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On a scorching summer day, a parked car can feel like a furnace. New research from Lisbon shows that the effect isn’t just uncomfortable, it could be warming entire neighbourhoods.

And the colour of your car could be making it worse.

In a new study published in City and Environment Interactions, scientists found that dark-coloured vehicles radiate far more heat than light ones, raising nearby air temperatures by several degrees.

Scaled up across thousands of parked cars, this hidden factor could significantly worsen the urban heat island effect, when cities become much hotter than their surroundings.

Dark cars act like radiators

Márcia Matias and colleagues at the University of Lisbon measured the air temperature around two cars, one black and one white, left outdoors for more than five hours under a clear summer sky. At 36°C, the black car raised air temperatures nearby by as much as 3.8°C compared to the asphalt beside it. The white car had a far smaller impact.

The difference comes down to the light the colours reflect. 

White paint bounces back between 75 and 85 per cent of incoming sunlight. Black paint reflects just 5 to 10 per cent while absorbing the rest. And unlike asphalt, which is thick and slow to heat, a car’s thin metal shell warms quickly and releases heat straight into the air.

“Now picture thousands of cars parked across a city, each one acting like a little heat source or a heat shield,” says Matias. “Their colour can actually shift how hot the streets feel.”

What exactly is the urban heat island effect?

According to Copernicus, the EU’s Earth observation programme, an urban heat island is a city area that is significantly warmer than its rural surroundings due to human activity and infrastructure.

Paved ground absorbs and stores heat while dense buildings reduce air circulation, trapping warmth. Cars, air conditioning and industrial activity add even more heat.

At night, the effect is strongest. Cities can stay up to 10°C warmer than the surrounding countryside, as concrete, asphalt, and steel slowly release the heat they stored during the day. During summer, surface temperatures in European cities can soar 10-15°C higher than in rural surroundings, where plants, forests and fields cool the air.

With around 70 per cent of Europeans living in urban areas, that difference makes the urban heat island effect a pressing public health concern.

Why Europe is especially vulnerable

Europe has been battered by record-breaking heatwaves in recent years, with temperatures topping 40°C in multiple cities this summer alone.

A study last summer found climate change had tripled the death toll from one extreme heat event.

Heat stress doesn’t just cause discomfort. Repeated exposure can accelerate biological ageing, affect mental health and leave children more vulnerable to dehydration, respiratory illness and even death. Older people and people with pre-existing conditions face the highest risks.

In cities such as London and Paris, where night-time temperatures can stay up to 4°C higher than surrounding areas, the lack of relief after sunset only compounds the danger.

How cities are fighting back

Across Europe, some cities are now racing to adapt. Some, like Barcelona, have designated climate shelters – public buildings such as libraries, schools or museums that stay open during heatwaves to provide cooler spaces for residents.

Others are greening their streets. In the Dutch city of Breda, riversides have been transformed into gardens and concrete tiles have been replaced with grass and trees. Now, 60 per cent of the city is green space. By 2030, local leaders aim to make Breda one of the most nature-rich cities in Europe.

These projects take time and investment. That’s why quicker, cheaper strategies, such as boosting urban reflectivity, are gaining attention. Cars, as this study suggests, could be part of that toolkit.

The researchers calculated that repainting dark cars in Lisbon to lighter shades could double the reflectivity of certain streets from around 20 to nearly 40 per cent and lower near-surface air temperatures on hot, windless days.

Sarah Berk, a climate researcher at the University of North Carolina, calls the approach “novel” since most research into cooling cities has focused on reflective roofs or lighter pavements. “Vehicles are a surprisingly overlooked piece of the urban heat puzzle,” she says.

Fleets of taxis, delivery vans or municipal vehicles could be especially effective candidates for lighter paint jobs, Matias adds.