More trees in cities could significantly reduce the number of heat deaths in summer. This is what an international research team reports after analyzing 93 major European cities, including seven major German cities. If the treetop coverage in the cities were doubled from the current average of just under 15 percent to 30 percent, the temperature there in summer would drop by an average of 0.4 degrees. That could reduce the number of heat-related premature deaths by almost 40 percent, like the ones in the journal The Lancet presented study found.
The residents of cities in southern and eastern Europe, where it is particularly hot in summer, would particularly benefit. Extreme heat impairs health and increases mortality, writes the team led by Tamara Iungman from the Institute for Global Health in Barcelona, referring in particular to cardiovascular diseases. Older people and children are particularly at risk. Cities are considered heat islands because they heat up more in summer and cool down more slowly than surrounding rural areas.
Since the heat stress will increase due to global warming, experts have been advising for years to take countermeasures in urban planning, among other things by expanding the treetop cover to 30 percent in the residential areas of a city. Trees provide shade and moisture.
In 2015, around 6,700 deaths in the cities surveyed were due to the heat island effect
In order to assess the consequences of this measure, the team evaluated data from 93 European cities, including Berlin, Hamburg, Munich, Cologne, Düsseldorf, Frankfurt and Leipzig. From June to August 2015, the researchers examined temperatures and death dates for each city. They also calculated the cooling effect of expanding tree cover and the consequences for mortality.
Accordingly, the cities analyzed were on average 1.5 degrees warmer than their surroundings in the summer of 2015, with the difference being significantly greater during the day than at night. The heat island effect was strongest in Cluj-Napoca, the second largest city in Romania: It was 4.1 degrees warmer here than in the rural area.
For the summer of 2015, the team attributed around 6,700 premature deaths to the heat island effect – in a total population of 58 million people aged 20 and over. 2,644 of them, a good 40 percent, could have been avoided if trees on 30 percent of the area had softened the summer climate. This is also an average value: the researchers found hardly any effect for northern regions such as Sweden, the Baltic States or northern England. In contrast, the effect was even more pronounced in the cities of southern and eastern Europe, where it is hotter in summer and tend to be more densely built-up.
For German cities, the team found a moderate impact. It puts the heat island effect for Hamburg – where the treetop cover is currently just under 24 percent – at a good 0.8 degrees. In Munich, with a tree canopy coverage of a good 20 percent, it was a good 1.2 degrees. However, it is not just about the absolute tree area in a city, says study leader Mark Nieuwenhuijsen, but about a fair distribution across all residential areas. “We encourage urban planners and policy makers to implement green infrastructure adapted to the local environment and combine it with other measures to maximize health benefits.”
These include the use of other surface materials such as granite instead of asphalt, a lower building density and the greening of roofs and facades. The study does contain weaknesses, write experts in an accompanying commentary The Lancet, but the results are valid enough to be implemented immediately in urban planning. In addition, such analyzes are also urgently needed for the large metropolitan areas in China and India or in Africa.