Cool roofs could have saved lives during London's hottest summer, say researchers

The paper, published in Nature Cities, analysed the cooling effect that roofs painted white or other reflective colours would have on London's ambient temperature between June and August 2018, the city's hottest summer. From June through August, the average temperature around London was 19.2 degrees C, about 1.6 degrees warmer than average for that time of year.

Urban environments tend to absorb a lot of heat and are usually a few degrees warmer than the surrounding region, an effect known as the 'urban heat island'. Painting roofs white or reflective colours would absorb less radiant energy from the Sun than traditional dark roofs, effectively cooling the city.

The researchers found that had cool, light-coloured roofs been widely installed throughout London, it could have cooled the city by about 0.8 degrees C on average, preventing the heat-related deaths of an estimated 249 people -- equating to around 32% of the 786 heat-related deaths during that period.

In the same paper, the researchers also found that had rooftop photovoltaic solar panels been similarly widely adopted, they would also have cooled the city by about 0.3 degrees C. This would have prevented the deaths of an estimated 96 people across the city, or 12% of the heat-related deaths during that summer.

The researchers used a complex 3D computer model to simulate the outcomes of different urban environments. They calculated what the average urban temperatures were during the hot 2018 summer (cross-checking it against actual measurements from the time) and then compared the temperature differences if all roofs in London were given a reflective coating, if all roofs were covered in rooftop solar panels and what the temperature of a hypothetical non-urbanised London would be.

The team also estimated the economic impact of the increased mortality rates of the two scenarios. The 96 lives saved by the adoption of rooftop solar panels would have reduced the economic burden on the city by about £237 million, while the 249 lives saved by adopting cool roofs would have reduced the city's economic burden by about £615 million.

In addition, had rooftop solar panels been widely installed, the researchers estimate that the total electricity that could have been produced during that three-month timeframe would have been as much as 20 terawatt-hour (TWh), more than half the energy usage of London during the entire year of 2018.

Lead author, Dr Charles Simpson (UCL Bartlett School Environment, Energy & Resources) said: "If widely adopted, cool roofs can significantly reduce the ground-level air temperature of a city. The resulting cooling effect across the city would save lives and improve the quality of life for residents throughout the urban area. Solar panels have great benefits as a source of renewable power, so it's good to see they won't make the city hotter."

Combating urban heat is growing in importance as the world continues to warm because of climate change. Though unusual at the time, hot summers like the one in 2018 are projected to occur more frequently because of the warming climate. In addition, the UK is particularly vulnerable to the effect as an estimated 83% of the country's population lives in urban areas.

Dr Simpson added: "As the effects of climate change manifest more and more, people living in cities will need to find new ways to adapt. Our research shows that cool roofs could be an effective way to mitigate the heat-trapping effects of urban environments."

Co-author Professor Tim Taylor of the University of Exeter said: "The need for our cities to adapt to climate change is clear. Changing our roof spaces offers one potential solution. We need to encourage action like this, to reduce the burden of excess heat on people living in urban areas and capture potential co-benefits, including energy generation."

Recent preliminary research by members of the team found that during the three hottest days of 2018, wide adoption of cool roofs would have lowered the city's average temperature by about 1.2 degrees C, while rooftop solar panels would have lowered the average temperature by about 0.3 degrees C. This new research extends those modelling efforts throughout the whole summer of 2018, the hottest on record for London.

The research developed as part of the HEROIC: Health and Economic impacts of Reducing Overheating in Cities project based at UCL and Exeter, and supported by Wellcome Trust and NERC.