Is applying heat-reflective paint to every building and sidewalk in a city a cool solution?

Heat-reflective coatings (cool paints in short) are proposed as a fast-response measure to mitigate urban warming by increasing the albedo of roofs, facades, and roadways, and thereby lowering air temperature. Small-scale field studies and numerical simulations covering entire cities have shown promising results. However, holistic assessments that consider other aspects of the urban ecosystem are missing. Increased reflected solar radiation could reduce thermal comfort and increase glare. While the urban heat island effect will decrease, local micrometeorology will be affected, and along with a potential increase in biogenic and evaporative emissions of reactive chemical species, air quality could deteriorate. Similarly, reducing cooling demand could save some carbon dioxide emissions, but it could also lead to higher temperatures and prolonged dry spells in adjacent regions. These and other potentially unintended consequences of the widespread use of heat-reflective paints are examined in the context of Singapore, where all public housing buildings will be coated with them by 2030. Aiming to maximize the thermal benefits of applying cool paints to selected surfaces, assessments that include not only variations in surface and air temperatures but also changes in the energy balance partitioning, air quality, and thermal comfort, derived from both direct measurements and numerical simulations, are suggested.