Gabriele Manoli envisages natural ways of cooling cities

More green spaces can help lower temperatures in urban zones – but not everywhere. The effectiveness of heat-reduction strategies in cities varies depending on the regional climate. Through carefully targeted planting, a city like Phoenix in the USA could achieve cooler temperatures than the surrounding countryside, where conditions are almost desert-like. By comparison, a city surrounded by tropical forests, such as Singapore, would need far more green spaces to reduce temperatures, but this would also create more humidity. In cities located in tropical zones, other cooling methods are therefore expected to be more effective, such as increased wind circulation, more use of shade and new heat-dispersing materials. These are the findings of a study led by Branco Weiss Fellow Gabriele Manoli, published recently in Nature.

Gabriele Manoli and his colleagues from ETH Zurich, Princeton University and Duke University studied data from some 30,000 cities worldwide and their surrounding environment, taking into consideration the average summer temperature, the population size and the annual rainfall. “We already know that plants create a more pleasant environment in a city, but we wanted to quantify how many green spaces are actually needed to produce a significant cooling effect,” he says. “There is no single solution. It all depends on the surrounding environment and regional climate characteristics.”

The main benefit of the study is a preliminary classification of cities, in the form of a clear visualisation guiding planners on possible approaches to mitigate the urban heat island effect. Manoli is currently analysing data from other periods of the year and is studying which types of plant are most suitable for reducing temperatures.

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