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Combating global warming with white paint

Supposedly, we could combat a non-trivial chunk of global warming by painting all road and roof surfaces white. Even Nobel prize-winning Secretary of Energy Steven Chu champions this idea. He says "It’s the equivalent of reducing the carbon emissions due to all the cars in the world by 11 years."

I was idly thinking about this, and wondering why this has any global effect. Sure, a white house will stay cooler inside than a dark one. But if the light is reflected into the atmosphere and trapped there by green house gasses, you haven't won anything. Similarly, even if a dark object absorbs heat, it'll radiate that heat back out when the air cools off. Is there a difference (for the planet overall) between reflecting it right away as light and radiating it later as heat?

First off, keeping the house cooler helps simply by reducing the need to spend energy on air conditioning or fans.

Second, it turns out that the atmosphere allows far more visible light through than it does heat radiation. (See e.g. here.) So heat radiated back out tends to stay in the atmosphere, while 70-75% of reflected, visible sunlight will pass back out into space.

The one bit left unaddressed is the cost of producing, transporting, and applying all that paint.



( 6 comments — Leave a comment )
Jul. 27th, 2009 10:22 pm (UTC)
And the fact that you wouldn't be able to see a darn thing if you were driving on a white road in bright sunlight. Plus it would be almost impossible the keep the roads white for very long anyway.
Jul. 28th, 2009 05:15 am (UTC)
Yeah, I was going to say that you could try to engineer a material that scatters light really well, so that you don't get directional glare, but I wonder how well the micro-texture would survive. (Tho WoS says below that glass mixed in could help.)
Jul. 28th, 2009 04:17 am (UTC)
The bit that does make sense here is that asphalt doesn't *have* to be black. In Milwaukee, for example, almost all of the asphalt is a whitish grey, about the color of chalk or light sand. No paint involved, it's just how they mix the local asphalt.

I've also seen asphalt with large amounts of ground-up glass. It tends to be very sparkly and reflect a lot of sunlight and heat. Granted, you get a bit of glare on sunny days, but it works fine in more northern latitudes.

Jul. 28th, 2009 05:13 am (UTC)
IIRC, the newer, quieter asphalt (like that on SR-520 east of the bridge) has material from old car tires mixed in.
Jul. 28th, 2009 09:24 pm (UTC)
I was just thinking recently that what we really need is a chameleon-like coating for building roofs: white for summer, black for winter. Perhaps it could be temperature-sensitive, and change colour when the temperature drops below (say) 70 degrees?

A much better alternative to all of this, though, would be much more efficient and much cheaper solar panels, so we can do something truly useful with all that free energy. Hey, imagine if the roads themselves could be used to generate electricity, combining solar with some kind of power generation from cars rolling over the surface. Cool!

- S
Jul. 28th, 2009 10:15 pm (UTC)
The chameleon idea is really clever! Could you get on that? :)

There's a conceptual proposal to power wind turbines along freeways from the air displaced by vehicles. It's not clear to me if this doesn't make the vehicles less energy efficient.
( 6 comments — Leave a comment )



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