Energy Secretary Dr. Steven Chu recently made headlines by calling for the widespread use of "cool roofs" as a smart way to combat climate change.
The idea was oversimplified in the news media as simply "painting the world white," but that is not what Secretary Chu suggested. In fact, it is a caricature of what could be an important way to offset our carbon emissions. Secretary Chu is correct in suggesting we pursue cool roofs, and I hope more people will learn about this new strategy and consider adopting it for their homes and businesses.
If nothing else, a white or cool roof will save you up to 20 percent on your air conditioning bill and it's hard to argue with that. Over its lifecycle, a new white roof costs no more than a traditional roof.
The basic idea behind cool roofs is simple and recognized for centuries by the ancient Greeks and Egyptians. Dark colors absorb more heat than light colors. It's for this reason that people living in the tropics wear light-colored clothes and the same reason you don't lean on a black car on a hot day. Similarly, darker colored roofs retain more of the heat from sunlight within our atmosphere. But light roofs reflect more of that light straight back into space. Therefore, making roofs lighter in color increases their solar reflectivity and directly offsets CO2 emissions.
The potential savings are both huge and surprising. My colleagues and I have estimated that replacing urban roofs with solar-reflective materials in tropical and temperate regions of the world would offset 24 billion tons of CO2.
Let me explain. The average US roof is approximately 1,000 square feet and lasts for about 20 years. A white roof produces a one-time offset of 10 tons of CO2 and would eliminate emissions from one car for more than 2.5 years. Considered on a national scale, the equivalent would be eliminating two billion tons of CO2 emissions or removing 20 million cars off the road for 20 years. From a global perspective, replacing dark roofs with cool ones would be equivalent to taking half the world's cars - 300 million vehicles -- off the road for 20 years or reducing 24 billion tons of CO2 emissions for the same period.
That may sound too good to be true, but it is possible.
Because most large, modern cities have dark roofs, roads, and parking lots, they tend to run 5-10% hotter and create the "urban heat island" effect. Cool roofs mitigate the "urban heat island" effect and improve outdoor air quality and comfort. Light-colored roofs have other benefits. Most importantly, they lower temperatures inside of homes and businesses, thereby reducing the need for air-conditioning during the hot summer months. That translates to additional savings in CO2 emissions.
Since 2005, California building standards have required that any flat roof on a new building be a white roof. The California will soon also require new residential roofs to have cool colors as a way to reduce cooling costs. In an effort to cut its power costs, the city of Phoenix recently invested $28,600 of its $4.3 million in housing funding from the American Recovery and Reinvestment Act to apply reflective white paint on the roof of a public housing complex.
Simply put, a cool roof will save money for homeowners and businesses through reduced air conditioning costs. The real question is not whether we should move toward cool roof technology: it's why we haven't done it sooner.
Kudos to Dr. Chu for examining the science and embracing this sensible approach in combating climate change.
Dr. Art Rosenfeld is a member of the California Energy Commission.