Covered in heat-trapping asphalt and concrete, cities can become urban heat islands that can stay hot even into the late-night hours. According to some studies, the heat-stress index for many European cities will double by 2050, when compared with adjacent rural areas. By 2100, some cities may even see a temperature increase of 8 degrees Celsius.
While adding more trees and creating larger areas of shade seems like a perfect way to counteract the rising temperatures associated with climate change, it’s important for urban planners to understand potential health impacts and learn what, where, and how to plant.
“Greening up cities is not really a straightforward measure,” said Galina Churkina, a researcher with the Institute for Advanced Sustainability Studies in Potsdam. “If one is not careful enough with the plants, you can get other effects that you do not expect.”
According to a recent study in Berlin, during an intense heatwave in 2006, trees contributed up to 60 percent of the ozone formation. Certain trees, like plane and poplar trees, can generate isoprene, an organic molecule with natural rubber compounds that can vaporize easily in the heat. However, when combined with nitrogen oxides from the fumes from cars and factories, this can become ozone.
During heatwaves, Churkina said, ozone is one of the most direct threats to urban populations, particularly for infants and elderly people. Unless we can combine greening efforts with cuts to emissions from both industry and traffic, the results will be less than desirable.
Dense canopies can also spell trouble for urban areas since taller trees can block the breezes that help cool the air and dissipate any noxious fumes. Along narrow streets, planners might be better off incorporating “living walls” or low hedges.
According to Paris’ Chief Resilience Officer Seb Maire, another concern planners will need to consider is the growing risk of insect-transmitted diseases like dengue fever and malaria – which rises with the temperature. City-dwellers can help decrease some of these risks by eliminating potential mosquito breeding grounds, like pools of water that can accumulate in plant pots.
“If we have a mosquito with a deadly disease in ten years, we won’t care about being hot,” he said.
Maire said cities still have “a real opportunity” to make these improvements, especially since much of the infrastructure that will exist 50 years from now hasn’t even been constructed. Detailed climate models are helping scientists develop tactics to mitigate some of these negative impacts.
According to urban climate scientists in Vienna, if all suitable roofs were covered in plants or another material that could reflect at least 70 percent of incoming solar radiation, the city could see a 29 percent reduction in the number of days exceeding 30 degrees Celsius.
This, along with other measures currently being investigated by scientists, could allow for the maintenance and improvement of quality of life for urban dwellers, said Maja Zuvela-Aloise, with Austria’s Central Institution for Meteorology and Geodynamics – even in the face of global warming.
“There’s a lot of potential for mitigation,” she added.