How do white roofs cause global warming?

Question

Another user asked the question, and I think the question is important enough to deserve it's own post.

I've heard that white roofs drive away clouds and cause global warming. How can this be?

Answer 1

This counter-intuitive advice is actually the result of research. Unfortunately journalists are rarely qualified to cover any sort of science news, and this was no exception. I'll try to explain the result in more detail, and in a way people can understand.

Most people know that dark colored objects absorb light and become warm in the sun. The darker the color, the more light objects tend to absorb. Black asphalt and dark roofs are familiar examples. When they warm in the sun, they in turn warm nearby air.

Rather than rising immediately, this warm surface air tends to get stuck under colder, heavier air. On a large scale this is called an inversion, but on small scales such things happen everyday. The warm air is less dense, and will rise given the opportunity. Most commonly a building or bare tree will provide an easy path for the warm air to rise, and this tends to pull the entire pocket of warm air into a thermal column. These columns can last minutes or hours, depending on conditions.

Thermal air columns rise until they hit the condensation point, where they stop and form white, puffy clouds (cumulus clouds).

Now consider what happens when you replace that dark roof with a very reflective white roof. Rather than heating up, the roof "reflects" solar energy right back into the atmosphere. Some of the light escapes, but much of it is absorbed by air gasses and black carbon soot. The small amount of light escaping the atmosphere would be more than offset by the clouds formed by dark roofs.

Clouds reflect light just like a white roof does, but they do so from higher in the atmosphere, and they're much larger than a roof. Overall white roofs cause more energy to stay in our atmosphere, and contribute to global warming.

However, if you live in a hot, sunny area where clouds never form regardless of your choice, a white roof will reflect some solar energy back into space. In this scenario, you can paint your roof white with a clean conscience.

A better alternative is to cover your roof with solar panels. This converts solar energy into electricity which you may otherwise purchase from a coal or diesel power plant. It keeps the solar energy from heating your home, and doesn't reflect as much energy back into the atmosphere. It allows clouds to form, and is the most sustainable solution. A green roof might also be a good answer, if your building is strong enough to support it.

Answer 2

The answer is, in contributes to the planet surface albedo, it reflects light and therefore heat back into the atmosphere where it is trapped in the weather system. Whether it has real impact or not is debatable. If everyone on the planet painted their roof's white, it might increase our collective albedo by something on the order of less than 0.001 %

The effect it will have or more local, shifting the weather patterns, similar to how metro Florida where the rain clouds always rain on schedule around the cities, but no longer seem able to surmount the updraft caused by all the pavement.