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Computer modeling indicates white roofs may be a cool idea


January 28, 2010

A construction crew paints a white roof in downtown Washington, D.C. (Image: Maria Jose-Vinas, American Geophysical Union)

A construction crew paints a white roof in downtown Washington, D.C. (Image: Maria Jose-Vinas, American Geophysical Union)

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.

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag. All articles by Darren Quick

Comme on ! Do you really need a scientist to tell you that white roofs are cooler thant dark ones? What did you learn at school? !!!

Ariel Dahan

How long do you intend to continue the \"global warming\" scam?


Reflecting light and heat off a white surface definitely gives a cooler surface, but I don\'t think that the heat will be reflected back into space, but only into the surrounding air. Also where does this leave solar panels, which appear to be black? one idea to have cooler cities would be to replace tarmac roads with block paving. This is already used in pedestrianised areas,and would be satisfactory to drive on as the roads would probably have a speed limit on them of 20-30mph. They would be very easy to repair if necessary, and they would not deteriorate into pot holes, which is a perennial problem in the UK. They would be self draining as well.


Global warming is not a scam. The scientific evidence supporting anthropogenic forcing (human caused climate change) is as overwelming as the evidence supporting the link between cancer and cigarettes. Denial of climate change is like saying the earth is flat. The science of climate change is complex, which is why many people do not understand. Those people who claim to have contrary evidence have not been reading the entire research, and are not experts on climate science. Denial of climate change is closely linked to political beliefs and existing personality, such as having individualistic views instead of egalitarian views. Local weather conditions do not accuratey represent global climate change. Evidence of previous warming over 1000 years ago does not disprove evidence of current anthropogenic warming. We have just had the hottest two decades on record, back to back. While average temperatures have increased a little, in some areas the heating is far worse, such as in the polar caps. Wake up!


Dear Pscyh-research( did you mean Psych?) We have just had the worst winter for many years. USA had terrible nationwide blizzards. \'Evidence of previous warming over 1000 years ago does not disprove evidence of current anthropogenic warming.\' No, and it doesn\'t prove it either.


I did have a simple method to cool buildings inside. I foot diameter (or even more depending on the availability) mud covers are placed side by side with gaps plastered with mud. They are convex shaped and coated with white calcium (we in India call white wash). Here part of the sunlight falling is reflected back because of white colour and then it passes through 1 cm thick clay and then air. Still air acts as insulator. Experiments revealed there is on an average a drop of 5 degrees Celsius compared to normal solar incidence without these covers. Simple White colour itself reduces 2 to 3 degrees Celsius. That is why in the olden days all houses were white washed outside. In Gulf most people wear white dress because to get relief from high sunshine in summer. In India in the olden days Every Congressman used to wear Gandhi Cap (White cloth cap). In Rajasthan traditionally people cover head with lengthy white cloth rolled over head. Traditional methods offer promise in many cases and it is for Scientists and Technologists to study them to adopt.

Dr.A.Jagadeesh Nellore (AP), India


Interesting concept, not sure on the global warming scenario. I myself as a business owner for a Metal Roofing and Siding company. I have seen white metal roofs help out tremendously in savings on homeowners electric bills. If anyone has any insight on white roofs relecting back into space, I would be more than interested into hearing how it is possible. Thanks, James

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