Painting the roofs of buildings white may reduce the impact of the urban heat island effect by as much as 33 percent, significantly cooling off cities and helping society adjust to the changing climate, suggests a new study soon to be published in Geophysical Research Letters.

The study's authors used a newly developed computer model to simulate the amount of solar
radiation that is absorbed or reflected by urban surfaces. The model simulations, which provide scientists with an idealized view of different types of cities around the world, indicate that, if every roof were entirely painted white, the world's cities could cool by an average of about 0.4 degrees Celsius (0.7 degrees Fahrenheit), with the cooling influence being particularly   pronounced during the day, especially in summer.

In addition, white roofs would have the effect of cooling temperatures within buildings.
As a result, depending on the local climate, the amount of energy used for space heating
and air conditioning could change, which could affect both outside air temperatures and
the consumption of fossil fuels such as oil and coal that are associated with global warming. Depending on whether air conditioning or heating is affected more, this could either magnify or partially offset the impact of the roofs.

The new computer model is designed to assess the impacts of a changing climate on urban populations and explore options for countering rising temperatures. This urban canyon model simulates temperature changes in city landscapes, capturing such factors as the influence of roofs, walls, streets, and green spaces on local temperatures. Oleson has successfully linked it to a computer simulation of worldwide climate, the NCAR-based Community Climate System Model, thereby enabling researchers to study the interactions between global climate change and urban areas.

"Our research demonstrates that white roofs, at least in theory, can be an effective
method for reducing urban heat," says Keith Oleson, the lead author of the study and a
scientist at the National Center for Atmospheric Research (NCAR). "It remains to be seen
if it's actually feasible for cities to paint their roofs white, but the idea certainly warrants
further investigation."

The authors emphasize that their research should be viewed as a hypothetical look at
typical city landscapes rather than the actual rooftops of any one city. In the real world,
the cooling impact might be somewhat less because dust and weathering would cause the
white paint to darken over time and parts of roofs would remain unpainted because of
openings such as heating and cooling vents.

"It's not as simple as just painting roofs white and cooling off a city," Oleson says.

The research indicates that some cities would benefit more than others from white roofs,
depending on such factors as the city's location and design:

  • Roof density: Cities where roofs make up more of the urban surface area would cool more.

  • Construction: Roofs that allow large amounts of heat from the Sun to penetrate the interior of a building (as can happen with metal roofs and little insulation) are less effective in cooling outside temperatures when painted white.

  • Location: White roofs tend to have a larger impact in relatively warm climates that receive strong, year-round sunlight.

While the model did not have enough detail to capture individual cities, it did show the change in temperatures in larger metropolitan regions. The New York area, for example, would cool in summer afternoons by almost 1.1 degrees Celsius (2 degrees Fahrenheit).

Researchers say they can't yet replicate the architecture and design of specific cities. Instead, they created abstractions of cities in the model, using classes of population density, urban design, and building construction. Oleson and his colleagues plan to continue refining the model to provide more information for policymakers concerned about protecting urban populations from the risks associated with heat waves and other changes in climate.

"It's critical to understand how climate change will affect vulnerable urban areas, which
are home to most of the world's population," says NCAR scientist Gordon Bonan, a co-
author of the study.