Extreme heat in cities – a growing threat to public health

Extreme heat is the weather-related phenomenon that causes the most deaths in Sweden – and cities are particularly vulnerable. Dense development and hard materials such as asphalt, concrete, and glass contribute to high radiant temperatures that can turn entire neighborhoods into so-called heat islands. In some places, a sunny summer day with an air temperature of 25°C can result in a radiant temperature of up to 50–60°C.

– If you plant a tree in the same location, you can cut the radiant temperature in half. But it’s important to choose the right type of vegetation. Sparse tree canopies and shrubs have less impact on thermal stress, says Erik Johansson, Associate Professor in Housing and Residential Development at Lund University’s Faculty of Engineering.

At the same time, trees with large canopies can reduce ventilation, making nighttime cooling more difficult. Kristina Blennow, Professor of Landscape Planning at SLU, explains that digital models are used to analyze how heat affects the city:

– In the models, we can simulate shadows at different times of day and assess how materials absorb sunlight. This gives us a measure of local heat stress. But what reduces daytime temperatures, like narrow buildings, can contribute to nighttime heat retention. That balance is important to consider.

By tailoring urban planning – using vegetation, water, and favorable wind conditions – it’s possible to create microclimates that make our urban environments more resilient to heatwaves.

Severe impact on vulnerable groups

Heat stress particularly affects the elderly, pregnant women, children, and people with chronic illnesses. For people over 80, daytime heat is the greatest risk, while mortality among those aged 45 and older is linked to high nighttime temperatures.

– Heatwaves have become longer in recent decades. We've also seen more tropical nights where temperatures don't drop below 20°C – meaning the body doesn’t recover after a hot day. Model studies show this kind of extreme heat is expected to increase in the future. We must take this very seriously, says Kristina Blennow.

Green schoolyards in Stockholm

Since 2015, the City of Stockholm has run the Green Schoolyards project, aimed at replacing asphalt surfaces with greenery. The goal is to create health-promoting environments that support education and contribute to climate adaptation. Each year, several schoolyards are upgraded through dialogue with students and staff.

– The concept is based on nature-inspired environments with paths, islands, and boardwalks where play is central. We collaborate with researchers and other stakeholders to test and evaluate new methods, says Tanja van de Meulebrouck, architect and urban planner specializing in children's physical environments in primary schools at the City of Stockholm.

Challenges include creating durable greenery that withstands wear and tear from children, ensuring long-term maintenance, and designing the spaces with safety in mind.

– We want to realize the full potential of the schoolyard – as the children’s space in the city, a cool oasis with biodiversity and educational support for sustainable development, says Tanja van de Meulebrouck.

Knowledge gaps among decision-makers

At SLU, the future platform SLU Urban Futures is working to compile knowledge on how cities can adapt to a warmer climate.

– We want to raise awareness, strengthen preparedness, and promote more systematic efforts to create cooler, more resilient, and socially just cities, says Amanda Gabriel, hub coordinator for SLU Urban Futures and university lecturer in environmental psychology at SLU.

She emphasizes that nature-based solutions are often both long-term and cost-effective – yet still underutilized.

– Even though we have a lot of knowledge, awareness is lacking among decision-makers. We need to move beyond simplified assumptions like ‘green is always cool’ and develop more site-specific, research-based strategies.