White paint saves the planet?

Business Week recently published an odd little essay by Greg Blonder, someone I've not come across before. He posits that painting your roof white will do more to offset anthropogenic climate change than installing photovoltaic panels. Seriously. The science appears to be pretty sound, but it has the whiff of a thought experiment rather than a practical idea.

Here's the essential section:

...imagine a solar photovoltaic panel. Unlike burning coal or oil, the production of photovoltaic electricity does not add to the stock of global warming gases permeating our atmosphere. The panel's surface is pitch black--all the better to absorb sunlight and convert it into electricity. On average, a panel that's 1 square meter in size will receive 300 watts of sunlight over a 24-hour period. In turning that sunlight into electricity, about 80% of that energy is lost due to the inefficient conversion process.

But if the remaining 20% is used to replace the equivalent amount of fossil fuel needed to produce that electricity, the result would be equivalent to eliminating--every single day--a square-meter column of CO2 gas extending from earth to outer space. Put another way, each day that one panel would offset the equivalent of those extra 2 watts of global warming per square meter of earth [the total due to greenhouse gases and other anthrpogenic effects].....

But here's the rub. If, instead of a black solar panel absorbing light and producing electricity, you simply painted that square meter white, it would reflect back into outer space perhaps 50 of the 300 watts incident from the sun. So it would take about 25 days for the solar panel to catch up with the more efficient reflection of sunlight that the white-painted panel would provide in a single day.

Hmmm. So, if millions of homeowners simply painted their roofs white, we could cool the planet enough to mitigate climate change? In theory, I suppose. In addition to the reflectivity, white roofs should save a few kilowatt-hours in air-conditioning in those climes where air condition is a big deal.

The problem, as Blonder admits, is "you can't run a washing machine on white paint." None of that paint actually reduces greenhouse gas emissions. And so we'd still have to worry about increasing emissions, which would eventually overtake the white-paint effect.

Second, white roofs are only really good at reflecting solar radiation when they're clean. That means a fair bit of maintenance to keep the roofs operating a peak efficiency. (Of course, solar panels have to be clean, too.) Third, what is the source of the paint? Latex may not be the best for roofs, and what's the latex supply like even if it is? Oil-based paints pose their own environmental problems and would only further tighten the petroleum market that many belief is already in "peak" territory.

But given all these caveats, I like Blonder's thinking. White paint may not be a seriously good option for mitigating climate change, but increasing the planet's albedo by painting roofs white, could have a measureable negative effect on the total amount of global climate forcing. More low-tech, counter-intuitive suggestions are just the thing we need (as opposed to foolish, old-style solutions the likes of "clean" coal," seeding the oceans with nutrient-limiting phtyoplankton food or giant mirrors in space.) I'll give the last word (before we get to the comments) to Blonder:

It offers a bigger, faster, and surer contribution to global warming reduction than more photovoltaic cells. And it would save on fuel costs. One estimate puts the annual national energy cost savings of more reflective roofs at $750 million, not including similar savings for lighter roads and parking lots. Perhaps such white roofs and parking lots deserve an energy subsidy from Congress. Perhaps white paint deserves to be traded on the carbon exchanges.

Perhaps white will become the new green.

Probably not. But bonus marks for counter-intuitive thinking.


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But isnt the long wavelength radiation that is reflected by the earth's surface what is heating the atmosphere for the most part? I am not sure I understand how merely reflecting more radiation of the surface helps reduce global warming.

I've alays advocated this, although one needs to distinquish between an instantaneous forcing (painting a square meter white creates minus whatever watts of forcing immediately, and as long as it reflects sunlight), a PV panel initialy produces no change in GHG forcing, that accumulates over time as the saved CO2 emmisions accumulate. So you have to put in some time constants, like how long the paint lasts, how long the PV panels last, and how long the CO2 stays in the atmosphere.

Your third point is not logically sound in conjunction with opposition to replacing solar panels with white paint. Yes, the energy costs and CO2 emitted in making something should be considered.

But, as it stands now, solar panels take more energy to make then they will produce in their lifetime. (The doesn't even count the creation of batteries to store the power they generate.) Solar panels are an overall energy sink. White paint most definitely has lower creation CO2 production than solar panels, so it's not a valid objection.

You're right that this sounds like a thought experiment. It would be nice to have concrete information that it works.

What if we installed mirrors instead, or aluminum sheets?

Blonder's idea is a real blunder.
Is he aware that the reflected rays would need to penetrate more than 120 km of dense radiation-scattering atmosphere before they would escape into "outer-space"? Attempting to reflect those rays back through the atmosphere would actually contribute to warming!
Here's a hint for Blonder: It isn't the Sun that is causing global warming - It's the atmosphere!

By HPLC_Sean (not verified) on 18 Jun 2007 #permalink

I get the feeling that none of the comments here are by climate scientists. Me neither, but here goes. Changing the albedo of the earth would certainly affect climate (for instance by painting it white). The scale here is kinda crazy though. The surface area of the earth is just over a half billion sq. km or 0.5 million billion sq. meters. Even a few million home would have a incredibly tiny impact on the overall albedo of the planet.

His point was that solar panels, with their low efficiency, also do very little. It seems like an apples to oranges argument though. It seems to me you would have to work out how much carbon would have been produced by another electricity-generating method, say burning coal, and calculating how many watts of heat energy that carbon would trap over its lifetime in the atmosphere.

Or how about we keep the solar panels and paint the asphalt driveway white instead? Creativity is good, though this articles seems like more propaganda against solar panels.

By Theodore Wilson (not verified) on 18 Jun 2007 #permalink

Hey we should be combing all practical steps.
- Making the earth more reflective will slow down global warming. Period. The idea that a black earth keeps us cool is kooky.
- any complex of buildings is going to have areas available for both solar and reflection. e.g. A house plus garden sheds, driveways, etc.
- If a larger urban area lowers its "heat island" effect then you get a double wammy as less carbon producing fuel is consumed to air condition the place.
- I don't not for a second believe the claim that PV panels don't "repay" the energy cost of manufacturing for one second that's an old cynic's cliche

This whole idea seems to be the equivalent of painting over your car's rusty fender. It doesn't fix the source of the problem, and eventually the rest will win.

solar panels take more energy to make then they will produce in their lifetime.
Source? If this were true, it would be a fascinating and important bit of information I've never heard.

The parts of my house's roof that are pitched have white shingles. I don't know if any of the reflected light makes it all the way back into space, but it does keep my house cooler in the summer. This part of the country has more summer than winter, so I picked white roofing rather than black as the best way to lower my energy bills (which also lowers the electric company's production of CO2, though I had not heard of global warming many years ago when I made that decision). The parts of the house with a dark-colored flat roof get much hotter, so a light-colored coating (not latex paint) would be a good idea there, if I knew of such a thing (and it was not expensive).

(I have a white car for the same reason.)

Well since I'm here i might as well clear up some confusions. The reflected light is shortwave and a fair amount of it will go straight back out to space (on a sunny day), as you could see easily enough from satellite photos. The greenhouse effect of the atmosphere works on the longwave emitted due to the planet's temperature (much lower than that of the sun!). So there is no question that the effect is real, but as others have pointed out, it's small in global terms. That doesn't mean it is not useful in the context of of urban heat island/building design issues in hot places.

Green roofing (by which I mean plants!) is another useful idea along similar lines, which also helps smooth out peaks in rainfall run-off. But white paint is a lot cheaper.

Er, rest = rust.

The effect of painting your roof white is small on a global scale, but so is the effect of solar cells, or even giving up all greenhouse-gas emitting activities altogether. The net impact of brightening human construction worldwide could actually be pretty significant, I suspect.

I remember reading about a study recently that estimated that planting more forests in high latitudes would actually have a net warming effect on the earth, because the decrease in albedo would counteract the increased carbon fixation. Which means we should plant more trees in the tropics, not stop planting trees.

By Matthew L. (not verified) on 18 Jun 2007 #permalink

But if we painted everything white, just imagine the disastrous increase in the incidence of snow blindness!

Let me know before we start doing this, so I can start buying stock in Luxottica, the corporation that owns the Ray-Ban brand. ^_^

By Feral Kitten (not verified) on 18 Jun 2007 #permalink

California is already requiring this as a way of reducing A/C load. Flat roofs must be painted white. Angled roofs are required (next year, I think) to use new tiles that are "white" in the infrared spectrum. They're any color you want in the visible spectrum, but the infrared bounces off.

They're also looking at changing the pigments in asphalt to reduce urban heat island effect.

I had an idea for a more ambitious version of this involving pulverising huge amounts of limestone and dropping it from aircraft over the central Australian deserts.

Then I think I sobered up.

By Ian Gould (not verified) on 18 Jun 2007 #permalink

A recent analysis (http://coriolis.lbl.gov/~hashem/share/CoolRoofsAndGlobalCooling.pdf) looks at the relationship of increasing the albedo of the urban areas to the rate of global warming. Increasing the albedo of urban areas is worth about 1/3 year of global warming. At current prices for CO2, it is worth over $100B. I also suggest visiting http://HeatIsland.LBL.gov.

Note to moderator: I do not know how to provide a link for the article. Can you teach me, please.

Renee :

... solar panels take more energy to make then they will produce in their lifetime

I've been hearing this claim since the 1980s. In that time, solar panels have undergone several improvements in efficiency, and several significant manufacturing changes. Yet the claim has never budged. And it's never been referenced. I'd like you to explain why it should be believed.

Speaking of roofs I just wrote an article on "greenroofs" for Inter Press news wire. Among the benefits mentioned is a reduction in the heat island effect. I don't know if this is conjecture, a working hypothesis, or "greenroofs" just happen to look pretty. But its definitely a design trend among architects.


>infrared bounces off

but there's not all that much infrared in the incoming sunlight --- and what's needed is something that will heat up and be "bright" in the infrared but in some narrow band to which the atmosphere's transparent so the outgoing energy actually leaves the planet successfully. Something that 'fluoresces' in a band where the atmosphere's transparent. I think ...

By Hank Roberts (not verified) on 07 Jul 2007 #permalink

I spent many years as a roofer. Painting the roof white, won't work out so well, with most products on the market. It just won't last. However, there is no reason that one can't put white shingles on. Even standard asphalt shingles come in white (though of course, they are made from, well, oil). But there are many ways to put white on ones roof. You can even put the white on and use solar panels. For most roofs, solar panels take up a relatively small percentage of the roof.

Enrique -

Green roofs do reduce the "heat island" effect. They are don't reflect or amplify any heat, the way that asphalt shingles do, nor do they absorb it. It creates a roof that is temperature neutral, for the most part. Also, they tend to absorb rainwater, rather than allowing it to runoff into the sewers or storm drains. The sweetest ones, collect unabsorbed water into a catch basin, to help maintain the roof during dry seasons. I have seen pictures of a building in France, that used a similar principle, to side a building. Bye, bye AC.

Please see the USDOE NREL study about energy payback for PV. Scientists state that based on models and empirical data the payback is currently 1 - 4 years. With assumed life expectancies of 30 years, "37% to 97% of the energy that PV systems generate won't be plagued by pollution, greenhouse gases, and depletion of recourses (PV FAQs: What is the energy payback for PV? http://www.nrel.gov/docs/fy04osti/35489.pdf)


when sunlight strikes the earths surface it can do three things:

1 get reflected straight back into space

2 get absorbed and re emitted as heat (infra red light)

3 absorbed by plants and also re emitted as heat.

when the sun's light falls on the polar ice caps most of the light gets reflected straight back into space because white reflects the visible spectrum (the visible spectrum is the majority of the sun's output - this is why most things are geared up to see using those frequencies). the visible light is barely absorbed by the atmosphere/ co2 and thus doesn't heat the atmosphere. the polar ice caps are cold because not only do they have little light falling on them but the majority of the light is reflected back into space.

when the sun falls on a jet black road most of the light is absorbed and re emitted as heat. it is this, the infra red frequency/heat that is absorbed by the co2 molecule. the atmosphere is more likely heated up via heat than by visible light, thus if you have more co2 or more black surfaces then the more the earth's atmosphere heats up.

to counter this heating effect, if the planets roofs were painted white a tremendous amount of enrgy from the sun would be reflected staright back into space thus making the atmosphere cooler than it would otherwise be. the double whammy of white paint is that a house/ building that reflects energy off it is most likely a much cooler one which means if you are running air conditioning the air conditioning will work easier and this means less coal will be burnt to provide power to the air con.

By wookiemeister (not verified) on 25 Dec 2008 #permalink

what a narrowminded dork

By wookiedork (not verified) on 25 Dec 2008 #permalink


the very notion of white paint must be horrifying i'm sorry that white paint makes you so angry.

as an edit: the percentage of black body radiation versus visible radiation is about half , half.

by reflecting at least half of that radiation away from your house it will be much cooler. in some countries they paint their houses white to reflect the sun's energy.

if you can't accept that white paint (or similar) will keep your house cool and reflect energy back into space then all the science text books in the world won't help you.

i wish you well on your life journey "wookiedork"

By wookiemeister (not verified) on 26 Dec 2008 #permalink

Ok, I live in Puerto Rico, a tropical island in the cariean, we have sun ligth almust 365 days at year, roofs are made from concreet and it get very hot when there is no white paint on top, the other problem is the roof dasign, it sould be in angle whith windows on top, so hot air can escape. hot air gets acumulate at the higest point and the cold air coms down if the roof is orizontaly flat the hot air can not escape and there is an other problem, algy, micro algy grow wen the rain fall and water guet stoc like litle lagoons over the roof, the micro algy grow very fast and when the roof gets dry the dark algy gets black, absorving radiation from sun, so people need air conditioner that produce more hot at the out side part of the recidence, so not only the concreet dark roofs but also the streets, high ways, dark colored cars and every over exposed dark oject or surface maded by man. Yes the over exposed dark surfaces produces a negative efect that increase global warming.

I wouldn't paint the roof white, but here in the land of tar roofs (Philadelphia,PA), I know that there's white finishing products for roofs that work much better. The last time I had work done on my roof, I actually paid for a white coating because I don't have an air conditioner, and I'm not especially interested in getting one.

nice blog.. liked it very much.... a great post which reveals the facts about study recently that estimated that planting more forests in high latitudes would actually have a net warming effect on the earth, because the decrease in albedo would counteract the increased carbon fixation.