Will white roofs really slow global warming?
I'm feeling a little bit dated. No, it's not because I go around singing tunes from the '60s. It's my roof. I have one of those dark, shingled affairs. Energy Secretary Steven Chu advises to "make it white."
What's wrong with dark roofs? Simple: they absorb most of the rays from the sun that happen to hit them each day, and those rays ultimately show up as heat that brings warmer temperatures for your home, your neighborhood, and your city. Not good for urban heat islands -- and if you run your air-conditioner a lot, not good for global warming.
White roofs, on the other hand, reflect rather than absorb most of the sunlight, and that means less heat, lower temperatures, and less energy needed for air-conditioning.
So white roofs are a great idea, right? Well, depending on your regional climate, there might be a catch, so maybe hold off calling your local roofer for some brighter shingles.
Sure, having a white roof in the summer is great because it cools when we all want some cooling. But what about the winter, when we actually want the heat from the sun? Is it really better to lose the winter heat in order to lose the summer heat?
Keith Oleson of the National Center for Atmospheric Research in Boulder and colleagues attempted to answer that very question. They coupled a global climate model to a so-called urban canyon model (to simulate the effects of changing roof surfaces on near-surface air temperatures), and reported their results last week in Geophysical Research Letters.
As expected, Oleson et al found that white roofs definitely provided cooler temperatures during the summer months but also caused cooler temperatures in the winter. And there's the rub. The cooler it is in winter, the more energy (and presumably fossil fuels) will be needed to heat homes and buildings. Given that emissions from energy consumption contribute far more to global warming than urban heat island effects, this is a problem.
The question then becomes which is more important: the extra heating in winter or the cooling savings in summer?
Oleson's calculations indicate the former: overall, if white roofs became ubiquitous, the extra energy needed for heating in the winter would exceed the energy savings in the summer. And, assuming that most heating and cooling comes from burning fossil fuels, that would mean an overall increase in global carbon dioxide (CO2) emissions. Clearly not an ideal recipe for addressing global warming.
(One caveat to Oleson's calculations: his team's model adopts the current global distribution of heating and cooling, and, in case you're wondering, most of the current cooling occurs in the United States. The researchers note that the benefits of white roofs will grow as the use of air-conditioning around the world grows. Of course, rising A/C use, unless it's from carbon-free energy sources, which is a tough bet for now, is probably not a great recipe for addressing global warming either.)
So is the white roof craze just plain crazy? Well, no. It makes a lot of sense in the summer, just not so much in the winter.
One solution would be to have the roofers out to your house twice a year to change your roof from white to black and back to white, and so forth, to get both energy savings and temperature benefits. (See NYT graphic [pdf] showing top five states where CO2 emission cuts would be greatest.)
If you don't care for that solution, some students from the Massachusetts Institute of Technology have struck upon a better idea. A group of recent M.I.T. grads, as reported on the school's web site, has developed a temperature-sensitive tile -- it's black when temperatures are cold and white when temps are warm. It's a chameleon roof tile -- so adding the Greek word for heat to the English word for the colorful lizard, they named their invention Thermeleon.
Cool, huh? And warm! But don't get too excited. The Thermeleon still needs some work so you won't be able to find it at the local Home Depot or Lowe's.
But there are similar products on the market already heating up competition, such as these temperature-sensitive glass tiles, though they haven't quite made it onto a roof yet. White or "cool" roofs, however, encouraged in commercial building codes in California, Florida, and Georgia, are available and are taking off in some areas here in the states and abroad. (More info on cool roofs from Energy Star here and here.)
If I lived in California or Florida, where winter heating is negligible, I'd definitely "make it white" with one of those cool roofs. But here in North Carolina, where we experience chilly winters, I will remain dated with my dark, shingled roof, awaiting technology and the marketplace to catch up with the idea.
Originally posted at www.thegreengrok.com.
Follow Bill Chameides on Twitter: www.twitter.com/theGreenGrok
Do you have any ideas on what may be influencing this year's severe cold weather and heavy snow falls on the East Coast and in Europe?
Two things come to my own mind, but I don't have a lot of data to check them with.
First, I read somewhere that El Nino can influence the Jet Stream somehow, bringing severe winters further south in North America, while making for milder winters from Alaska through Canada to Maine. Any truth to that?
Secondly, there has been some discussion that the Gulf Stream ocean current has been weakening, perhaps due to higher temps in the polar regions and melting of fresh water ice. Since the Gulf Stream is said to be a strong determinant for the climate from the Caribean to Europe, any weakening of this current would seem to be able to allow colder temps in the areas it flows near. One study mentioned an observed 30% weakening of the Gulf Stream since the 1970s. There was also a recent article that coral near Florida was dying due to cold temps this winter. Since it would be cold water temps, and not cold air temps, that effect the coral, it seems to me another possible indicator for a weakening of the Gulf Stream.
Any comments would be most welcome!
So I wouldn't recommend waiting for new roofing materials to be developed.
Walls covered with windows, greenhouses, glassed-in porches, catch the slanted sunlight and can provide good heating in winter daytime. But no way can the roof reflections in winter be as powerful as in summer.
It is amazing how hot the greenhouse/patio can get in winter, but a quick look at the attachment makes it obvious that most of the incoming rays from the sun are hitting the side walls, not the roof. The sun is too low in the sky to do much heating on the roof, because the slope on the attachment is too flat to catch much of the suns rays in winter, and the nights are longer than the days.
In the winter, when we want daytime heat, we open the oversized door between the house and the attachment, and in the summer, when we want nightime cool air, we again open the door between the house and the attachment. It is very convenient.
We also have a fireplace between the house and the attachment, so that when we burn firewood from our home-grown trees, it warms the attachment, instead of the outside of the house.
http://www.dailymail.co.uk/news/article-1250872/Climategate-U-turn-Astonishment-scientist-centre-global-warming-email-row-admits-data-organised.html
Hasn't it been cold in Durham this winter? BBBRRRR I'm ready for spring.
These evaluations are easy to do using existing energy modeling packages such as the eQUEST software suite available for free download from the DOE website. Using parametric analysis, design engineers can identify the precise albedo that optimizes annual energy consumption, electricity vs. gas consumption, energy cost, and/or carbon footprint.
I suspect that a better "one-size-fits-all" solution for roofing surfaces would be photovoltaic panels, ideally an with integral water loop for preheating domestic and/or heating hot water when panel surface temperatures permit.
I would love to see the cost of photovoltaic roofing panels come down some more.
The reverse is true in June, with 15 hours of daylight and the sun very high in the sky for most of the day, so a lot of benefit from a reflective roof.
In Greece, they have been painting their roofs white for thousands of years. and they use architecture to encourage drafts. Maybe we are finally learning from what the ancient Greeks knew.
I live in Southern California.
Our heating bills are negligible only in comparison to more northern climes. Our heating bills are much higher in winter than in summer, although they pale in comparison to the rest of the country. A neighbor told me what she used to pay in the midwest, and I can't imagine paying that much for heating. It is shocking.
a stair case roof with black verticals, and white horizontals.