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The Easy Fix That Isn’t: White Roofs May Increase Global Warming

Installing white roofs (or painting them white) has been  promoted as a way to help slow global warming. New research shows that white roofs may actually add to global warming. Credit: NNSA/flickr.

[Update, Nov. 8, 2011: WhiteRoofProject.com has posted a new blog in response to some of the coverage of Jacobson’s recent study on white roofs, including mine below. They offer some important points that I didn’t cover in my piece, so their blog is worth reading. In particular, previous research has shown that in some specific regions, like those near the equator, white roofs probably offer more benefits than drawbacks, from an emissions perspective – something Jacobson’s computer models haven’t yet accounted for.

I should stress that Jacobson’s study is just one piece of research in a broad field, but it’s an important study nonetheless. More than anything, it points out that the science isn’t settled on white and cool roofs, and that more research can help figure out where and how they can be put to good use.]

If you’re interested in staving off climate change without trying too hard, painting your roof white seems like a complete no-brainer. It’s far cheaper than trading in your SUV for a Prius, and it turns the laws of physics to best advantage. Dark roofs absorb sunlight that heats up your house, office tower, or apartment building. That means you’re bound to crank up the energy-intensive air conditioner to keep pace in the summer months — and since electricity in the U.S. comes largely from fossil fuels, the net result is more heat-trapping greenhouse gas emissions, and more global warming.

But a white roof does just the opposite. It bounces sunlight right back into the sky, just as light clothing helps you stay cool in the summer. Cooler buildings need less air conditioning, which translates to fewer emissions of heat-trapping gases. That’s why Energy Secretary (and Physics Nobel prizewinner) Steven Chu endorsed the idea back in 2009 and why cities like New York and Philadelphia have launched white-roof projects.

Unfortunately, what seems obvious is not always true, and a new study available online and soon coming out in the Journal of Climate reveals some potentially bad news for white roofs. When Stanford University engineer Mark Jacobson, and his grad student John Hoeve modeled the total climate response to white roofs and other urban surfaces, they found the lightening may actually cause more global warming.  

Here’s why: the sunlight that bounces off white roofs doesn’t all fly out into space. A lot of it is absorbed by particles of soot and other dark-colored pollutants that float around in the atmosphere (those same particles are already responsible for a good portion of global warming). The particles heat up, just like your house would have, and the net result is a warmer atmosphere. You house might be cooler, but it would be at the expense of heating the planet.

In short, says Jacobson in a press release: "There does not seem to be a benefit from investing in white roofs. The most important thing is to reduce emissions of the pollutants that contribute to global warming." So much for trying to take the easy way out.

On the other hand, says Jacobson, there is another way to use your roof in the fight against climate change: cover it with solar panels. The panels intercept sunlight before it hits the roof, so your house doesn’t heat up so much. They don’t bounce the light back into the atmosphere where it can heat up soot particles. And they generate at least some electricity without emitting greenhouse gases. It’s not quite as cheap as painting your roof. But unlike that feel-good solution, it’s actually likely to be effective. 

« Climate in Context

Comments

By Monrudee Gruenberg (Pattaya, Thailand 20150)
on November 1st, 2011

Apparently Ms. Kenward is not a scientist and it is doubtful she even read the paper. I quote from the paper referenced in the article:

“A worldwide conversion to white roofs, accounting for their albedo effect only, was calculated to cool population-weighted temperatures by ~0.02 K but to warm the Earth overall by ~0.07 K. White-roof local cooling may also affect energy use, thus emissions, a factor not accounted for here. ***As such, conclusions here regarding white roofs apply only to the assumptions made.***(emphasis added).”

The commonly used tool to fool the masses is computer models that are only as good as the assumptions programmed into them. This feeble attempt at science, didn’t even bother to calculate the additional energy used to cool the hotter buildings. The paper also purports that the reflected energy from the white roofs is converted to long-wave infrared via soot particles, totally neglecting the cooling, shading effect of the particles in the first place that volcanic eruption data clearly shows is more relevant.

Lies, lies and more lies. Nothing at all resembling science here.

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By Alyson (Climate Central)
on November 2nd, 2011

While the model calculations in this paper didn’t account for how lighter roofs will affect the energy used to cool and heat buildings, Jacobson (the lead author) didn’t ignore this point in his paper. As he points out, lighter roofs will help keep buildings cooler in the summer (requiring less air conditioning and reducing emissions) but they keep them cooler in the winter months too, which means the buildings typically need more heating. That extra heating also increases greenhouse gas emissions. Previous studies have found that “white roofs increased winter heating more than decreased summer air conditioning,” meaning white roofs may increase overall emissions.

More research will undoubtedly look at integrating these two different aspects (the emissions and the effects of more reflected light) but in the meantime, this finding shows that white roofs aren’t a silver bullet for mitigating warming.

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By Aubrey Enoch (71837)
on November 4th, 2011

We are experimenting with a green roof retrofit system to reduce solar heat gain. The photosynthetic shading of living and work spaces by building support structures for climbing vines can provide tremendous benefit in reduced cooling costs. We welcome contact with anyone pursuing application of photosynthetic shade. We invite anyone to use any part of our designs. Visit us at solarincome dot com.
We are all breathing the same atmosphere and sunshine is the only income we have got

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By Timothy Chase (Seattle, WA)
on November 5th, 2011

From your essay above:  “Here’s why: the sunlight that bounces off white roofs doesn’t all fly out into space. A lot of it is absorbed by particles of soot and other dark-colored pollutants that float around in the atmosphere (those same particles are already responsible for a good portion of global warming). The particles heat up, just like your house would have, and the net result is a warmer atmosphere. You house might be cooler, but it would be at the expense of heating the planet.”

The problem as it is explained in the article itself isn’t so much that the temperature of the atmosphere heats up per se.  That after all wouldn’t be that much of a problem since the warmer atmosphere would be higher up than the surface, reducing the distance that energy has to travel to make it to space.

Rather, given a cooler surface and warmer atmosphere at the relevant altitudes you reduce moist air convection, responsible for heat and moisture transport.  In the troposphere moist air convection plays an important role in heat transfer.  But such convection largely depends upon air at the surface being warmer than the air above it and warm air’s tendency to rise, resulting in instability.  Reducing the temperature differential reduces this instability.

Furthermore, the moisture that is normally transported as the result of such moist air convection goes into clouds, reducing the amount of sunlight that reaches the lower atmosphere and the surface.  So by increasing the albedo, at least above the cities where black carbon pollution is more common and thus has a greater tendency to absorb radiation from the surface, we actually end up reducing the albedo further up where it would be more effective.

Anyway, not saying that the journal article is right.  I believe for that you would need some serious number-crunching - which they purport to do.  But it is plausible.

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By Dave (Missoula)
on November 7th, 2011

This result seems to go counter to what I understand as a major climate problem - that the decrease of (white) snow and ice due to higher temperatures in polar regions leads to greater absorption of solar radiation on the ground, and thus higher temperatures overall. Does the paper address this?

If white roofs are increasing warming by reflecting light into the sky and warming soot, then does the lack of white snow in polar regions then decrease warming overall? I have a hard time swallowing that!  A white membrane roof keeps my house so cool that I never use AC even on 100 degree summer days. In the winter this roof also keeps snow on the roof, which I believe adds insulation value (after many cozy nights spent in snow caves in the mountains.)

If the foregone winter solar heating gains of a white roof really are more consequential than the lost insulation value of snow on a dark roof, then would it make sense to have another removable dark roof covering to install in the winter? Rather than simply disparaging efforts to improve efficiency and reduce carbon emissions through building design as “taking the easy way,” you might suggest more beneficial actions based on new information. I think the writer may have been the one taking the easy way there!

Finally, I’d like to note that it is quite possible to do both “easy” things AND hard things simultaneously. I don’t think Chu et al were suggesting that white roofs alone would solve the climate issue.

Thanks for the article!

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By SqueakyRat (02903)
on November 16th, 2011

Dave from Missoula—good point about snow and albedo. it really is pretty hard to see how lowering albedo could be a positive feedback for warming while white roofs—increasing albedo—also contribute to warming. Something doesn’t add up here.

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By proudskeptic (Portland, Oregon 97229)
on November 17th, 2011

Much ado about nothing. Keep your roof whatever color it is and quite spending money on ridiculous studies.

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By Matt Farkas (Appleton WI, 54913)
on August 8th, 2014

It looks like Jacobson modeled taking into consideration at least soot, and maybe increased GHG concentrations above cities? It seems like above cities there would be a lot more “layers” of particlces for IR radiation to bounce around in, vs. the at the poles. This would account for increased wamring from higher albedo cities vs. the poles. Once again, the complexity of global warming rears it’s ugly head.

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