In recent years we have repeatedly heard that planting trees is one of the solutions to combat climate change and rising temperatures in urban environments. Something that is quite necessary, since data indicates that in 93 European cities, the heat island effect was blamed for 4.33% of deaths in the summer of 2015. Thus, bringing the tree cover of our cities to 30% would cool the average temperature by about 0.4°C and prevent more than 2,600 premature deaths.
It’s not that simple. It may seem right to start planting trees like crazy, but the truth is that reality is much more complex than the simple sum of branches. A series of recent studies here warn that the actual performance of trees depends heavily on humidity, road shape and climate. And the most surprising thing is that under certain conditions the plant can create a feeling of excess heat.
A Spanish case. To understand how trees interact with the city under limited conditions, a team of researchers from ICTA-UAB and CIEMAT carried out a full simulation focused on Barcelona’s Eixample. Research, published Urban climateused a modified version of the BEP-tree model to deal with two different situations, namely Balmes Street with very few trees and Aribau Street with many and tall trees.
Results. The researchers were able to confirm that, when the soil is moist, the plants will be able to cool the environment with great efficiency. Numerically they found that air temperature could be reduced by an average of 1.2ºC and the thermal stress index could be reduced by 3.2ºC. And if we go to the street level, the temperature felt by a pedestrian walking on the street at the most important points has dropped to 12.3ºC.
Even when the soil dries out and the tree shuts off the flow of water to protect itself, the pure shade cast by its crown preserves between 71% and 97% of this cooling effect.
Bad news. Thermodynamics holds several surprises and the model reveals that, in a scenario with very dry soil during a heat wave, a road with dense trees has an air temperature of 0.3ºC higher than a road with almost no trees. And this is explained because in dense urban fabrics, such as the typical grids of many cities, excess tree mass can act as a “lid” that slows ventilation and traps heat radiated by buildings and asphalt.
To this we must add the sauna effect that occurs in the late afternoon on heat wave days, when trees can increase pedestrian thermal stress. And the reason is simple because the moisture that leaves release into the air, added to the high temperatures, reduces our ability to sweat and cool down.
A bath of reality. These conclusions align with what has been observed in other parts of the world, such as in Zurich. This case shows that, although watered trees relieve thermal stress on a typical summer day, even densely watered trees are not sufficient to alleviate human suffocation during extreme heat waves.
At the global level, the situation is not as good as advertised green wash. A study has been published the nature It assessed global coverage and concluded that, while current trees mitigate about half of the heat island effect, the actual benefits to humans are only 0.15 degrees Celsius. Furthermore, these climate benefits are deeply unequal, overwhelmingly concentrated in wealthy countries and privileged suburbs.
Picture | Malcon Oliveira
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