The Urban Heat Island Effect and Its Growing Impact
The summer of 2025 has once again been marked by a series of intense heatwaves that have affected large parts of Europe. These extreme weather events have had the most severe impact on urban areas, where temperatures are significantly higher than in surrounding rural regions due to the Urban Heat Island (UHI) effect.
This phenomenon occurs when cities become much warmer than their surroundings because of the materials used in construction and the concentration of buildings. According to Niels Souverijns, a climate expert at VITO in Belgium, cities can be two to four degrees Celsius warmer than rural areas. Data from the European Commission’s Joint Research Centre shows that surface temperatures in cities can sometimes be as much as 10-15°C higher during the summer months.
With almost 40% of Europe’s population living in cities and around 70% residing in urban areas, the UHI effect poses a major public health concern. A study by NOVA National School of Public Health in Portugal found that hospital admissions increased by 18.9% on heatwave days, putting significant pressure on healthcare systems.
Vulnerable groups, such as the elderly and young children, are particularly at risk. “Especially the elderly with underlying health conditions, such as respiratory or cardiovascular illnesses, are worst affected,” said Souverijns. The problem is compounded by the fact that the UHI effect often persists into the night, preventing people from cooling down properly and leading to further health complications.
Factors Contributing to the Urban Heat Island Effect
The UHI effect is driven by several factors, including the widespread use of heat-absorbing materials like asphalt and cement. These materials absorb sunlight during the day and release it at night, keeping city temperatures elevated. Wim Thiery, a climate scientist at VUB, explained that this leads to higher daytime temperatures and slower nighttime cooling.
Tall buildings and narrow streets also contribute to the problem by trapping heat and limiting natural wind flow. In many cities, this issue is more pronounced in poorer urban areas characterized by dense and unregulated development. These neighborhoods often lack green spaces, proper insulation, and access to cooling options, making them more vulnerable to the UHI effect.
In wealthier areas, there tends to be more vegetation and open spaces. For example, in Brussels, there is a clear correlation between income levels and access to natural areas, with the poorest communities suffering from a lack of trees and parks. Souverijns noted that the size of a city and the amount of concrete and buildings also play a role in the severity of the UHI effect.
Traffic is another contributing factor, as vehicle emissions create a layer of greenhouse gases that trap heat over urban areas. This exacerbates the already high temperatures in cities.
Green Solutions for a Warmer Future
As climate change continues to drive up global temperatures, the UHI effect is expected to worsen. Thiery warned that heat days in Europe could become 4°C warmer than average, increasing the risk of extreme heat events. However, cities can take steps to mitigate these effects.
Experts suggest measures such as creating wind corridors, designing green roofs and façades, planting more vegetation, and incorporating water features into urban planning. “In essence, we have to redesign cities to remove as much concrete as possible,” said Souverijns.
Brussels, which is particularly vulnerable to heatwaves, has taken proactive steps to address the issue. The city provides residents with interactive maps showing locations of drinking water points, museums, fountains, and shaded green spaces. Frederik Ceulemans, alderman for climate in the City of Brussels, emphasized efforts to plant trees, soften surfaces, and create greener, shadier public spaces.
Additional interventions, such as shaded structures, are being tested to further reduce heat exposure. However, both experts agree that adaptation alone is not enough. “We have to be realistic that the impact of adaptation is limited,” Thiery said. In densely populated areas, there is simply not enough space for sufficient greenery.
The Need for Mitigation
To truly address the UHI effect, mitigation must be a priority. Souverijns stressed the importance of reducing CO2 emissions as quickly as possible. In the City of Brussels, sustainable building renovations, mobility plans, and energy strategies are being implemented to cut emissions by 55% by 2030 and achieve climate neutrality by 2050.
Experts warn that if emissions are not reduced to zero, cities could face a “tsunami of heat” that makes them increasingly unlivable. Already, some cities in India and equatorial regions are experiencing extreme heat that hampers daily life. As the situation worsens, the need for comprehensive action becomes more urgent.