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?

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.

 

[LeonhardEuler]

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.

I see what you're saying, but I wasn't making that argument. I said "If there is any energy reduction...". I was avoiding the question of whether there was any such benefit, and just pointing out the consequences that would result if it did exist.

 

[vanesch]

This is food for thinking. How does this then apply to something like DESERTEC ?
Clearly, the CSP plants will have a much darker color than the original desert ?

However, there seems something to be missing in the whole balance.
The Earth receives about 176 PW from the sun, and the potential unwanted greenhouse effect comes from a drive of the order of a few watts per square meter of Earth surface, which corresponds to about 1% of the average solar influx if I'm not mistaking. Now, humanity as a whole consumes about 16 TW of technical energy (mainly fossil fuels), which is less than 1/10000 of the solar power, while the potentially problematic greenhouse effect is rather of the order of 1 % so 100 times more. In other words, the extra "heating power" due to the greenhouse effect that we think will give us trouble, is of the order of 1.7 PW or something (1% of the total solar power we receive) - we're talking orders of magnitude.

Now, suppose that all human power consumption (the 16 TW) is now produced by black solar power plants (CSP plants for instance). Their blackness corresponds then to 16 TW of absorbed solar energy. Meaning that this 16 TW is *peanuts* as compared to the 1.7 PW we would like to reflect back in the sky.

So I don't see how black solar panels on the roofs could ever be a problem (unless it means that they produce 100 times more power than humanity needs) ; but by the same token, I don't see how painting roofs white is going to change anything either.

We'd need to reflect back about 100 times more energy than we are consuming. We'd need to paint an area white that is about 100 times bigger than the solar farm that would power humanity.

 

[sylas]

This is food for thinking. How does this then apply to something like DESERTEC ?
Clearly, the CSP plants will have a much darker color than the original desert ?

Yes, changes and landcover and changes in vegetation have a substantial effect, by altering albedo.

However, there seems something to be missing in the whole balance.
The Earth receives about 176 PW from the sun, and the potential unwanted greenhouse effect comes from a drive of the order of a few watts per square meter of Earth surface, which corresponds to about 1% of the average solar influx if I'm not mistaking.

Sounds about right. I tend to think in terms of values per square meter. The total solar input is about 340 W/m². 30% or so is reflected, and so Earth absorbs roughly 240 W/m². Some of that is absorbed by cloud and the atmosphere; we get roughly 184 W/m² at the surface; of which some more is reflected to give approx 160 W/m² absorbed at the surface, on average.

(Numbers from Trenberth and Keihl, cited and illustrated in [thread=307685]msg #1[/thread] of "Estimating the impact of CO2 on global mean temperature".)

Doubling of CO2 would give an additional 3.7 W/m²; this works out to about 2% of the solar flux seen at the surface where it could be reflected by surface cover. Actual CO2 increases from human activity are less than a doubling, so far. Your estimate seems a good guide to me.

Now, humanity as a whole consumes about 16 TW of technical energy (mainly fossil fuels), which is less than 1/10000 of the solar power, while the potentially problematic greenhouse effect is rather of the order of 1 % so 100 times more. In other words, the extra "heating power" due to the greenhouse effect that we think will give us trouble, is of the order of 1.7 PW or something (1% of the total solar power we receive) - we're talking orders of magnitude.

Yes indeed. The impact of actual direct energy production is a drop in the bucket. However, reducing your energy consumption has a much stronger impact on the Earth's energy balance indirectly. Most of our energy for power is from fossil fuels; and so when you consume less energy, you reduce the greenhouse impact. That is how energy use makes an impact, if you feel so inclined.

There's another reason why heat from consumed energy is not so important. That gives you a certain energy per unit CO2. But once the CO2 is in the atmosphere, it gives a certain power per unit CO2 of continuous additional heating.

I said I would try my own numbers. And, by the way, Chu appears to have been referring to an upcoming publication by other scientists; not his own calculations directly. The paper is not yet available. When it is, we can compare our own estimates with their methods.

Here are my numbers:

Heating effect of one ton of carbon dioxide

The atmosphere weighs about 10 tons per square meter. (This is air pressure)

The atomic weight of air averages out at about 29, carbon dioxide averages out at about 44. There's about 385 ppm(v) of carbon dioxide. Carbon dioxide therefore weighs in at about 10 * 44 / 29 * 385 * 10^-6 = 5.84 kilograms, per square meter.

Surface area of the Earth is about 5.15 * 10¹⁴ square meters.

The extra energy feeding to the surface from additional carbon dioxide is a logarithmic relation; you get 3.7 W/m² per doubling. The rate at which this increases from increasing CO2 levels is given by a derivative. Let S be the surface area, and C be the total tonnage of carbon dioxide in the atmosphere. Let P be the difference in energy by adding an additional tonnage D. The derivative dP/dD, at D=0 for the present, gives the rate at which available power increases with additional CO2. We have

\begin{align*}<br /> P &amp; = 3.7 S \log_2 \left( \frac{C+D}{C} \right) \\<br /> \frac{dP}{dD} &amp; = \frac{3.7 S}{\log_e(2) (C+D)} \\<br /> C &amp; = 5.84 \times 10^{-3} \times S \\<br /> \frac{dP}{dD}_{[D=0]} &amp; = \frac{3.7}{0.693 \times 5.84 \times 10^{-3}} \approx 914 W<br /> \end{align*}​

That is; one ton of CO2 in the air is about an additional 914 W of heating; almost a kiloWatt.

To compare with how much energy you actually get to use from burning fuels; numbers vary depending on the fuel and application. For example, using coal for power generation, you get a bit less than 1 kilogram of CO2 for about 1 kWhr of generated power.

Hence, 1 ton of CO2 is emitted to get enough energy to power a 914 W heater about 1000 hours, or around 42 days.

There's another wrinkle here. Not all emissions remain in the atmosphere. About half gets flushed into other reservoirs, especially the ocean. The trend of increase in atmospheric CO2 is roughly half the rate of emissions.

The effect of painting a roof

Moving a roof from an albedo of 0.2 to 0.7 seems entirely feasible. We have about 184 W/m² available to be reflected. Reflect half of that from a 100 m² roof, and you are saving power at a rate of 9200 W. That should offset about 10 tons of CO2.

These numbers are annual global averages. You would of course reflect nothing at night, and much more in the day. But the global annual average works out as calculated.

Now looking at the reference given by Ivan Seeking back in [post=2292644]msg #19[/post], I see that the study by Akbari et al, apparently quoted by Professor Chu, speaks of retrofitting a roof to give at least 0.4 extra albedo, with 100m² of roof off-setting about 10 tons of CO2. The calculations will, I suppose, be available when the paper comes out soon.

But in the meantime, their estimate fits well with the technique I have used... except that I used CO2 actually in the atmosphere, rather than CO2 emitted. 10 tons in the atmosphere results from about 20 tons emitted.

We'd need to reflect back about 100 times more energy than we are consuming. We'd need to paint an area white that is about 100 times bigger than the solar farm that would power humanity.

I don't think we really care very much about the energy being consumed itself. It doesn't make much difference to Earth's energy balance. What makes more difference is the changes in the impact of the Sun's energy as the composition of the atmosphere changes.

Note that in Professor Chu's comparison, he compares a certain area of reflective surface with cars being used for a certain period of time. That is; a reflective surface gives you a certain wattage of reflected power, continuously. Consuming fuel gives you a certain fixed amount of energy. The emitted CO2 gives you a certain wattage of absorbed heat, which continues to have its impact year upon year long after you've finished with the energy it produced.

Cheers -- sylas

 

[vanesch]

I don't think we really care very much about the energy being consumed itself. It doesn't make much difference to Earth's energy balance. What makes more difference is the changes in the impact of the Sun's energy as the composition of the atmosphere changes.

I think I agree with what you write, but I think you missed the point I was trying to make. As I didn't want to look into numbers of surfaces, changes in albedo and all that, I wanted to reduce this problem to its most elementary constituent, which is the ratio between the "greenhouse gas forcing" power, and the "solar power plant power", in orders of magnitude.

As you confirm, with CO2 doubling, we can expect a greenhouse gas forcing which is of the order of 2% of year-average solar power on the ground, or about 1% of the solar power on the top of the atmosphere. That's the kind of heating that makes for the unwanted heating, of the order of 1.7 PW.
Humanity's energy consumption is about 100 times smaller than this, namely 16 TW or so.

Now, what probably misled you in what was my argument, is that by coincidence a big part of the greenhouse gas forcing is probably caused by human CO2 emission to produce this 16TW, but this is besides the point. Let us imagine for sake of argument that the forcing has nothing to do with human energy consumption or anything ; that we just want to get rid, one way or another, of 1.7 PW of "too much" solar heating.

Now, let us consider that we equip all roofs with (dark) solar panels, be it photovoltaic, or thermal solar panels. They would have an "extra heating" effect, but if all these solar panels are providing humanity with their energy, namely 16 TW, that would mean (assuming them to be 100% efficient for sake of argument) that their surface is such that they take about 16 TW of extra solar energy. So they "heat the earth" by 16 TW extra. Peanuts in comparison to the 1.7 PW.

Now, on the other hand, suppose that we change them into white surfaces. They won't work anymore as solar panels then. Then we can expect that they *reflect* 16 TW. Also peanuts in comparison to the 1.7 PW we want to get rid of.

So that's why I said: in order for this to set off the greenhouse gas forcing, the needed surface that has to be whitened must be about 100 times larger than the surface we would need to cover with solar panels that can provide humanity with all its energy.

This is independent of the fact whether it was this energy which was the cullprit of driving the greenhouse gas emissions. I was just comparing the surface needed to provide humanity with solar power, and the surface needed to reflect back in space a 100 times bigger power.

This reasoning was triggered by the discussion of whether, as an individual, one should put solar panels on one's roof (in order to produce energy, but they are dark), or whether one should paint one's roof white (in order to reflect sun power).

The conclusion is that you should put solar panels on your roof which give you the energy you need, and then paint a 100 times larger area white. (if you are representative of the human power consumption) Because we need to reflect about 100 times more energy to fight the greenhouse gas forcing than we need energy for our own consumption.

Another way to put this, is that in order to reflect back about 2% of surface-reaching solar power, we need, well, to paint white about 2% of the Earth's surface. Earth's surface is about 510 10^6 km^2 which means we need to paint white:
about 10^7 km^2 or about 3000 km x 3000 km.
In order to power humanity with solar power, we need in principle a 100 times smaller area, about 300 km x 300 km (assuming 100% efficiency).
This is the earth-size version of the solar panel on the roof and the area that needs to be painted white.

BTW, the surface that needs to be painted white, 10 million square km, is the surface of the entire USA. The surface of the USA needs to be painted white in order to offset the forcing of a CO2 doubling.

In fact, it needs to be bigger, because we won't go from albedo 0 to albedo 1, but this might even be compensated with solar power plants not being 100% efficient, so their ratio might still remain 100.

Very very rough rule of thumb estimation of course.

 

[archis]

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

Lets start calculations with how many roofs you will have and with what reflecting properties. I don't know, you don't know and i doubt the nobel guy knows too. I am very sceptical about this, becouse their is enough areas like Arctica and Antarctica which reflects light + the sea + deserts. In all this reflection sum i think roof area is like
waterdrop in the sea. Altough to support this idea is bether then doing nothing and then we could see test results.

 

[turbo]

Lets start calculations with how many roofs you will have and with what reflecting properties. I don't know, you don't know and i doubt the nobel guy knows too. I am very sceptical about this, becouse their is enough areas like Arctica and Antarctica which reflects light + the sea + deserts. In all this reflection sum i think roof area is like
waterdrop in the sea. Altough to support this idea is bether then doing nothing and then we could see test results.

You are missing a big part of the picture. Highly reflective roofs don't just bounce more visible light back to space. They reduce the amount of energy absorbed by the building. This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

We don't need to modify the albedo of huge areas of the world to achieve these efficiencies - just reduce the cooling load building-by-building. As I mentioned earlier, when I bought this place, it had a dark asphalt-shingle roof, and the place was really tough to cool, even with a large 220V AC unit. I had 1" of Styrofoam installed over that roof, capped with reflective standing-seam Galvalum roofing. Now we can keep the place cool with just a couple of small portable 120V AV units that vent through windows, and our summertime electric bills are much smaller. I live in Maine, and the summers are generally not real brutally hot, but imagine the savings in electrical generation and transmission if such roofs were installed all across the deep south. Summer brown-outs could be a thing of the past.

 

[vanesch]

This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

This is correct, but has more to do with more environmentally-friendly building than with painting everything white.

From the link of the OP:

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.

(emphasis mine)

If it were to use less air conditioning, there wouldn't be any reason to paint roads white. I don't know of any culture yet which cools pavements with airconditioning

So clearly, the message of Chu is about increasing albedo to compensate for CO2 forcing.

Another indication is that he equals the change of color of roofs and so on to a *fixed quantity* of CO2 (and hence forcing): cars times 11 years. If it were to reduce consumption, such as using less AC, he would have talked in terms of emissions PER UNIT OF TIME (so many tonnes of CO2 per year).

 

[archis]

You are missing a big part of the picture. Highly reflective roofs don't just bounce more visible light back to space. They reduce the amount of energy absorbed by the building. This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

Good point but just now i see some paradoxes. I say let's paint all buildings black in the north to lover the costs of the house warming in the winter. You are worried about conditioning costs, energy in summer i am worried about house heating in the winter. There where i come from we put heath isolations in walls and the roofes to fix both of these problems.

 

[Skyhunter]

Good point but just now i see some paradoxes. I say let's paint all buildings black in the north to lover the costs of the house warming in the winter. You are worried about conditioning costs, energy in summer i am worried about house heating in the winter. There where i come from we put heath isolations in walls and the roofes to fix both of these problems.

What you are forgetting is that the Sun's rays are not as direct, nor do the days last as long during NH winter when buildings need to be heated. A slightly warmer roof for 8 to 10 hours a day in winter would have far less impact on energy consumption than the cooler roof for 14 to 20 hours in the summer. Canadians use air conditioning in the summer the same as Floridians do.

I have heard Chu mention the reduced cooling costs when discussing "paint your roof white". Citing this one example is somewhat out of context IMO.

Do a quick google of "Chu white roof air conditioning" and you will find that all the discussions include reduced energy requirements for cooling.

http://physics.suite101.com/article.cfm/obamas_energy_secretary_suggests_white_roofs"


http://climateprogress.org/2009/05/27/energy-steven-chu-white-roofs-geo-engineering-adaptation-mitigation/"

It was a geo-engineering scheme that was “completely benign” and would keep buildings cooler and reduce energy use from air conditioning, as well as reflecting sunlight back away from the Earth.

 

[vanesch]

What you are forgetting is that the Sun's rays are not as direct, nor do the days last as long during NH winter when buildings need to be heated. A slightly warmer roof for 8 to 10 hours a day in winter would have far less impact on energy consumption than the cooler roof for 14 to 20 hours in the summer. Canadians use air conditioning in the summer the same as Floridians do.

I have heard Chu mention the reduced cooling costs when discussing "paint your roof white". Citing this one example is somewhat out of context IMO.

As I said before, this part of the idea is not bad, but it doesn't have necessarily anything to do with the roof. After all, the best use of the roof is not to paint it black or white, but to install solar panels on it, energy-wise. The other thing to do, is to make more environmentally-friendly buildings (thermal insulation and so on).

So painting the roof is already questionable (use panels instead of paint). But painting the roads and the pavements can only have one goal, unrelated to airconditioning or energy use: it is to change albedo.

Well, I challenge you to point out a gross error in the rough estimate I made, which shows that one needs the entire surface of the USA to be painted white in order to offset the CO2-doubling forcing, so this proposal is akin to proposing to take buckets of water out of the sea to compensate for sea-level changes. Of course "it helps a (very little tiny) bit". But "does it matter" ?

 

[skeff]

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.

Hi!
There are open, international, databases of "emission factors" for greenhouse gases for anything you can think of. My database tells me the following:

1. Asphalt, Road&Pavement = 0.14 kg CO2equiv/kg asphalt
and in unit area (metres squared): 134 kg CO2equiv/m2 asphalt

2. Concrete, Road&Pavement = 0.13 kg CO2equiv/kg concrete
and in unit area (metres squared): 188 kg CO2equiv/m2 concrete

The source of this data is from something called ICE, which I don't really know what is, so I cannot verify this. Anyway, if this is relatively correct, seems asphalt wins because of structral differences, when comparing area covered with the same function of road and pavement.

 

[mgb_phys]

seems asphalt wins because of structral differences, when comparing area covered with the same function of road and pavement.

The asphalt coating is much thinner, more of the structural load is taken by the roadbed. Concrete is poured much thicker and takes a lot of the load, needing less material in the roadbed - so concrete freeways are overall cheaper to build.
The concrete surface also lasts longer.

 

[DaveC426913]

After all, the best use of the roof is not to paint it black or white, but to install solar panels on it, energy-wise.

The energy from those panels adds to the net energy of the Earth; it will come out somewhere as waste heat. White roofs put that energy back into space.

The asphalt coating is much thinner, more of the structural load is taken by the roadbed. Concrete is poured much thicker and takes a lot of the load, needing less material in the roadbed - so concrete freeways are overall cheaper to build.
The concrete surface also lasts longer.

If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start. Concrete is the largest single material produced by Mankind. It produces 5 to 10% of the world's CO2 emissions [http://en.wikipedia.org/wiki/Concrete#CO2"]*.

*(according to Wiki)

 

[mgb_phys]

If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start.

Wooden freeways?
You could use the snap-together wooden flooring from Ikea, then you wouldn't have to cone off miles of freeway for months on end to lay a bit more asphalt.

 

[skeff]

...
If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start. Concrete is the largest single material produced by Mankind. It produces 5 to 10% of the world's CO2 emissions [http://en.wikipedia.org/wiki/Concrete#CO2"]*.

*(according to Wiki)

I'd be really skeptical about that number, 5-10% of net CO2 emissions being caused by concrete. As mgb_phys hinted at, time is essential in the balance of CO2 emissions for a given product or system. What I'm looking at, is plants for a certain kind of processes. The concrete for construction of a plant, is vanishingly small, when set up against the energy used for driving the process during the lifetime of the plant/construction. Chemicals and energy are the main sources of CO2, and usually you are talking about energy in the production of those chemicals too, so it all boils down to how you create energy. It all boils down to fossile fuels.

Biomass incineration, hydropower, saltpower. That's the solution. Forget about moving your car or airplane around, those stationary plants we depend on can reduce their footprint enormously.