A new study by researchers at the University College of London discovered that painting roofs white or covering them with a reflective coating would be more effective at cooling cities like London than vegetation-covered “green roofs,” street-level vegetation, or solar panels.
The findings suggest that widespread use of air conditioning in London’s dense city center could increase the outside temperature by up to 1 degree Celsius. The research utilized a three-dimensional urban climate model of Greater London to assess the impact of various passive and active urban heat management systems during the hottest days of the summer of 2018.
Cool roofs could potentially reduce outdoor temperatures by an average of 1.2 degrees Celsius across the city, with some locations experiencing a decrease of up to 2 degrees Celsius. While other systems, such as street-level vegetation and solar panels, may provide less significant cooling effects, they offer additional environmental benefits.
Despite the negligible net cooling effect of green roofs on the city, they still offer advantages such as water drainage and wildlife habitats.
The utilization of air conditioning, which displaces heat from indoors to outdoors, is projected to raise the outdoor urban temperature by approximately 0.15 degrees C citywide and up to 1 degree C in densely populated central London. Additionally, the researchers identified that the expanded use of air conditioning units could be sustainably powered by the widespread implementation of photovoltaic solar panels.
To fully assess the potential impact of each approach, the team simulated widespread adoption across residential, commercial, and industrial buildings throughout Greater London.
“We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures, and found that cool roofs were the best way to keep temperatures down during extremely hot summer days,” lead author Dr Oscar Brousse (UCL Bartlett School Environment, Energy & Resources) said. “Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”
It is essential to address the impact of warming temperatures in cities, which make residents more vulnerable. The Urban Heat Island effect can lead to discomfort and increased mortality during hot spells. City planners and designers are prioritizing the exploration of passive cooling methods like cool roofs, green roofs, and expanded urban vegetation, as well as active methods such as air conditioning for building interiors.
The study provides valuable insights into urban heat management by comparing various passive and active heat mitigation actions. Cool roofs, for example, not only cool the outside urban environment but also contribute to cooling the inside of buildings by reflecting rather than absorbing heat.
The impact of green roofs on urban temperatures varies significantly throughout the day. They can potentially lower daytime temperatures by an average of 0.5 degrees Celsius, providing much-needed relief during the warmest times. However, the roofs retain heat overnight, offsetting the daytime cooling effect.
Conversely, replacing grass with deciduous tree cover in city greenspaces can lower nighttime temperatures but may have mixed effects during the day and could increase air humidity, potentially influencing residents’ comfort.
Journal reference:
O. Brousse, C. Simpson, A. Zonato, A. Martilli, J. Taylor, M. Davies, C. Heaviside. Cool Roofs Could Be Most Effective at Reducing Outdoor Urban Temperatures in London (United Kingdom) Compared With Other Roof Top and Vegetation Interventions: A Mesoscale Urban Climate Modeling Study. Geophysical Research Letters, 2024; DOI: 10.1029/2024GL109634
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