The summer of 2025 has again been marked by several successive heatwaves, which have gripped large swathes of Europe.
Nowhere are the effects of this extreme weather more intense than in the continent’s urban areas. This is due to the Urban Heat Island (UHI) effect, which significantly raises temperatures compared to surrounding (rural) areas, intensifying heatwaves.
“This urban heat island effect is impacting cities all over Europe, and this makes temperatures about two, three, sometimes even four degrees warmer in cities compared to the rural environments,” Niels Souverijns, climate expert at VITO in Belgium, told Euronews.
Surface temperatures in cities were sometimes up to 10-15°C higher than in their rural surroundings during summer, data from the European Commission’s Joint Research Centre shows.
These figures are particularly concerning given the large number of people affected. Almost 40% of Europe’s total population lives in cities, while around 70% resides in urban areas.
The phenomenon is a major health concern for city authorities across Europe. One study by NOVA National School of Public Health in Portugal found that hospital admissions increased by 18.9% from the baseline on heatwave days, straining healthcare systems.
Vulnerable groups — like the elderly and young children — are particularly at risk. “Especially the elderly with underlying health conditions, for example, respiratory illnesses or cardiovascular illnesses, are worst affected,” said Souverijns.
As the phenomenon plays out mostly at night, people don’t cool down properly when going to sleep, which also has negative health effects.
Concrete jungle
This urban heat island effect is the result of several factors, ranging from the widespread use of materials to the concentration of buildings and heavy traffic.
“The fact that the city is warmer than the surrounding area is due to the materials used in the city,” climate scientist Wim Thiery (VUB) told Euronews. These include asphalt and cement, which “absorb heat when the sun shines, retain it and then radiate it at night”.
As a result, during the day, the maximum temperature in cities is warmer, but at night, the temperature also cools down less.
Tall buildings and narrow streets also drive this phenomenon, as they heat up the air that gets trapped between them, blocking the natural flow of wind that would otherwise assist in cooling the air.
In many cities, this is particularly true in poorer urban areas characterised by dense and sometimes unregulated urbanisation. This factor, in combination with poor housing conditions, lack of insulation and reduced access to cooling options such as green spaces, makes the urban heat island effect a socio-economic issue.
“In the richer neighbourhoods located just outside city centres, there tends to be more greenery,” Thiery said. This is the case in Brussels, for example, where the wealth divide correlates strongly with access to natural areas, with the poorest suffering an acute shortage of trees and parks.
Souverijns noted that the magnitude of the urban heat island is strongly correlated with the size of the cities and the amount of concrete and buildings located there. “So bigger cities, which have more concrete and more buildings, tend to be more impacted.”
Finally, the amount of traffic in a city also has an impact. Pollution from cars, for example, can act as a type of greenhouse gas layer over a city, trapping hot air.
Green solutions
Climate scientist Wim Thiery emphasised that, in the future, the urban heat island effect is expected to worsen. “Due to climate change, temperatures are increasing by more than the average temperature of 2°C in general, with heat days being 4°C warmer in Europe,” he told Euronews.
Cities can, however, adopt a range of measures to counteract the urban heat island effect. This includes creating wind corridors for ventilation, designing green roofs and façades for buildings, planting more vegetation and making better use of water.
“In essence, we have to redesign cities to remove as much concrete as possible,” said Souverijns.
The Brussels region is particularly vulnerable to heatwaves compared to other Belgian municipalities. The difference can be as much as 10° compared to rural areas.
One of its 19 communes, the City of Brussels, provides residents with additional information about precautions or behaviours that are best to adopt on hot days, including through interactive maps with which they can find drinking water points, museums, fountains, shaded green spaces and other places where they can protect themselves from the heat.
“More structurally, we are of course committed to physically adapting and improving our living environment by planting trees, softening surfaces, making squares and streets shadier and greener, bringing water into the city and creating green-blue networks,” Frederik Ceulemans, alderman for climate in the City of Brussels, told Euronews.
“There are additional interventions that we want to test soon, such as shaded structures, whether temporary or permanent.”
‘Adaptation alone is insufficient’
However, both experts agree that adaptation alone is not enough. “We have to be realistic that the impact of adaptation is limited. Very locally, where trees are planted, the impact can be noticeable, but on one street further, it will be warmer again,” Thiery said.
That is the reality of cities: there is no room for sufficient green spaces in densely populated neighbourhoods.
He added that, if authorities want to stop this issue, mitigation must be top of mind. This was mirrored by Souverijns. “The urban heat island effect will also increase towards the future with additional climate change. So it’s also important to reduce our emissions as fast as possible.”
In the City of Brussels, the focus is on sustainable renovations of buildings, which account for 60% of CO2 emissions in the region, as well as sustainable plans for mobility and energy. The region aims to reduce CO2 emissions by 55% by 2030 and become climate-neutral by 2050.
If CO2 emissions are not reduced to zero, experts warn that “a tsunami of heat” will descend onto cities, which could become unlivable.
“We already see these kinds of unlivable cities, mainly in India and in countries close to the equator. Here, people are struggling to live in the city and go about their daily business,” Souverijns said.
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