Due to their evaporation, green spaces with trees have a greater cooling effect in cities than green spaces without trees. This was shown in a study by a team led by the German geoecologist Jonas Schwaab from the University of ETH Zurich. In urban green spaces with lawns, meadows or flowers, the cooling effect is two to four times as low as in areas with trees. “During the day, parks with trees across Europe have a significantly higher cooling effect than parks without trees,” said Schwaab.
Unlike other studies on the influence of vegetation on temperature, the researchers looked at a large region. They evaluated data on 293 European cities in order to see the influence of vegetation on temperatures in different latitudes. These included 36 German cities from Lüneburg to Passau. The surface temperature measurements were taken from satellites. The study appeared in the journal Nature Communications.
The difference between green spaces with and without trees can therefore be clearly seen in all European regions. One of the reasons for this is that deep roots allow trees to absorb more water that can evaporate. Especially during hot and dry periods, trees have a greater cooling effect than green areas without trees.
The temperature difference is particularly large between tree areas and built-up areas. In Central Europe, for example in Germany, Switzerland, Austria, the Netherlands or Belgium, areas overgrown with trees are eight to twelve degrees cooler, said Schwaab. He emphasizes that these are surface temperatures; in air close to the ground, the temperature difference is significantly smaller.
In southern Europe, the difference between tree areas and built-up areas is smaller, says Schwaab. One of the reasons for this is that the soils there are less moist than in Central Europe and that trees tend to evaporate less as a result. Trees in more southerly climes have another important cooling effect: they provide shade, says Schwaab. And with irrigation, the cooling effect through evaporation is often great in southern regions.
The study is not suitable for comparing cities and the influence of greenery on temperatures, says Schwaab. For this, more precise measurements would have to be made in individual city districts. According to the study, the underlying information about the tree population did not include individual rows of trees or scattered trees. The satellite measurements were also practically only carried out under a cloudless sky. For more precise information on individual cities, it must also be taken into account that trees are not entirely useful for the urban climate, said Schwaab. For example, trees in the wrong place could reduce the supply of cold air in cities. The cooling effects would have to be investigated further, write the researchers.