Repainting surfaces and structures in white is often presented as an effective mechanism to counteract heat waves in urban areas. Recent studies have confirmed its benefits, showing that it can reduce surrounding temperatures by up to two degrees Celsius. In any case, researchers from University College London (UCL) found that white paint is more effective than other alternatives like solar panels placed on roofs to provide shade and absorb solar radiation or boosting vegetation cover.
The scientists conducted their study, published in the journal Geophysical Research Letters, using three-dimensional climate models based on data from summer 2018, the hottest summer on record for the British capital. The models indicated that if widely adopted throughout London, white-painted or reflective (also known as "cool roofs") surfaces could lower external temperatures in the city by an average of 1.2 degrees, reaching as much as two degrees in some areas.
The principal investigator, Dr. Oscar Bruce, explained that "cool roofs are the best solution to keep temperatures low on the hottest summer days, and while there are other methods with similar beneficial effects, none have matched the same level of heat reduction."
- **Better than Boosting Vegetation**
The capacity of solar panels or greening streets to reduce heat does not typically exceed an average of 0.3 degrees Celsius. Similarly, enhancing vegetation on rooftops, while beneficial for stormwater management and biodiversity, "does not have a significant impact" on heat reduction, according to the study authors. Additionally, air conditioning, which transfers heat from inside buildings to the outside, can raise overall city temperatures by about 0.15 degrees, and this increase can reach one degree in central London, contributing to the formation of urban heat islands.
The study emphasizes that "by reflecting rather than absorbing heat, cool roofs have a dual advantage in reducing temperatures, not only in the external urban environment but also inside buildings."
- **Repainting All Areas?**
In another study dating back to March, conducted under real conditions in a region of Singapore where roofs, walls, and road surfaces were repainted white, it was found that overall temperatures could drop by up to two degrees in the afternoon, with a noticeable decrease in temperatures experienced by pedestrians of up to 1.5 degrees Celsius. This is attributed to the "whiteness effect" (or albedo), where lighter surfaces exhibit a greater ability to reflect light and, consequently, heat. For example, a white roof made of thermal plastic can reflect 80% of sunlight.
These methods, recommended by the Intergovernmental Panel on Climate Change and tested in various Mediterranean countries such as Greece, are currently under examination in cities that were previously temperate but are increasingly facing heat waves.
Can a complete repainting of cities in white provide a solution for reducing temperatures? The answer remains relative. In cities that have opted to cover buildings and also paint the road surfaces in pure white, the results have been mixed.
For instance, in Los Angeles, where part of the sidewalks was repainted white in 2017, pedestrians reported feeling warmer, as the temperature only dropped at ground level and not at the level of individuals, alongside visual fatigue from looking at the white surfaces. Similarly, the city of Lyon, France, concluded a two-year trial of white-painted streets last August, citing high costs and rapid soiling of the streets.
In contrast, when white is used only on building rooftops, the results have been promising. In the French city of Grenoble, after repainting the "Bivorck" social hall white, the result was positive with a four degrees Celsius temperature drop, according to an independent study. In Tremblay-en-France, white paint on the roof of the "Jean Guimier" hall resulted in a five degrees Celsius temperature reduction.