Heatwaves claimed the lives of tens of thousands of people in Europe’s capital cities this summer, as they experienced record temperatures.
In London, emergency wards filled with elderly patients injured after fainting in the heat. Meanwhile, the dry weather created subsidence and left homes with cracks.
In Paris temperatures exceeded 40°C in late June and people struggled to carry on their lives, and in Madrid, heatwaves burned thousands of hectares on the outskirts of the city.
How trees can help
Thami carried out preliminary research on the results of extreme heat on European cities this summer. The study looked into how many buildings have adequate shade from trees. The results were striking.
In London, Paris and Madrid over 90% of buildings did not have enough shade around them to prevent urban heat island effects. These were among the worst results in cities we studied, but even the best performers had a majority of residents without adequate heat protection – cities like Hamburg (56% unprotected) and Barcelona (70% unprotected).
This builds on past research that shows how major cities around the world – including in Europe – tend to struggle to reach useful levels of tree canopies to create shade close to homes and workplaces. It suggests that that homes should have a view of three trees and be within a 300 m walk of a park.
Early results for my research suggests that huge portions of European cities are exposed to avoidable heat island effects.
Map showing tree cover and heat in European cities
How urban heat islands work
On many days during summer 2026 it didn’t get much cooler in the evening. This is because of what is known as the urban heat island effect. Cities typically lack shade, due to high rates of tree removal, sparse planting, and poor tree health and there are vast amounts of asphalt, clay and concrete. These are almost the perfect materials to soak up and hold heat from all that direct sunshine. Unshaded asphalt can reach over 70°C on heatwave days.
Even after sunset, cities stay hot because roads, buildings and footpaths begin to radiate their absorbed heat back at us as they cool. The urban heat island effect can add significantly to ambient temperatures, making a day that is already very hot (perhaps 35°C) even more dangerous in high-asphalt, low shade locations (over 40°C).
What could help?
Tree shading is an effective strategy to address this urban heat, with a range of other mental and physical health benefits to local residents. Trees prevent solar radiation reaching heat-absorbing materials such as asphalt, and also keep the sun off our bodies. Tree shade has been shown to reduce the heat absorbed by pedestrians by up to 18°C.
Healthy trees also push water out of their leaves as they photosynthesise, which soaks up heat as it evaporates. This evaporative cooling effect adds an extra few degrees of cooling, in addition to shading people and preventing heat island effects, even during heatwaves as long as trees still have access to water.
Where trees are needed
Planting more trees in cities in the wake of this summer’s heatwaves is going to help cool urban areas down. The thinktank, the Create Streets Foundation, found the temperature on an exposed paved area in London during one heatwave was 55°C but was 25-30°C in a shaded area metres away.
Looking at a city such as London, you might argue that it already has plenty of trees: around 8 million.
But trees are only effective tools against urban heat when they’re close to homes. A lush shady park 800m away isn’t going to prevent dangerous localised heat island effects for you and your neighbours.
The other crucial factor is that there are thresholds of shade cover that have to be reached for trees to actually offer substantial cooling. A few isolated trees creating small islands of shade aren’t enough. Adding more tree canopy has been shown to be effective at preventing urban heat islands when it reaches 30-40% overall shade cover, within 60m of homes and workplaces.
It will also be important to plant trees that are best able to cope with increasingly hot temperatures in Europe.
Adding just a few degrees to an already hot day sharply ramps up the danger that heat poses. In locations where heat islands bump up the ambient air temperature from 33°C to 37°C, or 35°C to 41°C, there’s an increase in patients attending hospital admissions’ rooms due to heat related illness. When air temperature increases above 35°C the human body’s ability to manage the heat is greatly reduced; cardiovascular strain, dehydration, potentially leading to heatstroke and the need for medical intervention.
When it makes nights hotter there are other ramifications. It is during the cooler night hours that the body loses heat to reset for the following day. If the body is still trying to cope with warm temperatures, 24°C and above, this impacts recovery and impairs sleep quality. This means we start the next day in deficit, both physiologically and cognitively, to cope with another hot day. Repeated exposure to days of high heat mean that the deficit builds over time.
Adding more trees works
Medellin in Colombia is an example of what happens when more plants are added to urban streets.
In just a few years, millions of plants and over 12,500 trees were added to form a network of lush greenways across Colombia’s second largest city.
This project has resulted in significant drops in temperature (of 4.5°C in some areas, and 2°C citywide).
There are signs that Paris and London are aware they need to make changes. The recently announced Heat Ready London plan includes a focus on urban greening.
Paris is already starting to add more green space. The city’s school streets programme not only creates safe, car-free areas close to local schools, but also has added more trees. Evidence already suggests that surfaces in school streets are 5°C cooler than their traffic-heavy counterparts, with fewer trees.
As Europe prepares for more hot summers, planting trees close to homes needs to be high on the agenda to help people cope.
The post “More trees are needed to cool down European cities in heatwaves – but they must be planted nearer to homes” by Thami Croeser, Vice-Chancellor’s Research Fellow, Centre for Urban Research, RMIT University was published on 09/02/2026 by theconversation.com