(CNN) Blistering heat has returned to western Europe, as some countries like France enter into their third wave of the summer with temperatures expected to reach over 100 degrees Fahrenheit (37 degrees Celsius), while more than 80% of the US population will experience temperatures over 90 degrees (32C) within the next week, including in the Mid-Atlantic and Northeast. Around 100 million Americans have been under heat alerts for eight of the past 16 days.
This means hundreds of millions of people living in urban areas are again desperately trying to stay cool. The climate crisis is making extreme heat more frequent and last longer — but cities, without thoughtful design, can make life even hotter.
Air conditioners might keep the indoors cool, but they only add to heat outdoors. And in most cases, they are adding to the climate crisis by increasing planet-warming emissions. Public transportation may be unbearable on a hot day, but driving a car that runs on gas instead just worsens traffic, also adding to heat and emissions. A lack of trees means a lack of shade, and buildings made of dark materials bring hotter interiors, which means more air conditioning.
It's a vicious cycle, but there are other solutions.
Here's how eight cities are taking some of the heat out of their summers.
When it gets really hot, people with air conditioning might stay indoors, but not everyone has that luxury and — well, who wants to say in all the time?
For cities that aren't on the coast, parks that offer shade are a good option. Colombia's second-largest city, Medellín, however, has created an entire metropolis of shade with its award-winning Green Corridors project.
The web-like network has transformed 18 roads and 12 waterways into lush green cycling lanes and walkways that connect the city's parks and other frequently visited sites.
Temperatures have come down in these areas and their surrounds by as much as 5.5 degrees Fahrenheit (around 3 degrees Celsius), and officials hope that before 2030, it could shave off up to 9 degrees Fahrenheit (5 Celsius).
"Urban forests are the very best thing for city heat," Kathy Baughman McLeod, director of the Adrienne Arsht-Rockefeller Foundation Resilience Center (Arsht-Rock) at the Atlantic Council, told CNN. "Medellín has dropped the city's average summer temperature, which is remarkable."
By 2019, the city had planted more than 8,000 trees and over 350,000 shrubs. It also uses an area beneath a raised Metro line to collect rainwater that flows down from the bridge, capturing it in a system of pipes to help water the green belts.
Like in much of Europe, many in Vienna don't have air conditioning, so water is a big part of how the Austrian capital keeps cool.
For those who don't have time for a dip in the Danube, the city offers cooling parks with mist-spraying "trees" that people can either "shower" in, or just sit near to enjoy the cooler temperatures they bring their surrounds.
Children, who are generally more vulnerable to extreme heat than adults, are often seen playing in the city's splash pools or running around in pop-up water features — typically hosepipes with holes punched in them — that the city government brings out on the hottest of days, including in areas like Karlsplatz, a popular city square.
Vienna also has a huge number of water fountains for drinking to keep people hydrated — more than 1,100 for its population of 1.9 million — which is important in preventing heat-related illness.
"Air conditioning in homes may sound like a quick and easy solution. But it's not a long-term sustainable solution because of the source of the power and the waste heat that comes off the unit," McLeod said. "So thinking about how to get more airflow, use water features and get windows to open in some of the oldest buildings is key. The nature-based solutions are the best for extreme heat."
Parts of the Middle East are some of the hottest inhabited places on Earth. Temperatures in Abu Dhabi can climb to more than 120 degrees Fahrenheit (over 50 Celsius). Air conditioning is seen as a necessity, and people tend to spend a lot of time indoors.
But people here haven't always had air conditioning, and an ancient Arabic architectural cooling technique has made a comeback — with a modern twist.
Mashrabiya refers to the latticed screens often seen in Islamic architecture, sometimes surrounding a small balcony, that diffuse sunlight and keep buildings cool without completely blocking light. They are designed to encourage a breeze and offer a spot of respite from the heat within a building. The idea is essentially to stop direct sunlight landing on a building's exterior.
That's what inspired the design of Al Bahar Towers, a 25-story building wrapped in more than 1,000 hexagonal shades with built-in sensors that allow them to respond to the sun's movements. When the sun hits the shades, they unfold like an umbrella to ward off the heat. Without these measures, the outside of such a building in Abu Dhabi could reach as high as 200 degrees (around 90 Celsius).
The technique has helped reduce the building's need for air conditioning by 50%. Cool, huh?
In a lot of cities, catching the bus can mean a long wait. If it's really hot, the wait can be all the more punishing — unless, of course, that bus stop has been thoughtfully engineered to include natural shade.
Medellín in Colombia may have proved that urban forests, or simply planting more trees, can cool a city a down, but Miami's Dade county has put a lot of thought into exactly which parts of the city need cooling the most.
Neat Streets Miami, a board convened by the county council, recognized that bus stops had become real danger zones during heat waves, so they planted trees around 10 stops. They wrote a guide on which trees work best and where to plant them so that other areas could replicate the project.
And that they have. There are now 71 green bus stops in the country, most of them by communities that applied to the government for resources to green their own bus stops.
To make it more fun, the organizers also held a haiku poetry competition, and selected the best 10 to etch into the sidewalks by the original stops.
The trees have also
missed their bus — look how they wave
their many sad arms
— Ariel Francisco
Not every city has an ancient aqueduct at their disposal, but the Greek capital of Athens does. The Hadrian aqueduct was once used as a main source of water, using a system of pipes that worked with gravity to allow water to flow from its source to the city for human consumption.
The water today isn't drinkable, but the city is looking at ways to salvage the 800,000 cubic meters of water that flows off as waste into the sea each year. One use will be to irrigate new greenbelts to run all along the 20 kilometer structure, which should help take the heat out of the areas around it. The water will also be used for misting, like in Vienna.
Even for cities without infrastructure quite this old, Athens is a good reminder that defunct water systems can sometime be revived.
This one is a little more controversial.
Some cities have experimented with painting rooftops white to reflect sunlight and keep buildings cool, but Los Angeles went a step further and is painting entire roads white. Dark things like asphalt absorbs sunlight and emits that energy back into the air as heat. Painting the asphalt white would theoretically nip that process in the bud, and lead to cooler air temperatures.
The idea has some merit. Researchers Ariane Middel and V. Kelly Turner found that technique did cool the streets themselves by around 10 degrees. But there was a major knock-on effect. The same researchers also said it was likely the extra heat reflecting off the roads was being absorbed by ... people.
That means if you're a few blocks away, the white streets may help you feel cooler, but if you're on the street, you could actually feel hotter.
Nonetheless, LA is continuing with this program to see what works and what doesn't. It currently uses a grayish-white substance called CoolSeal, once used to help hide grounded aircraft from satellites, but it's possible that another type of paint could yield different results.
Painting rooftops has had greater success.
Results vary depending on the level of heat and materials a roof is made of, but in places like Ahmedabad in India, which gets seriously hot, cool roofs have shaved 3-8 degrees Fahrenheit of the heat in homes. According to Berkeley Lab's Heat Island Group, a black roof could be as much at 54 degrees (around 30 Celsius) hotter than a white roof.
Another option is the green roof. Cities all around the world have created "gardens in the sky" to cool down buildings.
The French capital gets seriously hot.
Temperatures there have surpassed 104 degrees Fahrenheit (40 Celsius) this summer, but the combination of high-rise buildings, limestone monuments and busy asphalt roads means it can feel even hotter.
The city has a strong urban heat island effect, where it is often 18 degrees hotter in the city center on a summer's day than it is in the Parisian hinterlands.
But Paris Mayor Anne Hidalgo has been credited with putting in place some of the world's most innovative measures to combat heat, and the city's heat plan is truly comprehensive.
The main result is a city full of "cool islands." Parisians can use an app called EXTREMA to guide them to more than 800 cool spots — parks, water fountains and air-conditioned museums, for example — and get there via a naturally cooled walkway. The idea is that a cool island is always a maximum seven minutes' walk away for everyone.
Like Vienna, Paris uses mist machines on hot days. It also has dozens of new "splash fountains," in addition to its many traditional fountains, which are very shallow pools with fountain-like effects.
Paris' heat plan involves a register that identifies the most vulnerable, so officials can check up on them by phone and offer advice on staying cool. Kindergartens get temporary air conditioners in their classrooms, and public parks and pools stay open for longer hours into the night. And like LA, Paris is trying to take the heat out of its roads and sidewalks by "demineralizing them," using more porous materials. Now that sounds like a plan.
The world has been naming hurricanes, cyclones and typhoons for decades for a reason: A named storm makes you sit up and pay attention. The southern Spanish city of Seville is taking that approach with heat waves, becoming world's first to do so.
The July heat wave there was named Zoe.
"Naming heat waves is a positive thing because it means we're recognizing how lethal they are, and that they're here to stay. It's not a fluke heat wave," the Arsht-Rock's McLeod said. "This is this is something we're going to be living with for a long time, no matter what we do with our emissions."
But there's more to what Seville is doing than naming. Arsht-Rock is working with Seville on a new categorization system for heat waves based on projected negative health outcomes. The idea is to avoid scientific jargon that most people don't understand and link alert levels to what a heat wave is likely to do to people.
A 2018 Brown University study of 20 heat warning systems in the United States found that only Philadelphia's heat warning system was effective in saving lives, partly because it uses health-based metrics.
"Besides physical interventions for heat, naming and categorizing heat waves is the best, most immediate thing you can do," McLeod said. "Because that's the key — heat is killing people, and that's because people are not aware of the magnitude of the problem."