Back in June, my former colleague and scientist Daniel Gorelick ― now a postdoctoral fellow in the Embryology Department at the Carnegie Institution for Science-Baltimore ― wrote an
entry in his Science Planet blog about how painting your roof white reduces global warming and conserves energy. It was so popular and and there were so many comments that we decided to publish it again. Here it is, with some comments from me at the end:
Paint Your Roof White and Reduce Global Warming
– By Daniel Gorelick, 9 June 2009
Image by trillbilly A house in Bermuda with a white roof.
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Painting your roof white will reduce global warming and conserve energy, according to
Steven Chu, the Nobel prizewinning physicist who now runs the U.S. Department of Energy (DOE).
In an interview with the British newspaper The Independent, Chu said:
“If you look at all the buildings and make all the roofs white, and if you make the pavement a more concrete-type of color than a black-type of color, and you do this uniformly … It’s the equivalent of reducing the carbon emissions due to all the cars in the world by 11 years.”
Let’s tackle energy conservation first. On a hot day, you’ll be much cooler wearing a white shirt than a dark shirt. This is because light colored objects reflect more sunlight. Dark objects absorb more sunlight than light colored objects; the absorbed light then radiates away from the object (or is emitted from the object) as heat. Your dark shirt is absorbing sunlight, and then releasing it as heat, which makes you feel hotter.
A dark roof on a building is like a dark shirt. The roof absorbs sunlight, and then radiates heat into the building. The temperature inside the building increases, and we use energy ― in the form of air conditioning ― to cool the building. Paint the roof white (or use a reflective material, like white tiles), the roof absorbs less sunlight, less heat is radiated into the building, the temperature inside the building doesn’t increase as much and we don’t need to use as much energy to cool the building. That’s how painting a roof white conserves energy. In the United States, the California state government has become a leader in encouraging the use of white roofs or cool roofs.
White roofs may also reduce global warming.
When sunlight is absorbed by a roof, the roof heats up and radiates heat in the form of infrared light, which is invisible to humans (it has a longer wavelength than red light). Infrared light is emitted from the roof and reaches the atmosphere, where it is absorbed by gases and re-emitted as infrared light - a continuous cycle of absorption and emission that traps heat in the atmosphere and increases the temperature of the Earth. Gases that absorb and radiate infrared light are called greenhouse gases - these include water vapor, carbon dioxide and ozone.
Atmospheric gases don’t absorb much visible light, which is why sunlight reflected from a white roof ― visible light ― can travel through the atmosphere and escape into outer space.
Image by redskunk These principles also apply to car roofs.
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All roofs reflect and absorb sunlight. Dark roofs absorb more sunlight and therefore emit more infrared light than white roofs, and so contribute more to an increase in atmospheric temperature.
Incidentally, light is also absorbed by the Earth ― the ground, the soil ― and returned to the atmosphere as infrared light, where it is trapped as heat. This is the greenhouse effect. We can’t paint the Earth white to reduce global temperatures, but nature has helped us out a bit, in the form of ice. Polar ice caps and glaciers are like big, white roofs ― they reflect much of the incoming sunlight back into the atmosphere and out into space. Scientists and policymakers are concerned that melting ice will expose land, decreasing the amount of sunlight reflected back into space and increasing the amount absorbed by the Earth and trapped in the atmosphere as heat.
Researching this post I have found no reason why we should not be painting our roofs white (or using reflective tiles). Can you think of a reason not to do this? People might complain about having to look at a white roof, but does an aesthetic concern outweigh conserving energy and reducing global warming?
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In 2008, the Brazilian nonprofit One Degree Less partnered with Dr. Hashem Akbari, senior scientist at the DOE Lawrence Berkeley National Laboratory’s Environmental Energy Technologies Division, to promote international white-roof initiatives. Akbari’s Heat Island Group website tells all about high temperatures, cool roofs, vegetation and energy use. Around the world, the market for reflective roofing materials grows as regions, nations and communities look for ways to contribute to the effort to slow global climate change.

The Des Moines (Iowa) Public Library has a green roof. (USDA)
And speaking of vegetation, the movement to cover the roofs of homes and commercial buildings is growing worldwide. Read more about it in the following publications and websites:
Global Cooling: Increasing Worldwide Urban Albedos to Offset CO2 (PDF 356 KB) by Akbari and colleagues Surabi Menon, also at DOE’s Lawrence Berkeley National Lab, and Arthur Rosenfeld, a member of the California Energy Commission in Sacramento.
Green Roofs for Healthy Cities, United States and Canada
The World Green Roof Infrastructure Network, a network of green roof organizations in countries around the world
Cities Alive!, a conference held October 19-21, 2009, in Toronto to bring together global best practices and leading-edge developments in terms of policy, research and designs focused on green roofs, walls and other forms of green infrastructure.