February 02, 2010
White Roofs For Cooler Cities In Summer

Painting roofs white in order to cool the planet has been proposed previously. Now some scientists do some computer modeling of the effects of more reflective roofing in cities.

BOULDER—Painting the roofs of buildings white has the potential to significantly cool cities and mitigate some impacts of global warming, a new study indicates. The new NCAR-led research suggests there may be merit to an idea advanced by U.S. Energy Secretary Steven Chu that white roofs can be an important tool to help society adjust to climate change.

But the study team, led by scientists at the National Center for Atmospheric Research (NCAR), cautions that there are still many hurdles between the concept and actual use of white roofs to counteract rising temperatures.

Whiter buildings in cities are of special interest because cities are warmer (especially in the summer) than surrounding regions. The buildings and roads of cities absorb more sunlight than the same areas absorbed before humans built the cities. "Hot time, summer in the city."

White roofs would make a substantial difference.

"Our research demonstrates that white roofs, at least in theory, can be an effective method for reducing urban heat," says NCAR scientist Keith Oleson, the lead author of the study. "It remains to be seen if it's actually feasible for cities to paint their roofs white, but the idea certainly warrants further investigation."

The study is slated for publication later this winter in Geophysical Research Letters. It was funded by the National Science Foundation, NCAR's sponsor.

One third of the urban heat island effect could be eliminated by painting all city roofs white (orby using white materials to make the roofs). Okay, so what would it take to remove the other two thirds of the effect?

Cities are particularly vulnerable to climate change because they are warmer than outlying rural areas. Asphalt roads, tar roofs, and other artificial surfaces absorb heat from the Sun, creating an urban heat island effect that can raise temperatures on average by 2-5 degrees Fahrenheit (about 1-3 degrees Celsius) or more compared to rural areas. White roofs would reflect some of that heat back into space and cool temperatures, much as wearing a white shirt on a sunny day can be cooler than wearing a dark shirt.

The study team used a newly developed computer model to simulate the amount of solar radiation that is absorbed or reflected by urban surfaces. The model simulations, which provide scientists with an idealized view of different types of cities around the world, indicate that, if every roof were entirely painted white, the urban heat island effect could be reduced by 33 percent. This would cool the world's cities by an average of about 0.7 degrees F, with the cooling influence particularly pronounced during the day, especially in summer.

I'm thinking white streets would get us part of the way there. Is it possible to develop a whiter concrete? Would such a concrete make streets too bright at mid day?

The extent of the urban heat effect on rural temperature monitoring stations that cease to be rural when economic development occurs around them is a problem with studies that attempt to track long term temperature changes. This problem became an issue in an important case of researchers accused of misrepresenting data from Chinese temperature monitoring stations (and see here for more details). Those two articles from The Guardian in England are worth a read.

Share |      Randall Parker, 2010 February 02 11:48 PM  Climate Engineering

Brett Bellmore said at February 3, 2010 3:43 AM:

I don't think it's going to get much backing from the global warming people; It's got the serious drawback that, if a big hunk of the urban heat island effect disappears, over the course of a decade or so, that will be another decade where their numbers don't show any warming. A real PR disaster from their perspective.

It is nice to see this idea taken seriously though; Years ago, anybody who suggested it automatically got treated as an idiot.

Of course, in addition to being selective about what climate you apply this solution in, you'd probably want to pay close attention to the difference between IR and visible spectrum properties of the paint, not just any white paint from Home Depot would work best.

Fat Man said at February 3, 2010 10:31 AM:

It is not that easy. 80% of US residences are roofed with asphalt composition shingles, which are fiber mats (wood or glass fibers) coated with asphalt and crushed rock. They are cheap and durable, but will not hold paint. Changing the albedo of cities by using different roofing materials will be a drawn out process.

Highways are another issue. Down south you can use concrete, and even asphalt bleaches out pretty quickly. But up here concrete is destroyed by the winters and the salt, and is hard to patch.

I think the best approach to UHI is planting lots of tall deciduous trees.

Lance said at February 3, 2010 11:36 AM:

In many of your posts on Global Warming, you present it as a bad thing that needs to be stopped or reversed. For many reasons, the warming of the planet has been a boon to humanity over time. Why would we want to reverse that?

etaoin said at February 3, 2010 9:27 PM:

One problem with white shingles is that natural mildew growth on the shingles would turn them black!

Lance said at February 4, 2010 7:36 AM:

What temperature should the planet be?

Bob Badour said at February 4, 2010 8:01 AM:

As the future owner of Canada Citrus Incorporated, I say South Florida and Manhattan can suck it.

masstexodus said at February 4, 2010 10:10 AM:

I put on some shingles made by Malarkey Roofing - they are light grey and have special 3M granules. The are also hail resistant. The roofs don't have to be white - they just need to be lighter than they are now.

Vader said at February 4, 2010 10:26 AM:

Gold is an excellent reflector of infrared radiation.

So pave the streets with gold. It will be a fitting symbol of the millenarianism driving a lot of the global warming activism.

doghair said at February 4, 2010 10:32 AM:

Years ago while working for a telecom company I was involved with solar loading thermal tests of several power supply enclosures (those metal boxes you sometimes see on telephone poles). We discovered a shiny dark surface will reflect more light than a dull white surface. Also, after two days in the indoor test chamber both enclosures were dusty and had very little difference in steady state internal temperature when the test was re-run. However the darker enclosure's rate of temp increase was greater.

While painting roofs white may cause a slight reduction in temperature, I doubt it will be sufficient to justify the cost.

David said at February 4, 2010 10:35 AM:

Who pays to clean the roof due to very costly and labor intensive cleaning requirements? What about the additional insurance burden or added wear?

Are the costs spread pro-rata on mulit-tenant facilities? Even if one tenant has much higher, and separately metered HVAC useage?

We all know a roof gets dirty, right?

What about the warranty? Will having crews on the roof cleaning invalidate the warranty?

Do the lab findings scale over time?

What about the heat island effect?

LB Philadelphia said at February 4, 2010 10:54 AM:

Many of the cities with the worst heat island effects have significant numbers of flat-roofed structures and very little tree cover. This applies to my own area, South Philadelphia, where the average tree cover is about 2.3%. In our situations, it's fairly easy to transition to white or silver roofs from the dark asphalt of the past.

For a real lesson in the heat island effect and the value of tree cover, nothing beats bicycling in the city and turning from a treeless, asphalt-covered street to one where the tree canopy arches over the entire street. Instant relief!

Douglas said at February 4, 2010 11:05 AM:

If you replaced the roof of your home last year, the albedo (reflectance) of the roofing material was a big part of the equation of whether or not it was on the "energystar" list that made it qualify for a tax rebate.
And I don't expect to need to clean my nice white metal roof anytime soon.
Thanks for paying for part of my new roof, y'all!

Donald said at February 4, 2010 11:35 AM:

As far as I'm concerned it is a childish notion. It is nothing less than an attempt to engineer climate change. Anybody that thinks they understand climate processes well enough to do that is living with a big delusion. It would be a correction to something we don't understand and like the proverbial blind man grasping an elephants trunk, prone to wild and erroneous concepts.

Instead of spending a ton of money on a fool's errand, we should be devoting our efforts and resources to understanding how the climate actually works.

Knowing that there are temperature differences between urban and rural areas is nothing like understanding how the total system works with all its complex and not evenly barely understood interrelationships. Boys and girls, this is not just about stamping out excess heat. If you want to build a house of knowledge, start with a foundation and leave the paint till the end.

Mark S. said at February 4, 2010 12:00 PM:

Here in Phoenix, the light colored concrete freeways are covered with a dark material to reduce noise pollution. There are tradeoffs everywhere with different people having different priorities.

Bruce said at February 4, 2010 12:13 PM:

Wouldn't it cost more to heat your home in the non-summer months if the roof was white? I can see white as a good choice for hot sunny climates. But it would be a bad choice for moderate climates.

crosspatch said at February 4, 2010 12:23 PM:

UCLA, I believe, did a study several years ago concerning "cool roof" and "cool pavement" technology and came to the conclusion that it would reduce temperatures in Los Angeles by a degree or so. That would reduce the energy consumption by climate control equipment significantly. Lawrence Berkeley also did a study on energy savings with "cool roof" technology and concluded that energy savings can reach up to 50% and save in both hot AND cold climates when a coating that uses ceramic insulating beads is used.

I got a lot of information from this company's site over the years. I have no affiliation with them, just think the product is a good idea. It also saves landfill space by being applied over existing roofs, extends their lifetime significantly:


Bobnormal said at February 4, 2010 1:09 PM:

I work for an air conditioning company,and our clients have white roofs, Every one of them.Two problems
as noted earlier,these roofs get dirty,not very white,so I'm not sure about the reflectives effects in that regard,
What I do know is,you can add 12-15 degrees to the ambient temperature standing on the roof as opposed to the parking lot,I've done the measurements,I saw the evidence with my own eyes,

Bob B said at February 4, 2010 1:55 PM:

You ask about the rest of the heat island effect after the roof and pavement reflectance issues are adressed. One major contributor is air conditioning equipment. Those mechanical units put out a lot of heat. You will never ratchet that back to zero unless they are not run. One of the concepts of LEED designed buildings is that in theory, natural ventilation thru operable windows and daylighting with photsensors to shut off the lights will reduce the air conditioning loads. It works when there is a certain temperature and humidity range. Places like Seattle, San Francisco, and Portland have that kind of temperature to make that work. Of course those are the kinds of cities where urban heat island effect is the least problematic.

th said at February 4, 2010 4:53 PM:

Concrete, bricks, and asphalt are heat sinks, shingles don't store heat, probably 10 minutes after the sun goes down the heat convection they create is gone.

Bruce said at February 5, 2010 11:53 AM:

"and save in both hot AND cold climates "

Since I don't have an air conditioner in my climate, a white roof would save me ZERO $ in the summer.

But in the winter I spend a lot to keep the house I am in at 68-70 degrees. How much extra would a white roof cost me - noting that sunshine is relatively rare in the winter where I live. :(

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