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.

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 , 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.

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 . 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?