From now on, the city's building code will require new residential construction to include "cool roofs" capable of reflecting sunlight.

This highly unscientific aerial sampling of Los Angeles homes illustrates the city's rooftop problem:

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Almost all of the homes here have dark-colored roofs covered in asphalt shingles or black tar, material that we now know is bad for both utility bills and the local climate. Dark-colored roofs absorb more sunlight than lighter colored ones do (in the same way that a black T-shirt ensures you'll bake on a summer day). As a result, a roof like this can perceptibly raise temperatures inside a building (driving up the demand for AC in the hot months) while contributing to the urban heat island effect around it.

This is why there's been growing demand for white roofs (or cool roofs), as one small step toward creating greater energy efficiency and addressing climate change. It's also why the Los Angeles city council unanimously voted this week to mandate "cool roofs" in its building code for all new residential construction. Homeowners rehabbing their properties will now be required by the city to change their roofs, too.

Los Angeles is the first major U.S. city to take this step, according to the advocacy group Climate Resolve that lobbied for the change. These new roofs will have to use material that naturally reflects sunlight instead of absorbing it (such roofs don't have to be white although they often are). The change, advocates hope, will reduce electricity bills, greenhouse gas emissions, and the likelihood of power outages, while improving air quality.

The move, implemented through the city's building code, also illustrates what climate change policy looks like at the local level.

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