American cities are getting unbearably hot. These ones are roasting the most

(CNN) — No, it’s not your imagination — summers in the US really are getting hotter. And longer.

Human-caused climate change is turbocharging heat all over the country, but it’s most intense in cities, where more than 260 million Americans live. That’s because buildings, roads and sidewalks radiate more heat than grass and trees, in what’s known as the urban heat island effect, which can add as much at 20 degrees Fahrenheit to urban temperatures.

All of the country’s 50 most-populated cities all have gotten hotter over the past 50 years, and all but three are experiencing more “extremely hot” days above 95 degrees, according to a data analysis by the International Institute for Environment and Development, shared exclusively with CNN.

Here’s which cities are on the frontlines.

Summer 2024 has been a scorcher

Summer this year has been punctuated by record-breaking temperatures, blackouts and train delays as tracks overheated, warped and threatened derailment. In one Arizona county, hundreds of people are believed to have died from heat-related causes. Paramedics in Phoenix now travel with body bags filled with ice in which to immerse people suffering heat exhaustion or heatstroke.

If you take the country’s 50-biggest cities and add up the number of days above 95 degrees, there have been at least 1,071 this summer — 161 more than the average over the past decade for the same period.

Those days aren’t spread evenly across cities. Las Vegas and Phoenix, for example, have both experienced more than 80 days over 95 degrees this summer, while coastal cities in California, like Los Angeles and San Fransisco, have been spared entirely.

Washington, DC, and Baltimore are both experiencing their hottest summers in at least a decade. Each has recorded at least 18 extremely hot days this year. Last summer, they logged just three and five, respectively through the end of July.

In terms of which cities have seen the biggest increase in days above 95 degrees, Las Vegas comes out on top. It has roasted this summer, with 83 days — 18 more than its average over the past decade — and every July day in the city was above 105, topping out at 120.

San Antonio, Texas, experienced 54 extremely hot days, 16 more than its 2013-to-2023 average, while Nashville, Tennessee has logged 23, also 16 days above average.

It’s not just this summer

There has been a leap in the number of extremely hot days in the past five decades: from 1,064 to 1,857 among the 50 most populated cities. That’s a jump from an average of 21 days per city to 37.

“The overall trend is clear: major American cities have experienced a significant rise in the number of extremely hot days,” said Tucker Landesman, a senior researcher with the International Institute for Environment and Development.

More days of extreme heat and temperatures rising by a few degrees may sound manageable, but even small increases, especially when coupled with a rise in humidity, can raise the risk of serious health emergencies, like heat exhaustion and even heatstroke, which can be fatal.

And those impacts are having an outsized impact on vulnerable groups.

“Recent years have proved heat is a killer and this can be particularly true for children, the elderly, and people on lower incomes or who live in substandard housing,” Landesman told CNN. “High temperatures are also linked with premature and still births, and other health problems during pregnancy.”

Heat is bigger in Texas

Over the long term, big cities in Texas, Arizona and inland California have been worst-hit, in terms of the rise in extremely hot days.

Of the 10 cities that recorded the biggest increase since the mid-70s, seven are in Texas.

In 1976, San Antonio experienced one week’s worth of extreme heat, but by 2023 it had jumped to nearly 13 weeks — an increase of 1,276%.

Austin went from less than three weeks to more than 12, while Houston went from just over one week to more than seven.

Beyond climate change, parts of Texas have become more vulnerable to extreme heat in the past five decades because its cities have rapidly sprawled and its urban centers have become more dense. And populations there are still growing fast: In 2023, San Antonio added 22,000 people to its population, more than any other city, followed by Fort Worth, which added more than 21,000.

In Austin, city officials are aiming to cover 50% of the city with tree canopy by 2050 to fend off some of the urban heat getting trapped in its concrete and asphalt.

“Too often, heat is talked about as just ‘the weather.’ But we’ve built our cities to be heat islands, often much hotter than the surrounding land, which exacerbates the effects of climate change,” Landesman said. “This effect can be especially acute in low-income areas that lack quality green space.”

Outside Texas, but also in the top 10 cities for the greatest rise in extremely hot days, is Tucson, Arizona, which went from 79 days to 132, as well as Fresno, California, from 60 to 92, and Denver, Colorado, from just 5 days to 31 — that’s a leap from one work week of extreme heat to six full weeks.

Of the 50 cities, only three — Los Angeles, Omaha and Kansas City — did not experience an increase in extremely hot days over the five-decade period.

Arizona cities are the hottest in the country

Arizonans living in urban centers are used to the extreme heat of the desert, but like much of the country, they too are having to adapt to even longer periods of heat.

Phoenix, Mesa and Tucson have the most days of extreme heat. Phoenix reached 158 days on average in 2023 — three weeks more than 50 years ago and 40% of the year.

Tucson’s increase was Arizona’s biggest, with 53 more extremely hot days — or seven more weeks.

Summers are getting longer

If you’re one of those people who thinks summer hasn’t started until you hit a truly hot day, then summers are starting earlier and finishing later.

In the decade from 1974 to 1983, the first extremely hot day of the season, on average, hit on June 22, and the last on September 4. But over the past decade, that period is starting more than a week earlier and is running about a week longer than it used to.

That’s why, if you happened to have lived in those earlier decades, summers may feel like they’re getting longer.

As heat proves deadly, there is growing awareness of the importance of adapting to a hotter world to avoid breaching temperatures that challenge human survivability, all the while reducing fossil fuel pollution as much as possible.

Landesman suggests cities expand green spaces, increase shade, provide public cooling stations and improve messaging about the dangers of heat to prevent serious consequences and costs to human health.

“As the world’s richest country, the United States has the resources and technology needed to address extreme heat,” Landesman said.

“We can’t air-condition our way out of this problem — in fact that would just make it worse, because the US gets so much of its electricity from burning fossil fuels. It would also be prohibitively expensive for the most vulnerable, and cities should be liveable for everyone, not just the wealthy.”

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