Heat-related deaths around the world are climbing up as more frequent, longer-lasting extreme heatwaves become a harsh reality due to climate change. Heat deaths are projected to increase by 370 percent if global warming continues at its current rate.

This heating effect is more pronounced in dense cities because of the urban heat island effect. But reflecting sunlight back into the sky using the technology found in reflective road signs and bike reflectors could help cool cities down, according to Princeton University engineers.

The technology, called retroreflectors, use a combination of prisms and reflective materials to create a marker that is visible from long distances, even in low-light conditions.

Equipping building walls and roadways with these retroreflective materials could reduce urban temperatures by up to 20°C, they report in the journal Nature Cities. It should also notably improve air temperatures and human skin temperatures.

“More people die from extreme heat in the U.S. than from any other weather-related event,” said Elie Bou-Zeid, professor of civil and environmental engineering. “There’s an urgent need to develop and deploy technologies that can help people stay cool.”

Cities are often several degrees warmer than surrounding rural areas. That’s because building materials such as asphalt and concrete absorb more heat than vegetation. Increasing the green spaces in cities in a well-planned way or making green roofs can help cool cities down. Another solution is to paint pavements and rooftops with reflective white paints. But the solar radiation can scatter in any direction, so painting walls and narrow streets in dense urban areas might not work, and could even make the heat effect worse if the radiation bounces around in the urban canyon.

Retroreflective materials, on the other hand, reflect light with minimal scattering, sending it back in the same direction it came from, so it would go out of the urban canyon. The materials can be made as coatings or sheets, and would be a low-cost and highly effective way to beat urban heat, the Princeton team says.

In the new paper, the Princeton researchers created the best retroreflector designs for various climate conditions, latitudes, seasons and urban geometries. Policymakers and urban planners should be able to use the guidelines to pick the best retroreflectors based on the latitude of a city, the aspect ratio and the orientation of streets, and should be able to calculate the cooling benefits of using the materials.

“On the basis of our analysis, we recommend retroreflective pavements for open, low-rise areas and propose specific retroreflective wall design strategies for compact, high-rise areas,” they write.

Source: Xinjie Huang et al. Optimizing retro-reflective surfaces to untrap radiation and cool cities, Nature Cities, 2024.