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More reflective surfaces, such as cool roofs and pavements, reflect radiation back into the atmosphere and into space and thus help cool the planet in two ways. At the scale of individual cities, they can combat the urban heat island effect, and at a continental scale, they can combat global warming. Of course, in air-conditioned buildings, cool roofs can also help lower energy bills by decreasing the need for air conditioning.

On the local scale, this study validated previous studies finding California and the greater northeast of the U.S. as good candidates for cool roofs. Cities such as Los Angeles, Detroit and New York saw summer temperatures drop by 0.30 to 0.53 degrees Celsius. “Half a degree Celsius makes a big difference in terms of air quality,” Millstein said.

As for the southeast, some rural areas in Oklahoma, northern Texas and parts of Louisiana and Florida saw increases in temperature whereas cities either stayed the same or cooled slightly. But because temperature affects the chemistry of the atmosphere, causing higher ozone levels and more smog, cool roofs can still play an important role in improving air quality. “Even when you take feedback into account, cool roofs are still beneficial for most places,” Millstein said. “With the exceptions, there may be more study needed. The southeast is certainly not ruled out as a candidate for cool roofs.”

On the global or continental scale, the findings also confirmed the benefit of brightening roofs and pavements. “Even with the feedbacks from decreasing clouds in certain locations, we still had more reflection overall,” Millstein said.

For each square meter of cool roof surface deployed, the increased reflectivity is equivalent to offsetting 175 kilograms of carbon dioxide. For the continental U.S., it would achieve a one-time offset of 3.3 gigatons of CO2, or about half of total U.S. emissions in 2009.

The researchers’ next step will be to study the role of air pollution in weather patterns and investigate how photovoltaics and other forms of renewable energy may be used to reduce air pollution.