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Explaining and Addressing Urban Heat Island


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While climate change will have a global impact on our overall climate, cities will suffer magnified consequences. This is caused by a phenomenon called the ‘Urban Heat Island’ (UHI) effect, where the temperature in a city is noticeably higher than in the surrounding rural area. UHIs are caused by built-up geometries and surfaces (typically made of asphalt, cement, bricks) that absorb and release heat differently than natural (vegetated) surfaces. Moreover, people, cars, and buildings generate additional heat making cities warmer with increasing human activities. When combined with the sort of heatwave that hit many parts of Europe at the start of July, UHIs can present a real threat to the elderly, sick or other vulnerable people.
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Today, 55% of the world’s population lives in cities, with projections showing that this will increase to 68% by 2050. As a result, mitigating the UHI effect will be critical to improving the quality of life of a significant portion of the world population. Scientists have long known that vegetation and green spaces can play a significant role in offsetting the UHI effects (e.g., transpiration by plants produces a cooling effect), but no attempts have yet been made to quantify how much vegetation is needed in different regions around the world.
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For the first time, this latest research provides a critical perspective on the relationship between the UHI effect, background climate, and green space development in cities. By analysing the UHI effect in more than 30,000 cities Dr. Gabriele Manoli (Chair of Hydrology and Water Resources Management, ETH Zurich) and colleagues were able to describe the magnitude of urban warming as a function of two key variables only: urban population and precipitation. The new model provides a mechanistic understanding of the processes leading to the formation of UHIs globally and, in return, could help to guide urban planning strategies to offset this effect.
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The first observation made by the authors shows that the relationship between the variables (population, precipitation and temperature in cities) is non-linear. As a general pattern, the intensity of UHIs increases as a city’s average annual precipitation increases because its surroundings become greener and cooler, but only up to a point. Beyond a precipitation level of about 1500 mm per year — similar to that of Tokyo — the magnitude of UHIs does not increase any further.
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Using data from two regions of the world, Europe and South-East Asia, the global model has been used to highlight how green cover could offset the temperature difference existing between cities and surrounding rural areas.

The model shows that in dry areas (e.g. Southern Europe) where annual precipitation is limited (< 700 mm. yr-1), 20-40% green cover could be enough to offset the UHI effect.

However, cities located in a hot and wet climate (e.g. South-East Asia) would require a far higher amount of green cover (up to 80% or more) to offset the urban heat island effect.
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In his most recent Environment Strategy the Mayor of London set a target to increase the city’s green cover by up to 50%. Based on the analysis, this represents an effective target which could reduce city warming. However, in a tropical city like Singapore a significantly higher proportion of the urban area would need to be replaced by green surfaces which, in practice, may be unrealistic. In these conditions vegetation may not generate enough cooling while increasing humidity (a factor that reduces pedestrian comfort), requiring a more systemic change to the way the city is designed.

Vegetation strategies will need to be combined with other solutions such as increasing albedo (i.e. reflecting light away from surfaces) or convection efficiency (i.e. creating better air circulation). Nevertheless, the importance of green spaces in cities must not be underestimated, as other benefits can be drawn from them such as improving biodiversity, air quality and water filtration.
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This new, global model of the relationship between UHIs, background climate, and vegetation provides a simple tool to assess the effectiveness of vegetation as a mitigation strategy. Yet, more comprehensive guidelines and city-specific studies are needed to counteract the impacts of population growth and climate change in cities all over the world.
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Find the full paper here:

This work was done at the Chair of Hydrology and Water Resources Management (ETH Zurich) in collaboration with the Crowther Lab (ETH Zurich), ETH-FCL, Duke university, Princeton university and funded by the Branco Weiss Foundation.

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