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A dark colored roof absorbs much more heat than a white roof. By absorbing the sun's rays, the material of the roof converts visible spectrum light to heat energy.
This matters to climate change in two key ways:
- This re-radiates into the atmosphere as infra-red, heating the atmosphere. (By comparison, much more of the visible light reflected by a white roof will pass out of the atmosphere.)
- Dark roofs absorb huge amounts of heat into buildings, especially when it's hot. This means more air conditioning is used to cool buildings, homes and workplaces becomes less comfortable, and those at risk of heat stress are in greater danger. Dark colors also radiate heat more easily in cold conditions, meaning greater heat loss from buildings and higher heating costs.
Estimates for the effect on electricity usage from painting the roof white are around 20 percent on a hot day - more if combined with other measures such as a passive solar retrofit. Reductions from peak usage on hot days are important because they also reduce the need for extra electricity generation capacity.
Millions of buildings in America have dark colored tar roofs, and are particularly prone to this heat effect - but the same effect can be felt under any roof that isn't white, and the darker the color, the greater the effect.
New York City's Mayor Bloomberg began a program to hire and train young people to paint New York's roofs white.