Image by TWW

TWW's house in Brazil, with a white roof, stands out among the neighbor's darker roofs.

Last week I discussed the science behind Secretary of Energy Steven Chu’s comment that making roofs white will conserve energy and reduce global warming.

This photo depicts this scientific principle in action: TWW in Brazil painted her roof white. It looks great, even among a sea of dark colored roofs.

Many thanks to TWW and other readers for submitting their comments. I’m working on a series of features explaining the science behind climate change, so stay tuned!

Image by trillbilly

A house in Bermuda with a white roof.

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.

Image by Cool Roof Rating Council

A white roof reflects more light and radiates less heat than a dark roof.

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.

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?

The U.S. Department of Energy is bursting with, well, energy, now that its new secretary, Steven Chu, has been sworn in.

Six of my colleagues at the American Association for the Advancement of Science have fellowships at Energy. I spoke to one, who thought that this appointment makes a positive statement about Obama’s approach to energy policy. If Obama had appointed oilman turned alternative energy and natural gas promoter T. Boone Pickens, for example, that would have set a different tone, as would the appointment of a lawyer, or a businessman tied to the energy sector.

Chu’s appointment to the Cabinet-level position of energy secretary means a “scientist has a voice in the Cabinet,” my colleague said. Obama also can consult with John Holdren, the scientist appointed to direct the White House Office of Science and Technology Policy.

Chu won the Nobel Prize in physics in 1997 for using lasers to cool and trap atoms. In the ensuing decade he has been an outspoken advocate of renewable energy sources. He formerly headed the Lawrence Berkeley National Laboratory, overseeing 4,000 employees and a $650 million budget – so he’s no stranger to government bureaucracy. As a scientist in academia (he spent some of his career at Stanford), he realizes the importance of government funding for basic research.