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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.
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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.
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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?
Why would a promising young scientist leave the lab to spend a year working for the United States government? Daniel Gorelick is here at the State Department trying to figure that out.