Could cool roofs or solar panels have saved lives in London's hottest year?

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As the planet warms, urban areas are emerging as critical environments with unique characteristics that can both help and hinder efforts to mitigate the effects of climate change. Most people now live in cities, but urban areas tend to be warmer than the surrounding landscape because of a phenomenon known as the “urban heat island effect.” It’s an issue that’s going to become more serious as stronger and more frequent heatwaves are likely around the world because of rising global temperatures. Even in temperate climates like the UK, people are not well prepared or adapted for hot weather so heatwaves can cause health problems and early deaths, particularly in cities. Facing this morbid reality, a wide range of solutions will be needed to mitigate the health burden related to heat in urban environments.

One way to reduce urban heat is by reflecting away more of the incoming radiant energy from the Sun. Using more reflective and lighter coloured materials in will reflect part of that heating energy back into space. Rooftops made of special reflective coatings – known as “cool roofs” –absorb less heat than conventional asphalted or tiled ones. Solar panels are mainly made for generating electricity, but can also change the heat energy that roofs absorb from the Sun. In both cases, the energy balance of the cities is changed, reducing the amount of heat that is further released by the built-up environment.

To test how the two technologies could protect against urban heating in Greater London, we developed a model of the buildings and atmosphere representing how cities impact the local climate, then tested different scenarios. One scenario consisted of deploying cool roofs over all buildings in London, the other had all rooftops with solar panels. Both were compared to typical roof materials in London today. As a benchmark of potential future heatwaves, we used the summer of 2018, the year with the highest average temperature in London and across the UK.

Schematic showing how urban heat interventions were analysed.

According to our model, cool roofs cooled the air at street level by an average of 0.8 °C over the whole summer while rooftop solar cooled it by 0.3 °C. According to our statistical estimates, such a temperature reduction from cool roofs could save 249 out of the estimated 786 heat-related deaths in London, or 96 in the case of solar panels. This means that applying reflective surfaces on roofs could reduce heat-mortality in London by about a third under current exceptionally hot summers.

This work could influence how cool roofs and solar panels are supported by government policy in the UK. When avoided deaths are used to inform public policy, they are often quantified in economic terms by a "statistical value of life". In this case we found that over the summer of 2018, the cool roofs could have had a value of statistical life of £615 million, or the solar panels £237 million.

Could this ever be implemented over the whole of London? Practically speaking, it would be hard to change all the roofs like in these scenarios. Nevertheless, our study provides an upper estimate of what could be achieved by deploying those rooftop interventions. Also, while the physics model we used is plausible and accurate, other models, other weather, or other cities might see different results; so, we have to be cautious generalizing these results. Finally, cool roofs might increase the amount of heating needed in winter, which also affects people’s health: summer extreme heat is not the whole story.

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