New Arup research looks at hot and cool spots within Cairo, London, Los Angeles, Madrid, Mumbai and New York as design takes heat toll in city centers.
A new report by Arup has deployed innovative urban heat snapshot methods to map the parts of city centers most and lead affected by the warming effects of urban design.
Why are cities hotter?
Climate change is making many cities dangerously hot, with the number of cities exposed to extreme temperatures of 35°C and above expected to triple by 2050. This is in part because cities have been designed – inadvertently – to be hotter.
By pushing out nature, concreting streets and building high-rise structures with steel and glass, many of the natural cooling effects of nature are lost and heat is reflected and amplified. That makes the built environment a huge contributor to the build-up of heat in cities compared to their rural surroundings.
This is known as the urban heat island (UHI) effect but not everyone is impacted equally by heat. Some parts of a city will be more severely effected and it is often the most vulnerable who are most impacted by these urban heat island hot spots. Income inequality can play a role in the ability for people to be able to cool themselves, as well as being less likely to have greenery and shade from trees to keep their streets cool.
The concern that rising heatwaves are causing greater fatalities and other health problems, some cities have now taken to naming heatwaves to build awareness. The appointment of Chief Heat Officers is also helping to drive resilience planning but without good data, it can be hard to direct efforts as effectively as needed.
Cities have large variations within their centers
Arup have used their new UHeat data tool to analyse cities, and the assessments of six cities have shown remarkable variations in urban heat within and between cities.
Of the six cities assessed, Madrid proved to have the largest urban heat island effect of 8.5C, while Mumbai had the second highest UHI at 7C. New York and London had the lowest UHI, both being an average 4.5C warmer than surrounding countryside. Los Angeles and Cairo both had a UHI of 5C.
Each of these cities also saw significant differences between districts, some of which were significantly cooler than the city average and other significantly hotter. This includes in Madrid where Plaza Juan Pujol was 8C hotter than North of Casa de Campo.
Why does this matter?
The ability to assess in detail the UHI effect at a granular level not only helps to identify where action is most needed, but also what action is most likely to work in different districts by linking with other data.
For example, water and vegetation are known to have significant cooling effects on the urban environment, but the warmest and coolest districts in London had the same low levels of water. The significant difference between them was vegetation, with twice as much of it in the coolest district.
In a reverse of that, Mumbai’s hottest district Ghatkopar East had almost twice the vegetation levels of the city’s coolest district – Maharashtra Nature Park. But Ghatkopar East had no environmental water, which contrasted with Maharashtra Nature Park’s 74% water, suggesting water could have a more significant effect in the city.
It is hoped that such analysis might help to direct efforts of urban planners and policy-makers to help tackle urban heat islands as cities around the world face rising temperatures.
