Study results show that white roofs can worsen heat in neighboring areas

Painting roofs white to reflect sunlight is being touted as a promising solution to reducing the “heat island effect” in cities. However, according to a new study, this practice may lead to worse conditions in surrounding regions.

The researchers say their study “challenges conventional wisdom” by showing how the practice “can actually cause temperatures to rise, rather than fall as intended, due to its unintended effects on rainfall.”

Painting roofs and other surfaces in urban areas is part of a broader practice called light radiative management (LRM), a type of geoengineering designed to reduce an area's temperatures by controlling its surface's “albedo,” or the amount of reflected light radiation is increased. LRM is known for its potential to combat rising temperatures associated with climate change by lowering temperatures in a local environment – for example, by applying white paint in built-up areas, although it could also be applicable to farmland and other environments.

In one example, 17,000 homes in Ahmedabad, India, were painted white as part of a “low-cost measure” to reflect heat back into the atmosphere.

However, so far these practices have not been used on a large scale. The current study used models to determine what would happen if LRM were implemented on a “mesoscale” area between one and ten square kilometers.

The results suggest that there may be unintended disadvantages due to the effects of LRM on precipitation. LRM reduces precipitation and also soil moisture, which is important for evaporative cooling, in the region where it is used. The reflected light radiation in this area still reduces the temperature – but since the effect depends on the stationary land surface, the cooling stops abruptly at the LRM limit.

By comparison, the LRM influence on precipitation occurs in the atmosphere, which is “a turbulent, well-mixed fluid,” so the reduced precipitation extends beyond the boundary of the LRM region and overlaps with the region that is not benefits from the cooling. The temperature in this limit region therefore increases without the cooling influence of LRM or soil moisture evaporation.

This effect could have a significant impact, even if it only occurs in a band around the area. LRM over a 10 square kilometer region would result in an affected kilometer-wide border covering an area of ​​44 square kilometers. “With a population density equivalent to that of San Francisco, this would mean that approximately 300,000 people would be exposed to warming caused by LRM,” the study says.

The impacts may be increased if LRM is implemented near a vulnerable neighboring area. For example, a wealthy area that can better afford LRM might cool at the expense of a rise in temperatures in adjacent areas, so that “its neighbors could experience warming that worsens heat inequality.”

However, the researchers say other factors not accounted for in their results could alter the impact of LRM on nearby regions. Other studies have shown that rough surfaces – like those of cities – can attract rain. An LRM region that is systematically harsher than its surroundings may be able to counteract the declines in precipitation and thus the warming effect on its surroundings.

The study also describes how an LRM area of ​​sufficient size could create a sufficiently large benefit that could outweigh the negative outcomes, since the surrounding area is comparatively much smaller.

The study concludes that LRM must be implemented carefully and with consideration of the environment and is not a one-size-fits-all solution to rising temperatures in cities.