Obama's Energy Secretary Suggests White Roofs

Physics of Chu's Claim Reflective Surfaces Will Slow Global Warming

May 31, 2009

An examination of the physics behind energy secretary Steven Chu's proposal to paint roofs and roads white to slow global warming. Is it sensible or outlandish?

U.S. President Barack Obama's energy secretary, physics Nobel laureate Steven Chu, recently suggested that roofs and roads should be painted white, or other light colors, to slow global warming. Pundits on both sides argue the merits of this proposal.

This article explains the basic physics behind Chu's proposal and shows some illustrative calculations of the effect.

Physics of White and Dark Surfaces

White light is a mixture of all colors, so white surfaces reflect nearly all the light striking them. Black surfaces absorb nearly all the light striking them.

Buildings with white roofs will therefore be cooler in the summer than buildings with dark roofs because white roofs reflect more (absorb less) of the solar energy striking them. Similarly light colored clothing will be cooler than dark clothing in a sunny climate.

In hot climates where buildings require much more energy for air conditioning than for heating, painting roofs white will reduce the need for air conditioning and therefore save energy. In cold climates, however, where energy used for heating far exceeds that for air conditioning dark roofs will absorb more solar energy and reduce the energy needed for heating.

In intermediate climates, the ideal roof would be dark and absorb solar energy, when it is cold then become highly reflective, when it is hot. If a bright engineer were to develop a paint or roofing material that became reflective when the temperature increased, it would save considerable energy in all climates.

In addition to visible light, the Sun emits energy at wavelengths human eyes cannot see. So some paints and other materials can appear white to the eye but absorb significant energy at nonvisible wavelengths, such as ultraviolet or infrared. Similarly some materials appear dark but are very reflective at other wavelengths. So it may be possible for people who don't like white roofs to paint roofs a dark color but still have them reflect significant solar energy.

Approximate Illustrative Calculations

Calculations checking Chu's claim that painting roofs and roads white will have the same effect on global warming as removing all cars from the road for 11 years are beyond the scope of this article. However simple approximate calculations illustrate the effect of reflective roofs and roads on global warming. These calculations are based on simplifying assumptions and ignore complicating factors. They are intended as approximate estimates. Accuracy is sacrificed at the altar of simplicity, but simple estimations suffice to tell if Chu's proposal has merit.

The CIA World Fact Book states there are 4.2E6 kilometers (=4.2E9 meters) of paved roads in the US, and the total US land area is 9E6 square kilometers (=9E12 square meters). The total US annual (2007) electricity consumption is 4E12 kilowatt hours (=1.5E19 joules).

The rate solar energy strikes the top of Earth's atmosphere is the solar constant, which, as tabulated in many physics or astronomy textbooks, is 1370 joules per second per square meter. Because this calculation is approximate and the solar radiation away from the equator strikes indirectly, round the 1370 down to 1000.

There are just over 30 million seconds in a year (60X60X24X365.24). If it is daylight half the time and there are some rainy days, then sunlight strikes the roads roughly 10 million seconds each year.

Assume roads are an average of about 20 meters wide. Freeways are wider, country roads are narrower. Multiplying the width by the length gives the total area of roads in the US as about 8E10 square meters. Note: this number is roughly 1 percent of the total land area in the US.

Multiply the solar constant by the area of roads and the annual number of seconds of sunlight each year to find that roughly 8E20 joules of solar energy strike US roads every year. This amount of energy is difficult to grasp but it exceeds the total annual US electricity consumption.

If the roads are highly reflective, this energy immediately reflects into space. Black roads, however, absorb this energy, and reradiate it as infrared energy when the sun sets. Greenhouse gasses trap infrared energy contributing to global warming.

Assuming that the total area of buildings in the US is also roughly 1 percent of the land area, suggests that a roughly equal amount of energy also strikes buildings in the US every year. White roofs reflecting this energy would significantly reduce the amount of electricity required to cool the buildings in hot climates.

As converting solar energy into electricity becomes cost effective, roofs could supply a large fraction of electrical energy needs without the global warming caused by generating electricity with fossil fuels.

Until that time, painting roofs and roads white can, as Chu claims, slow the rate of global warming.

The copyright of the article Obama's Energy Secretary Suggests White Roofs in Physics is owned by Paul A. Heckert. Permission to republish Obama's Energy Secretary Suggests White Roofs in print or online must be granted by the author in writing.