Cool roofs reduce urban heat; green ones, not so much

Creating light-coloured, reflective “cool roofs” in urban areas could bring down temperatures in cities and save lives during heatwaves, a new British modelling study has found.

The report, published in the journal Environment International, notes that an August 2003 heatwave in Europe was thought to be responsible for 70,000 deaths. The figure was cited in a study from the French online journal Comptes Rendus Biologies. 

Multiple studies have concluded that climate change is expected to drive an increase in the frequency, intensity and duration of heatwaves.

Clare Heaviside, from the University of Oxford’s Environmental Change Institute, a co-author of the new report, says, “Climate change and increasing urbanisation 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.”

She adds that modelling studies such as this one can help find effective methods to reduce urban health risks in the future.

The researchers analysed heat-related mortality statistics from Britain’s West Midlands region and the city of Birmingham from the summer of 2006, along with two shorter heatwave periods in 2006 and 2003, playing particular attention to the “urban heat island (UHI) effect”, in which, in built up areas, temperatures are commonly observed to be higher than those in surrounding rural areas. 

The report cites a 2014 United Nations study that found about 54% of the world’s population currently lives in urban areas. The figure is projected to increase to 66% by 2050.{%recommended 7406%}

In Britain, where 82% of the population reportedly lives in urban areas, urbanisation has led to environmental changes. Forests, wetlands and cropland have been progressively replaced by urban surfaces such as buildings, roads, and other paved areas – all of which absorb and retain more heat from the sun than does vegetation. 

Energy use in cities, such as power for lighting, heating or cooling, can also add heat to the surroundings. 

“All these factors can lead to higher air temperatures in cities, an effect known as the urban heat island,” the researchers write.

The term “cool roof” denotes one that reflects sunlight, thereby reducing local temperatures. This can be achieved by a simple low-tech fix – painting it a lighter colour. The effect is cumulative: the more cool roofs, the greater the albedo, and thus the more solar radiation reflected rather than absorbed.

This can potentially reduce urban temperatures, the researchers say, and may also lower building energy consumption for cooling, both of which are projected to increase in future due to a combination of rising temperature and increasing incomes. 

The researchers found that over a summer season, the difference between simulated urban and rural temperature was 1.1 degrees Celsius on average, and 1.8  degrees when including only night times. The maximum difference was nine degrees.

“Our results suggest that the UHI contributes up to 40% of heat-related mortality over the summer period and that cool roofs implemented across the whole city [of Birmingham] could potentially offset 18% of seasonal heat-related mortality associated with the UHI,” the researchers conclude.

The figure corresponds to 7% of the total heat-related mortality recorded during the study period.

During heatwaves, modelling suggested that cool roofs could reduce city-centre daytime air temperature by 0.5 degrees on average, up to a maximum of about three  degrees. 

The researchers say cool roofs reduced average UHI intensity by about 23%, and reduced heat-related mortality associated with the UHI by about 25%, during a heatwave. They also found that temperature reductions varied according to the type of buildings on which cool roofs were applied. 

Targeting only commercial and industrial buildings contributed more than half of the reduction for heatwave periods. 

“Our modelling suggested that modifying half of all industrial/commercial urban buildings could have the same impact as modifying all high-intensity residential buildings in the West Midlands,” the researchers note.

Heaviside and colleagues also found that green roofs – surfaces covered in vegetation – “cannot match the hazard reduction potential of cool roofs unless they are well irrigated, but irrigation may place increased demand on water resources, which are likely to already be stressed in hot areas”.