Why does painting a roof white actually cool a building?

Painting a roof white cools a building because white surfaces bounce most sunlight straight back into the sky instead of absorbing it as heat. Scientists measure this with a number called albedo: the fraction of sunlight a surface reflects, from 0 for something that swallows every ray to 1 for something that reflects all of it. Fresh snow reflects about 90 to 95 percent of sunlight, while charcoal, one of the darkest substances around, reflects only about 4 percent; that's the same basic reason a white roof stays cooler than a black one on a sunny day.

This isn't just a curiosity for architects. Cities generally have lower albedo than the farmland around them, since dark roads and rooftops soak up heat, and that contributes to the 'urban heat island' effect that makes cities noticeably hotter than their surroundings. One 2022 estimate found that raising urban albedo worldwide by just 0.1 would produce a cooling effect equivalent to absorbing about 44 gigatons of carbon dioxide. In southern Spain, tens of thousands of hectares of greenhouses around Almería are covered in whitened plastic roofing, and a study found that this huge reflective surface measurably lowered the local surface temperature, even if the effect stayed fairly local.

Albedo matters on a planetary scale too. Earth's own average albedo is about 0.3, and that number, combined with the greenhouse effect, keeps the planet's average surface temperature around 15°C. If the whole Earth were covered in ice and far more reflective, scientists calculate the average temperature would plunge below minus 40°C; if it were covered entirely by ocean instead, it would warm to nearly 27°C. Ice itself drives a feedback loop: because it's so reflective, melting ice exposes darker water or ground that absorbs more heat, melting even more ice in turn.

The term 'albedo' isn't new. It was introduced into optics by Johann Heinrich Lambert all the way back in 1760, in his book Photometria, long before anyone was painting rooftops to fight climate change.