Previous studies have indicated that painting the roofs of buildings white could be a low tech way to reduce global warming by reflecting the sun's rays back into space. Now the first computer modeling study to simulate the impacts of white roofs on urban areas worldwide has added more weight to such a proposal indicating that painting every roof in a city entirely white could cool the world's cities by an average of about 0.7 degrees Fahrenheit or 0.4 degrees Celsius.

Cities are particularly vulnerable to climate change because they are warmer than outlying rural areas. Asphalt roads, tar roofs and other artificial surfaces absorb heat from the sun, creating an urban "heat island effect" that can raise temperatures on average by 2-5 degrees Fahrenheit (about 1-3 degrees Celsius) or more, compared to rural areas.

White roofs would reflect some of that heat back into space and cool temperatures, much as wearing a white shirt on a sunny day can be cooler than wearing a dark shirt. Using a newly developed computer model the study team led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado was able 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 urban heat island effect could be reduced by 33 percent, with the cooling influence being particularly pronounced during the day, especially in summer.

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.

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.

As Keith Oleson, the lead scientist of the team, says: "It's not as simple as just painting roofs white and cooling off a city."

The research indicated that some cities would benefit more than others from white roofs. Cities where roofs are packed closely together would cool more, while cities that contain a large number of 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) would cool less. The modeling also showed that white roofs tend to have a larger impact in relatively warm climates that receive strong, year-round sunlight.

The urban canyon computer model, developed and used by Oleson and his colleagues, is designed to assess the impacts of a changing climate on urban populations and explore options for countering rising temperatures. It simulates temperature changes in city landscapes, capturing such factors as the influence of roofs, walls, streets, and green spaces on local temperatures.

The new model does not yet have the power to replicate the architecture and design of specific cities. Instead, the research team created abstractions of cities in the model, using classes of population density, urban design and building construction.

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 two degrees Fahrenheit, or just over one degree Celsius.

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 paper.

Results of the team's research are slated for publication later this winter in the American Geophysical Union (AGU) journal Geophysical Research Letters.