Paint your roof white to reduce global warming

[Ivan Seeking]

...A global initiative to change the colour of roofs, roads and pavements so that they reflect more sunlight and heat could play a big part in containing global warming, he said yesterday.

Speaking at the opening of the St James’s Palace Nobel Laureate Symposium, for which The Times is media partner, Professor Chu said that this approach could have a vast impact. By lightning paved surfaces and roofs to the colour of cement, it would be possible to cut carbon emissions by as much as taking all the world’s cars off the roads for 11 years, he said...

http://www.timesonline.co.uk/tol/news/environment/article6366639.ece

That should read that it would yield the same benefit as removing all of the cars for 11 years...

https://www.youtube.com/watch?v=pLlxjYACa5U

 

[John Creighto]

I'm surprised (and somewhat skeptical) that the effect would be that significant. As for painting roads how about using concrete instead of asphalt.

 

[Ivan Seeking]

I'm surprised (and somewhat skeptical) that the effect would be that significant. As for painting roads how about using concrete instead of asphalt.

Given that this comes from Chu, I'll make a leap of faith. If nothing else, it would probably be significant to the heat island effect.

One concern that occurred to me wrt using concrete instead of asphalt is the amount of CO2 produced. I believe that concrete processing accounts for a significant portion of our CO2 output. But I don't know how much CO2 is produced by the processes related to asphalt.

 

[Ivan Seeking]

Consider for example a [mostly] random shot of Los Angeles.

http://img23.imageshack.us/img23/4199/99067936.jpg

 

[russ_watters]

It's certainly a useful idea, and it is near becoming a standard on commercial buildings, but I'm curious about this statement:

By lightning paved surfaces and roofs to the colour of cement, it would be possible to cut carbon emissions by as much as taking all the world’s cars off the roads for 11 years, he said...

It is very curious to talk about time in that context - if you're going to talk about 11 years of cars off the road, you also need to talk about the time the roofs and roads are exposed to the sun. Or is that supposed to say if they make roads and roofs white, they'd save in 1 year what taking all cars off the road for 11 years would save? Strange that Chu would make such an obvious error in his key point and not correct it.

I'd be interested in seeing the analysis of that, in any case.

...link is dead, by the way...

 

[russ_watters]

Consider for example a [mostly] random shot of Los Angeles.

I'm not capable of judging albedo from a photo like that! Too many different colors!

 

[Pengwuino]

Strange that Chu would make such an obvious error in his key point and not correct it.

Maybe he got tripped up and just said it incorrectly. Any calculation would obviously be done on a cars/year kinda figure and he'd be smart enough to not make a crazy calculation like that (which wouldn't even make sense to calculate)

 

[mgb_phys]

As for painting roads how about using concrete instead of asphalt.

Concrete freeways have been a requirement in LA for >10years because of this, a friend did a PhD on the effect at Caltech

 

[LeonhardEuler]

Maybe he got tripped up and just said it incorrectly. Any calculation would obviously be done on a cars/year kinda figure and he'd be smart enough to not make a crazy calculation like that (which wouldn't even make sense to calculate)

Actually, I've heard him repeat this claim in almost exactly the same words on the Daily Show with Jon Stewart, and I had the same confusion as russ at the time:

http://www.thedailyshow.com/watch/tue-july-21-2009/steven-chu

 

[russ_watters]

So I suppose we can assume that he meant we can leave the cars off the roads for 11 years and it'll save as much energy as if we painted all the roads and roofs white and left them that way for the next 4 billion years, until the sun burns out? :uhh:

Clarification needed!

 

[Pengwuino]

Actually, I've heard him repeat this claim in almost exactly the same words on the Daily Show with Jon Stewart, and I had the same confusion as russ at the time:

Wow really? That's weird... pretty big mistake to make repeatedly if its accidental.

 

[Skyhunter]

Given that this comes from Chu, I'll make a leap of faith. If nothing else, it would probably be significant to the heat island effect.

One concern that occurred to me wrt using concrete instead of asphalt is the amount of CO2 produced. I believe that concrete processing accounts for a significant portion of our CO2 output. But I don't know how much CO2 is produced by the processes related to asphalt.

The biggest carbon offset from painting your roof white is a reduction in energy expended for cooling. A lot of energy is used to heat and cool buildings. Living roofs are even better, they provide insulation, evaporative cooling, particulate scrubbing, and an environment for butterflys and other native fauna.

 

[sylas]

It is sensible to compare a number of years without cars with a number of white roofs. A number of years without cars is a certain amount less CO2, which is a forcing. So too, a certain number of white roofs is a rise in albedo: also a forcing. Here's a guesstimate.

The idea is this. I'll calculate a forcing, in W, from painting a roof white.

Then I'll calculate how roughly how much CO2 we get from cars, in eleven years, and make that a forcing. This should give a rough idea of how many roofs. Each step will involves back of the envelope estimates, so this should only give a very crude idea if the magnitude of the comparison is sensible. I'm doing this on the fly; I don't know what answer I'll get yet.

1. Effect of painting a roof white

Using some albedo numbers from the book Boundary layer climates, found online, I'll say that the albedo of a tile roof is 0.2; and that it can be made 0.9 by painting white.

I'll go with a roof size of around 150 m²; a medium sort of house, I think.

I'll assume that the solar incoming flux at the surface is 184 W/m², using the energy flow diagrams from other threads here, based on Trenberth and Kiehl.

Thus the change in energy flux for a white roof is 184*(0.9-0.2)*150 = about 20000 W; rounding to one figure.

Effect of removing cars

A quick google suggests to me that roughly 20% of CO2 emissions are from the transport sector, and about half that is from cars. So about 10% of the rise in CO2 levels might be from cars.

CO2 levels are rising at a bit over 2ppm per year, taking eleven years without any cars means roughly 2ppm CO2. The forcing for that, using a conventional formula I've described in these threads a number of times, is 3.7*log₂(387/385) = 0.028 W/m².

How many roofs?

The surface area of the Earth is about 5.15*10¹⁴ m². Hence the forcing in Watts for the cars is about 1.4*10¹³ W.

Divide by the impact of a white roof... and I get 7*10⁸ roofs... 700 million.

Now that is pretty crude, but it indicates an order of magnitude. Chu spoke of roads and pavements as well as roofs. I would think that home roofs in the USA only would be more like 70 million; but if all other buildings and pavement is included, the comparison seems roughly credible. I don't know the basis of Chu's estimate.

Cheers -- sylas

PS. I think skyhunter is probably correct though; the most important factor is probably efficiency and domestic energy use, rather than albedo.

 

[LeonhardEuler]

I see what you're saying, sylas, that makes a lot more sense now.

 

[mgb_phys]

Thus the change in energy flux for a white roof is 184*(0.9-0.2)*150 = about 20000 W; rounding to one figure.

The effect is even larger if you consider the electrical power needed to cool a building with AC that is receiving an extra 20KW of heat input.

 

[PRDan4th]

Given that this comes from Chu, I'll make a leap of faith. If nothing else, it would probably be significant to the heat island effect.

One concern that occurred to me wrt using concrete instead of asphalt is the amount of CO2 produced. I believe that concrete processing accounts for a significant portion of our CO2 output. But I don't know how much CO2 is produced by the processes related to asphalt.

Its true that manufacture of concrete releases large amounts of CO2 when CaCO2 is calcined into lime (CaO). When asphalt is produced from the bottom of the oil barrel in refining it contains the largest percentage of carbon in the product. Therefore, asphalt reduces CO2 that would otherwise be released if all of the oil was burned.

I agree with others regarding the CO2 reductions due to a significant energy savings from reduced cooling loads using lighter color roofs.

 

[turbo]

When I bought this place, it had a dark asphalt-shingle roof and it was a bear to cool even with a big 220V AC unit. I covered the shingles with 1" thick Styrofoam insulation and shiny silver galvalume standing-seam roofing. Now we can keep the place very cool with just a couple of small portable 120V AC units that vent through windows. Maine is a temperate climate, but if you applied that same type of insulation and roofing to houses all across the deep south from coast to coast, the savings in electricity for AC could be tremendous. It would be a great idea in SoCal, especially in areas susceptible to wildfires. BTW, the same material comes pre-painted if you want, and the roofing contractor can certainly get it in white or other light colors if you don't want a shiny roof.

 

[B. Elliott]

I've thought about this for years now, wondering why it wasn't a standard. I just assumed that black had some beneficial factor during the winter months that I wasn't aware of.

 

[Ivan Seeking]

This explores more of the specifics
http://www.energy.ca.gov/2008publications/CEC-999-2008-031/CEC-999-2008-031.PDF

See esp page 2

 

[chemisttree]

And, of course, white roofs never grow black mold and algae or get dirty from dust and pollution. We're supposed to whitewash our roofs how often? How's that going to look with my http://www.vchouseplans.com/" house?

Great idea, Chu! Brilliant! I spent a fortune on that roof...

 

[Ivan Seeking]

Have you ever heard of rain? It tends to wash things away.

Maybe you have a moldy roof, but most people do something about that.

 

[chemisttree]

They'd have to if it was brilliant white! And what the heck is rain?

Next thing he'll be sayin' is we can save a lot a gas if we just inflate our tires!

 

[Charlied]

I agree, painting a roof white will reduce the heating of the interior but if you reflect all that heat back through the atmosphere doesen't it give double exposure of that heat to the atmosphere?

 

[sylas]

I agree, painting a roof white will reduce the heating of the interior but if you reflect all that heat back through the atmosphere doesen't it give double exposure of that heat to the atmosphere?

No, because reflected light is much less well absorbed than emitted thermal radiation. The radiation you get on your roof comes from the Sun, and passes through the atmosphere because the atmosphere is transparent at those wavelengths. Reflect it and it goes straight back into space, just as it came down without being absorbed.

Basically, whatever energy is received from the Sun has to get back out to space again. You can either reflect it back, in which case it returns to space directly because the atmosphere is transparent to solar radiation, or else it is absorbed at the surface, leading to surface heating and emission of thermal infrared radiation. The atmosphere is opaque to large bands of thermal infrared, and so the atmosphere is heated by thermal emissions from the surface, much more effectively than by the incoming or reflected solar wavelengths.

Cheers -- sylas

 

[kldickson]

How many square miles of Earth does roof cover take up? How does it compare to the melting of polar ice?

I don't think increased albedo in urban areas will have a huge direct impact on heat absorption, but it'll decrease the amount of energy we use in cooling buildings.

 

[kldickson]

Slightly wacky idea, but institute a suggested roof color based on latitude. Pure white at the Equator and black at the pole, and a gradient.

It requires energy to both heat and cool.

 

[KingOfChaos]

The reason being that the white would reflect heat/light, making your house cooler so that your A/C doesn't have to run as much, thus reducing the amount of electricity created by fossil fuels.

kldickson is correct, that it would be better to have black roofs in certain areas of the world to do the exact opposite and absorb heat/light to reduce heating expenditures.

 

[Ivan Seeking]

The reason being that the white would reflect heat/light, making your house cooler so that your A/C doesn't have to run as much, thus reducing the amount of electricity created by fossil fuels.

kldickson is correct, that it would be better to have black roofs in certain areas of the world to do the exact opposite and absorb heat/light to reduce heating expenditures.

That depends. What is the benefit from a black roof and reduced energy demands for heat, as opposed to the reduced energy input to the Earth system from a white roof?

 

[kldickson]

You'll expend energy to heat it in the winter!

 

[Ivan Seeking]

You'll expend energy to heat it in the winter!

Yes, but how much of a difference will it [a black roof] make in the energy usage? From there, what is the net effect on the global temperature by reducing CO2 emissions and by not producing additional heat [electric, fire, whatever]. Compare this to the amount of warming avoided by reflecting the light back out into space with a white roof.

Using a completely arbitrary example: You may reduce the power input to the Earth system by 500 watts per square meters with a white roof, but only see an advantage on your heating bill corresponding to 50 watts per square meter, for a black roof. [Assuming these represent the average values over a given interval of time]

You may even find that having a black roof yields no advantage at all in cold weather. Perhaps the energy captured is immediately lost to the environment?

 

[LeonhardEuler]

Yes, but how much of a difference will it [a black roof] make in the energy usage? From there, what is the net effect on the global temperature by reducing CO2 emissions and by not producing additional heat [electric, fire, whatever]. Compare this to the amount of warming avoided by reflecting the light back out into space with a white roof.

Using a completely arbitrary example: You may reduce the power input to the Earth system by 500 watts per square meters with a white roof, but only see an advantage on your heating bill corresponding to 50 watts per square meter, for a black roof. [Assuming these represent the average values over a given interval of time]

You may even find that having a black roof yields no advantage at all in cold weather. Perhaps the energy captured is immediately lost to the environment?

This goes back to the point sylas made earlier. If there is any reduction in energy usage, that will always be more important in the long run. It's best not to think in terms of energy and power to make the comparison, but instead in terms of light reflection.

Painting your roof white increases the rate of reflection by a fixed amount (assuming it won't be covered in snow anyway). Reducing power consumption reduces carbon emissions, so that decreases the rate at which the rate of reflection is changed by carbon dioxide. In terms of the equilibrium mean temperature of the earth, in the first case you are lowering it, while in the second you are reducing the rate at which it rises.

 

[archis]

My personal opinion - this is a complite nonsense. What are his calculations? Is the roof area so big aginst the area of earth? From what materials will you make white roof? Will be the new matrials cost, energy effective, long living? How much energy wil you sepnd to clean them? What about spoiled architecture? Havnt seen any white roof in my life. Only matrial i know who could create this reflection effect is galvanized iron.

 

[sylas]

My personal opinion - this is a complite nonsense. What are his calculations? Is the roof area so big aginst the area of earth? From what materials will you make white roof? Will be the new matrials cost, energy effective, long living? How much energy wil you sepnd to clean them? What about spoiled architecture? Havnt seen any white roof in my life. Only matrial i know who could create this reflection effect is galvanized iron.

I also am a bit skeptical of the calculations; but I'll wait until the paper comes out to see the details. I'll probably try another order of magnitude calculation myself later.

However, there are two things that strike me about this.

The first is that the effect of albedo is indeed strong. You don't have to paint the whole Earth's surface. If you were to make the whole surface of the Earth white, you would easily cool the planet down to be the mother of all ice ages; a new "snowball Earth".

The second is that this is no panacea, or silver bullet. Having lighter more reflective roofing and paving is indeed enough to make a difference. It is nowhere near the whole Earth's surface, but it's enough to have a contribution that matters. The comparison Chu gave was similar to 11 years worth of car emissions, which is in turn (I estimate) roughly one years worth of total emission. I want to check the numbers on that; I suspect that's an upper bound on what is credible. In any case there's no question that it is nowhere near enough to reverse the main anthropogenic impact.

If people want to reduce the human impact, they should look at many different actions available, and not just one way to offset all the impact we have though the modern industry supported lifestyle. This is only one, which does help, but if you want to make a difference you need to look at multiple changes to the way you do things.

Cheers -- sylas

 

[Ivan Seeking]

This goes back to the point sylas made earlier. If there is any reduction in energy usage, that will always be more important in the long run. It's best not to think in terms of energy and power to make the comparison, but instead in terms of light reflection.

Painting your roof white increases the rate of reflection by a fixed amount (assuming it won't be covered in snow anyway). Reducing power consumption reduces carbon emissions, so that decreases the rate at which the rate of reflection is changed by carbon dioxide. In terms of the equilibrium mean temperature of the earth, in the first case you are lowering it, while in the second you are reducing the rate at which it rises.

The flaw in your reasoning is the statement that painting a roof black will decrease carbon emissions. We don't know that to be true to any significant degree, and I seriously doubt that you would see any measurable benefit in cold climates. As a minimum, we don't know if a black roof would yield any advantage at all or a corresponding reduction in ghg emissions.

Basically you are suggesting that a passive solar heater would work without a glass cover, while exposed to cold or freezing temperatures.

 

[Ivan Seeking]

My personal opinion - this is a complite nonsense. What are his calculations?

So you reject the claims of a Nobel-Prize winning physicist but have no calculations. Got it.