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White roofs may cool cities

Jan. 29, 2010
Courtesy National Science Foundation
and World Science staff

Paint­ing rooftops white may sig­nif­i­cantly cool off cit­ies and count­er some im­pacts of glob­al warm­ing, a study us­ing com­put­er sim­ula­t­ions sug­gests.

But the study team, led by sci­en­tists at the Na­tional Cen­ter for At­mos­pher­ic Re­search in Boul­der, Co­lo., cau­tions that there are still many hur­dles be­tween the con­cept and real­ity. 

A con­struc­tion crew paints a roof in down­town Wash­ing­ton, D.C. (Cred­it: Ma­ria Jose-Vi­n­as, Amer­i­can Ge­o­phys­i­ca Un­ion)
“It re­mains to be seen if it’s ac­tu­ally fea­si­ble... it’s not as sim­ple as just paint­ing roofs white and cool­ing off a city,” said the cen­ter’s Keith Ole­son, lead sci­ent­ist in the stu­dy. “But the idea cer­tainly war­rants fur­ther in­ves­ti­ga­t­ion.”

The find­ings are slat­ed for pub­lica­t­ion lat­er this win­ter in the jour­nal Geo­phys­i­cal Re­search Let­ters.

Cit­ies are par­tic­u­larly vul­ner­a­ble to glo­bal warm­ing 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.

The study team used a newly de­vel­oped com­put­er mod­el to sim­ulate the amount of so­lar radia­t­ion ab­sorbed or re­flected by ur­ban sur­faces. The sim­ula­t­ions, which give sci­en­tists an ide­alized view of dif­fer­ent types of cit­ies around the world, in­di­cate that, if eve­ry roof were paint­ed all white, the ur­ban heat is­land ef­fect could be re­duced by 33 per­cent. This would cool the world’s cit­ies by an av­er­age of about 0.7 F, with the ef­fect be­ing most pro­nounced in day­time, es­pe­cially in sum­mer.

The au­thors said their re­search is just 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.

White roofs would al­so tend to cool off build­ing in­te­ri­ors. 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 U.S. Na­tion­al Sci­ence Found­a­tion-fund­ed re­search al­so in­di­cated that some cit­ies would ben­e­fit more than oth­ers from white roofs. This would de­pend on such fac­tors as the city’s loca­t­ion and de­sign.

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Painting rooftops white may significantly cool off cities and counter some impacts of global warming, a study using computer simulations suggests. But the study team, led by scientists at the National Center for Atmospheric Research in Boulder, Colo., cautions that there are still many hurdles between the concept and reality. “It remains to be seen if it’s actually feasible for cities to paint their roofs white, but the idea certainly warrants further investigation,” said the center’s Keith Oleson, the lead scientist in the study. “It’s not as simple as just painting roofs white and cooling off a city.” The findings are slated for publication later this winter in the journal Geophysical Research Letters. 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. The study team 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 give scientists an idealized view of different types of cities around the world, indicate that, if every roof were painted all white, the urban heat island effect could be reduced by 33 percent. This would cool the world’s cities by an average of about 0.7 F, with the effect being most pronounced in daytime, especially in summer. The authors said 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. White roofs would also tend to cool off building interiors. 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 research also indicated that some cities would benefit more than others from white roofs, depending on such factors as the city’s location and design.