An extensive analysis of the factors affecting temperatures in urban heat islands (UHIs) in thousands of cities worldwide has been done by Gabriele Manoli and colleagues at ETH Zurich, Princeton University and Duke University. The team discovered that more green urban spaces can lessen the intensity of UHIs in drier climates, but planting trees is less effective in wetter, tropical environments. As climate change raises temperatures globally, the work provides crucial insights into how cities can mitigate local heating.
A UHI is a much-studied and common effect that occurs when a city is significantly warmer than its rural surroundings. This heating can lead to dangerously high temperatures that can harm the health of urban dwellers – particularly infants, the elderly, and the chronically ill – so predicting the intensity of UHIs is very important.
Previous research has shown that UHI intensity can be correlated with a city’s population and the amount of rain it receives. However, urban areas are so diverse and complex that a universal model that can predict heating in a specific city has remained elusive. This has made it difficult to develop strategies for mitigating the heating effect.
Urban textures
To meet this challenge, Manoli’s team developed simplified models to link summertime UHI intensity with the population and annual precipitation of about 30,000 cities around the globe. The researchers derived universal laws that considered urban populations, city infrastructure sizes and socioeconomic factors. This approach accounts for the fact that as a city grows, its structure and function are modified. Different building materials are used, for example, and human activity and energy consumption increase. The researchers also accounted for the “texture” of cities. This describes the height and densities of buildings, which affects the reflection of sunlight and convection of heat.
The modelling revealed that UHIs tend to be more intense in larger cities with more rainfall. In a wet region, the countryside surrounding a city will support more plant life. This reduces the temperature of the countryside compared to the city, thereby boosting the UHI.
Will hotter temperatures reduce urban heat island intensity?
However, the rainfall correlation does not extend beyond an annual precipitation of about 1500 mm (the annual figure for Tokyo, for example). This suggests that cities in drier climates can reduce their UHI intensities by expanding green spaces. This bodes well for London (about 600 mm), which plans to do just that. However, this is a less effective strategy in humid tropical cities like Singapore (2100 mm). In that city, far more green spaces would be needed to have a significant effect, highlighting the need for other cooling mechanisms.
The work of Manoli and colleagues provides urban planners with new guidance for future developments; particularly those in developing countries, where cities have seen far less UHI research. In rapidly expanding cities in tropical regions like Africa and South Asia, their findings suggest a need to increase wind dispersion and shade, while using heat-dispersing building materials which better reflect sunlight. As the global climate heats, the discoveries reveal important insights into how the worst effects of increasingly stifling urban heat can be avoided.
The research is described in Nature.
