Compared to the countryside, cities are usually a few degrees warmer because of the urban heat island effect. This effect is caused partly by the absence of moisture and vegetation in cities compared with rural settings, and because urban building materials absorb heat. During heatwaves, daytime temperatures can increase treacherously high in cities, resulting in severe health effects and increasing danger of mortality.
The concept of “cool” roofs is to manufacture roof surfaces more reflective to sunlight (for instance by coating roofs with a lighter paint color), thereby decreasing local temperatures.
Researchers used a regional weather model to study at how temperatures varied across the study city of Birmingham and the West Midlands, based on the extent of cool roof deployment. They examined the hot summers of 2003 and 2006, and learned that the intensity of the urban heat island (the urban-rural temperature variance) touched 9 °C for Birmingham city.
Earlier work has revealed that the additional heat linked with the urban heat island is accountable for about 40-50% of heat connected with the mortality in the West Midlands during heatwaves.
This newest study, reported in Environment International, proposes that implementing cool roofs throughout the city can lower peak daytime local temperatures by nearly 3 °C during a heatwave. This decrease in temperature could possibly offset about 25% of the heat-related mortality connected with the urban heat island during a heatwave.
The urban heat island effect is most distinct at night time since urban materials gradually release their stored heat overnight; however, the major benefits of cool roofs were witnessed to be during the hottest times of the day where sunlight was reflected away. The type of building made a difference too: altering only half of all the industrial and commercial buildings had the same impact on reducing temperatures as altering all the high-intensity residential buildings.
Climate change and increasing urbanization mean that future populations are likely to be at increased risk of overheating in cities, although building and city scale interventions have the potential to reduce this risk. Modelling studies like this one can help to determine the most effective methods to implement in order to reduce health risks in our cities in the future.
Dr Clare Heaviside, Study Co-Author, Environmental Change Institute, University of Oxford.