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So-called “urban greening” is being touted as a way of combating climate change and preventing the world from an environmental doomsday. However, a team of scientists led by Cardiff University researchers has shown the majority of cities around the world will not be able to reduce instances of heatwaves and flooding at the same time through such methods.
Consequently, flood protection was likely to be more successful in arid environments, whilst a cooling effect was more likely in more humid climates.
Lead author Dr Mark Cuthbert, from Cardiff University’s School of Earth and Environmental Sciences, said: “Our research found that the ability of urban greening to mitigate local flooding and excess heat is not automatic nor, in some areas, even possible.”
Urban areas each have unique climates which pose significant risks, even more so as climate change increases the likelihood and severity of extreme weather events.
Heatwaves within the world’s cities can be attributed to the urban heat island effect (UHI), which is caused by the predominance of concrete and steel, both of which absorbs and retains heat, coupled with a lack of cooling by water evaporating from plants.
Flooding is part of the urban stream syndrome (USS), whereby city structures and systems negatively affect the natural runoff of rainwater back into the environment.
Urban greening can reduce the UHI and USS effects in cities, as well as supporting local wildlife, reducing pollution and improving the general wellbeing of local populations.
In their study, the team used global climate model outputs and weather information from 175 cities around the world spanning 15 years of daily observations, from 2000 to 2015.
This data was used in conjunction with theories taken from soil science to calculate water infiltration into soils, which act like a sponge to reduce rainwater runoff, and the evaporation of water from plants, which can induce the desired cooling effect.
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Dr Cuthbert added: “Local and regional climatic conditions significantly impact the capacity of urban soils and plant growth to simultaneously defend against flooding and extreme heating.
“In fact, our findings indicate that in many, possibly the majority, of global cities, urban greening will not be able to mitigate cooling and flooding at the same time.”
The team also found increasing variability in rainfall patterns due to climate change may reduce the performance of thinner green structures, such as green roofs, more quickly compared to larger greened areas with thicker soils and root systems.
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Such considerations must be taken into account by urban planners in order to find the best solution for each individual city, with a balance needed between performance, cost and viability, the study argued.
Dr Cuthbert said: “While urban greening may not be a panacea, our results show what’s possible in designing the cities of the future.”
The research was led by Cardiff University in conjunction with scientists at the University of New South Wales, Karlsruhe Institute of Technology and Nottingham Trent University.
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