KU News Release

LAWRENCE — Anyone who has passed a scorching summer in a dense metropolis knows that cities act as “heat islands” — zones where asphalt, brick and concrete soak up heat and boost temperatures.

“Cities are known to be warmer than the surrounding environment,” said Johannes Feddema, professor of geography at the University of Kansas. “The geometry of the city — the buildings’ shapes — traps solar radiation. And water tends to be removed from the city environment, so it’s not available to evaporate and cool the city. Air pollution makes it warmer, too.”

Now, Feddema and other scientists from the National Center for Atmospheric Research have demonstrated that simply painting city roofs white would slash temperatures in urban areas. Their investigation, carried out through new computer modeling, shows that if every roof in a given city were white, the urban heat island effect would be about 33 percent less.

“Cities use a lot of blacktop and they absorb a lot of radiation,” Feddema said. “If you make a highly reflective surface, it won’t absorb as much radiation. By taking black roofs and changing them into white you can reduce the amount of energy that’s absorbed in an urban environment, reduce the urban heat island and also reduce the energy load on a building.”

The report by Feddema and his colleagues indicates that the world’s cities would cool off by an average of 0.7 degrees Fahrenheit each under all-white roofs.

Additionally, the KU researcher said that white roofs would reduce buildings’ interior temperatures and slash fossil fuel consumption in some places.

Feddema cautioned that painting roofs white is not a one-size-fits-all remedy for rising urban temperatures. In colder climates, for instance, white roofs actually would increase the need for interior heating and usage of coal and natural gas. In fact, if every roof on Earth were to be made white, world energy consumption would spike.

“You have to think carefully about where and when you’d implement a policy on white roofs,” Feddema said. “Globally, energy consumption went up in our model because even though you made hot places cooler, this was offset by the need to warm more places in the wintertime and use more energy. The need for heating was bigger than the cooling gains.”

Feddema has been involved in the design of the National Center for Atmospheric Research’s computer model for the past five years, building the model’s data sets including typical building characteristics for cities all over the world. KU graduate student Trish Jackson assisted him.

The study, which was funded by the National Science Foundation, was published in the Feb. 3 edition of Geophysical Research Letters.