Painting roofs white to reflect sunlight has been touted as a promising solution to reduce the “heat island” effect in cities, but a new study says the practice can create worse conditions in surrounding regions.
Researchers say their study “challenges conventional wisdom” by showing how the practice “can actually cause temperatures to increase, rather than decrease as intended, due to its unintended impacts on rainfall.”
Painting roofs and other surfaces in urban areas is part of a broader practice called light radiative management (LRM), a kind of geoengineering used to decrease an area’s temperatures by increasing its surface “albedo,” or how much light radiation it reflects. LRM has been noted for its potential to fight rising temperatures linked to climate change by reducing temperatures in a local environment—like by applying white paint in built areas, though it could be applicable to farmland and other environments, too.
In one example, 17,000 homes in Ahmedabad, India, were painted white as part of a “low-cost effort” to reflect heat back into the atmosphere.
But these practices have so far not been applied on a large scale. The recent study used models to determine what would happen if LRM were implemented over a “mesoscale” area of between one and 10 square kilometres.
The results indicate there could be unintended downsides because of LRM’s effect on precipitation. LRM reduces precipitation, and also soil moisture that is important for evaporative cooling, within the region where it is implemented. The reflected light radiation in that area still lowers temperature—but because the effect is dependent on the immovable land surface, the cooling stops abruptly at the LRM boundary.
In comparison, the LRM influence on precipitation takes place in the atmosphere that is “a turbulent, well-mixed fluid,” so the reduced precipitation extends past the boundary of the LRM area and overlaps with the region that does not benefit from the cooling. The temperature in this bordering area therefore increases without the cooling influence of either LRM or soil moisture evaporation.
This effect could have a significant impact, even though it only occurs in a strip around the area. LRM over a 10-square-kilometre region would result in an affected kilometre-wide boundary with an area of 44 square kilometres. “For a population density equivalent to San Francisco’s, this would imply roughly 300,000 people would be subject to warming caused by LRM,” the study says.
The impact can be compounded if LRM is implemented near a vulnerable neighbouring area. For example: a wealthy area that is better able to afford LRM could cool itself down at the expense of raising temperatures in adjacent areas, so “their neighbours may experience warming, worsening heat inequality.”
But the researchers say other factors could change how LRM affects nearby regions, ones that are not accounted for in their results. Other studies have shown that rough surfaces—like those of cities—can attract rainfall. An LRM region that is systematically rougher than its surroundings might be able to counteract the precipitation reductions, and by extension the warming effect on its surroundings.
The study also details how an LRM area of sufficient size could create a large enough benefit that, because the surrounding area is comparatively much smaller, could outweigh negative outcomes.
The study concludes that LRM needs to be implemented carefully and in consideration of its surroundings, and is not a one-size-fits-all solution to rising city temperatures.
