $25 billion and $60 billion by the end of the century, according to one 2017 study.

That’s a serious problem for many major U.S. transit authorities, which already face massive budget shortfalls after ridership plummeted to unprecedented lows in 2020 and 2021 due to the pandemic. Between September 2019 and September 2020, average ridership on commuter trains dropped 79 percent across the country, according to the U.S. Government Accountability Office.

The federal government jumped in, injecting around $69 billion in emergency aid to help keep transit systems afloat. But as that money begins to dry up, many of the nation’s largest transit systems still face significant budget deficits as ridership remains well below pre-pandemic levels.

That includes the Washington, D.C., transit system, which expects a funding gap of $519.3 million by fiscal year 2024, and the Boston metro area’s transit system, which faces a $236 million deficit that same year, followed by an even bigger $406 million deficit in 2025, according to a recent analysis by Governing.

New York City’s Metropolitan Transportation Authority, the nation’s largest public transportation system, also faces a budget shortfall of about $500 million by 2025. Although when state aid runs out the following year, that deficit could balloon to $2 billion if ridership remains at its current level and other funding sources don’t emerge.

Federal aid from the Bipartisan Infrastructure Investment and Jobs Act, which Congress passed last year, will help address some of those budget shortfalls. The legislation includes $39 billion for public transit systems and $66 billion for the nation’s interstate railways.

But despite being the largest federal investment in public transit in U.S. history, advocates say it still falls short of what’s needed. The nation’s public transit systems alone, not including interstate rails, had a deficit of $176 billion last year, and that gap is expected to grow to $250 billion by 2029, according to a 2021 report by the American Public Transportation Association. If accurate, the $39 billion from the infrastructure law, spread out over five years, would make just a dent in those estimates.

It’s a problem many climate and transit advocates say will only become harder and more expensive to tackle as the impacts of global warming accelerate. And some analysts believe the ongoing pandemic may even leave a lasting stigma on public transportation that could impact finances for years to come.

In Britain on Tuesday, as historic triple-digit temperatures broke three national records in the span of an hour, transit officials in some areas painted railway tracks white to help deflect the heat and prevent warping.

“Here in the U.K., we’re used to treating hot spells as a chance to go and play in the sun,” Penny Endersby, the chief executive of the country’s weather service, said in a video to the public. “This is not that sort of weather. Our lifestyles and our infrastructure are not adapted to what is coming.”

That’s it this week for Today’s Climate. Thanks for reading, and I’ll be back in your inbox on Tuesday.

That’s the percentage of Americans who will experience temperatures above 90 degrees Fahrenheit this weekend, including millions in the south-central U.S. expected to see triple-digit heat, the National Weather Service warned.

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